publications

Preprints

1. Trapped Transient Magnons in the Gapped Antiferromagnet Sr₃Ir₂O₇ D. G. Mazzone, D. Meyers, Y. Cao, J. G. Vale, C. D. Dashwood, Y. Shi, A. J. A. James, N. J. Robinson, J. Q. Lin, V. Thampy, Y. Tanaka, A. S. Johnson, H. Miao, R. Wang, T. A. Assefa, J. Kim, D. Casa, R. Mankowsky, D. Zhu, R. Alonso-Mori, S. Song, H. Yavas, T. Katayama, M. Yabashi, Y. Kubota S. Owada, J. Liu, J. Yang, R. M. Konik, I. K. Robinson, J. P. Hill, D. F. McMorrow, M. Forst, S. Wall, X. Liu, and M. P. M. Dean 2002.07301 (2020) [arXiv]
2. Discovery of Charge Density Waves in Cuprate Superconductors up to the Critical Doping and Beyond H. Miao, G. Fabbris, C. S. Nelson, R. Acevedo-Esteves, Y. Li, G. D. Gu, T. Yilmaz, K. Kaznatcheev, E. Vescovo, M. Oda, K. Kurosawa, N. Momono, T. A. Assefa, I. K. Robinson, J. M. Tranquada, P. D. Johnson, and M. P. M. Dean arXiv: 2001.10294 (2020) [arXiv]
3. Nature of the charge-density wave excitations in cuprates J. Q. lin, H. Miao, D. G. Mazzone, G. D. Gu, A. Nag, A. C. Walters, M. Garcia-Fernandez, A. Barbour, J. Pelliciari, I. Jarrige, M. Oda, K. Kurosawa, N. Momono, K. Zhou, V. Bisogni, X. Liu, and M. P. M. Dean arXiv: 2001.10312 (2020) [arXiv]

2020

1. Strain-Modulated Slater-Mott Crossover of Pseudospin-Half Square-Lattice in (SrIrO₃)₁/(SrTiO₃)₁ Superlattices Junyi Yang, Lin Hao, Derek Meyers, Tamene Dasa, Liubin Xu, Lukas Horak, Padraic Shafer, Elke Arenholz, Gilberto Fabbris, Yongseong Choi, Daniel Haskel, Jenia Karapetrova, Jong-Woo Kim, Philip J. Ryan, Haixuan Xu, Cristian D. Batista, Mark P. M. Dean, and Jian Liu Phys. Rev. Lett. 124, 177601 (2020) [Abs] [PDF] [HTML]

   We report on the epitaxial strain-driven electronic and antiferromagnetic modulations of a pseudospin-half square-lattice realized in superlattices of (SrIrO₃)₁/(SrTiO₃)₁. With increasing compressive strain, we find the low-temperature insulating behavior to be strongly suppressed with a corresponding systematic reduction of both the Néel temperature and the staggered moment. However, despite such a suppression, the system remains weakly insulating above the Néel transition. The emergence of metallicity is observed under large compressive strain but only at temperatures far above the Néel transition. These behaviors are characteristics of the Slater-Mott crossover regime, providing a unique experimental model system of the spin-half Hubbard Hamiltonian with a tunable intermediate coupling strength.

2. Scaling behavior of low-temperature orthorhombic domains in the prototypical high-temperature superconductor La_2−xBa_xCuO₄ T. A. Assefa, Y. Cao, J. Diao, R. J. Harder, W. Cha, K. Kisslinger, G. D. Gu, J. M. Tranquada, M. P. M. Dean, and I. K. Robinson Phys. Rev. B 101, 054104 (2020) [Abs] [PDF] [HTML]

   Structural symmetry breaking and recovery in condensed-matter systems are closely related to exotic physical properties such as superconductivity (SC), magnetism, spin density waves, and charge density waves (CDWs). The interplay between different order parameters is intricate and often subject to intense debate, as in the case of CDW order and superconductivity. In La_2−xBa_xCuO₄ (LBCO), the low-temperature structural domain walls are hypothesized as nanometer-scale pinning sites for the CDWs. Coherent x-ray diffraction techniques have been employed here to visualize the domain structures associated with these symmetry changes directly during phase transition. We have pushed Bragg coherent diffractive imaging (BCDI) into the cryogenic regime where most phase transitions in quantum materials reside. Utilizing BCDI, we image the structural evolution of LBCO microcrystal samples during the high-temperature tetragonal to low-temperature orthorhombic (LTO) phase transition. Our results show the formation of LTO domains close to the transition temperature and how the domain size decreases with temperature. The number of domains follows the secondary order parameter (or orthorhombic strain) measurement with a critical exponent that is consistent with the three-dimensional universality class.

3. Doping evolution of the charge excitations and electron correlations in electron-doped superconducting La_2-xCe_xCuO₄ J. Q. Lin, Jie Yuan, Kui Jin, Z. P. Yin, Gang Li, Ke-Jin Zhou, Xingye Lu, M. Dantz, Thorsten Schmitt, H. Ding, Haizhong Guo, M. P. M. Dean, and X. Liu npj Quantum Materials 5, 4 (2020) [Abs] [PDF] [HTML]

   Electron correlations play a dominant role in the charge dynamics of the cuprates. We use resonant inelastic X-ray scattering (RIXS) to track the doping dependence of the collective charge excitations in electron doped La_2-xCe_xCuO₄ (LCCO). From the resonant energy dependence and the out-of-plane momentum dependence, the charge excitations are identified as three-dimensional (3D) plasmons, which reflect the nature of the electronic structure and Coulomb repulsion on both short and long lengthscales. With increasing electron doping, the plasmon excitations increase monotonically in energy, a consequence of the electron correlation effect on electron structure near the Fermi surface (FS). Importantly, the plasmon excitations evolve from a broad feature into a well-defined peak with much increased life time, revealing the evolution of the electrons from incoherent states to coherent quasi-particles near the FS. Such evolution marks the reduction of the short-range electronic correlation, and thus the softening of the Mottness of the system with increasing electron doping.

2019

1. Depth-Resolved Modulation of Metal–Oxygen Hybridization and Orbital Polarization across Correlated Oxide Interfaces Paul C. Rogge, Padraic Shafer, Gilberto Fabbris, Wen Hu, Elke Arenholz, Evguenia Karapetrova, Mark P. M. Dean, Robert J. Green, and Steven J. May Advanced Materials 31, 1902364 (2019) [Abs] [HTML]

   Abstract Interface-induced modifications of the electronic, magnetic, and lattice degrees of freedom drive an array of novel physical properties in oxide heterostructures. Here, large changes in metal–oxygen band hybridization, as measured in the oxygen ligand hole density, are induced as a result of interfacing two isovalent correlated oxides. Using resonant X-ray reflectivity, a superlattice of SrFeO₃ and CaFeO₃ is shown to exhibit an electronic character that spatially evolves from strongly O-like in SrFeO3 to strongly Fe-like in CaFeO₃. This alternating degree of Fe electronic character is correlated with a modulation of an Fe 3d orbital polarization, giving rise to an orbital superstructure. At the SrFeO₃/CaFeO₃ interfaces, the ligand hole density and orbital polarization reconstruct in a single unit cell of CaFeO₃, demonstrating how the mismatch in these electronic parameters is accommodated at the interface. These results provide new insight into how the orbital character of electrons is altered by correlated oxide interfaces and lays out a broadly applicable approach for depth-resolving band hybridization.

2. Epitaxial growth and antiferromagnetism of Sn-substituted perovskite iridate SrIr_0.8Sn_0.2O₃ Junyi Yang, Lin Hao, Qi Cui, Jiaqi Lin, Lukas Horak, Xuerong Liu, Lu Zhang, Huaixin Yang, Jenia Karapetrova, Jong-Woo Kim, Philip J. Ryan, Mark P. M. Dean, Jinguang Cheng, and Jian Liu Phys. Rev. Materials 3, 124411 (2019) [Abs] [PDF] [HTML]

   5d iridates have shown vast emergent phenomena due to a strong interplay among their lattice, charge, and spin degrees of freedom, because of which the potential in spintronic application of the thin-film form is highly leveraged. Here we have epitaxially stabilized perovskite SrIr_0.8Sn_0.2O₃ on [001] SrTiO₃ substrates through pulsed laser deposition and systematically characterized the structural, electronic, and magnetic properties. Physical property measurements unravel an insulating ground state with a weak ferromagnetism in the compressively strained epitaxial film. The octahedral rotation pattern is identified by synchrotron x-ray diffraction, resolving a mix of a+b−c− and a−b+c− domains. X-ray magnetic resonant scattering directly demonstrates a G-type antiferromagnetic structure of the magnetic order and the spin canting nature of the weak ferromagnetism.

3. Phononic Helical Nodal Lines with PT Protection in MoB₂ T. T. Zhang, H. Miao, Q. Wang, J. Q. Lin, Y. Cao, G. Fabbris, A. H. Said, X. Liu, H. C. Lei, Z. Fang, H. M. Weng, and M. P. M. Dean Phys. Rev. Lett. 123, 245302 (2019) [Abs] [PDF] [HTML]

   While condensed matter systems host both fermionic and bosonic quasiparticles, reliably predicting and empirically verifying topological states is only mature for Fermionic electronic structures, leaving topological Bosonic excitations sporadically explored. This is unfortunate, as Bosonic systems such as phonons offer the opportunity to assess spinless band structures where nodal lines can be realized without invoking special additional symetries to protect against spin-orbit coupling. Here we combine firstprinciples calculations and meV-resolution inelastic x-ray scattering to demonstrate the first realization of parity-time reversal symmetry protected helical nodal lines in the phonon spectrum of MoB₂. This structure is unique to phononic systems as the spin-orbit coupling present in electronic systems tends to lift the degeneracy away from high-symmetry locations. Our study establishes a protocol to accurately identify topological Bosonic excitations, opening a new route to explore exotic topological states in crystalline materials.

4. Anomalous magnetoresistance due to longitudinal spin fluctuations in a Jeff = 1/2 Mott semiconductor Lin Hao, Zhentao Wang, Junyi Yang, D. Meyers, Joshua Sanchez, Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim, Daniel Haskel, Philip J. Ryan, Kipton Barros, Jiun-Haw Chu, M. P. M. Dean, Cristian D. Batista, and Jian Liu Nature Communications 10, 5301 (2019) [Abs] [PDF] [HTML]

   As a hallmark of electronic correlation, spin-charge interplay underlies many emergent phenomena in doped Mott insulators, such as high-temperature superconductivity, whereas the half-filled parent state is usually electronically frozen with an antiferromagnetic order that resists external control. We report on the observation of a positive magnetoresistance that probes the staggered susceptibility of a pseudospin-half square-lattice Mott insulator built as an artificial SrIrO3/SrTiO3 superlattice. Its size is particularly large in the high-temperature insulating paramagnetic phase near the Néel transition. This magnetoresistance originates from a collective charge response to the large longitudinal spin fluctuations under a linear coupling between the external magnetic field and the staggered magnetization enabled by strong spin-orbit interaction. Our results demonstrate a magnetic control of the binding energy of the fluctuating particle-hole pairs in the Slater-Mott crossover regime analogous to the Bardeen-Cooper-Schrieffer-to-Bose-Einstein condensation crossover of ultracold-superfluids.

5. Magnetism in iridate heterostructures leveraged by structural distortions Derek Meyers, Yue Cao, Gilberto Fabbris, Neil J Robinson, Lin Hao, Clayton Frederick, Nathan Traynor, Junyi Yang, Jiaqi Lin, MH Upton, D. Casa, Jong-Woo Kim, T. Gog, E. Karapetrova, Yongseong Choi, D. Haskel, P. J. Ryan, Lukas Horak, X. Liu, Jian Liu, and M. P. M. Dean Scientific reports 9, 4263 (2019) [Abs] [PDF] [HTML]

   Fundamental control of magnetic coupling through heterostructure morphology is a prerequisite for rational engineering of magnetic ground states. We report the tuning of magnetic interactions in superlattices composed of single and bilayers of SrIrO₃ inter-spaced with SrTiO₃ in analogy to the Ruddlesden-Popper series iridates. Magnetic scattering shows predominately c-axis antiferromagnetic orientation of the magnetic moments for the bilayer, as in Sr₃Ir₂O₇. However, the magnetic excitation gap, measured by resonant inelastic x-ray scattering, is quite different between the two structures, evidencing a significant change in the stability of the competing magnetic phases. In contrast, the single layer iridate hosts a more bulk-like gap. We find these changes are driven by bending of the c-axis Ir-O-Ir bond, which is much weaker in the single layer, and subsequent local environment changes, evidenced through x-ray diffraction and magnetic excitation modeling. Our findings demonstrate how large changes in the magnetic interactions can be tailored and probed in spin-orbit coupled heterostructures by engineering subtle structural modulations.

6. Ultrafast dynamics of spin and orbital correlations in quantum materials: an energy-and momentum-resolved perspective Y Cao, DG Mazzone, D Meyers, JP Hill, X Liu, S Wall, and MPM Dean Philosophical Transactions of the Royal Society A 377, 20170480 (2019) [Abs] [PDF] [HTML]

   Many remarkable properties of quantum materials emerge from states with intricate coupling between the charge, spin and orbital degrees of freedom. Ultrafast photo-excitation of these materials holds great promise for understanding and controlling the properties of these states. Here, we introduce time-resolved resonant inelastic X-ray scattering (tr-RIXS) as a means of measuring the charge, spin and orbital excitations out of equilibrium. These excitations encode the correlations and interactions that determine the detailed properties of the states generated. After outlining the basic principles and instrumentations of tr-RIXS, we review our first observations of transient antiferromagnetic correlations in quasi two dimensions in a photo-excited Mott insulator and present possible future routes of this fast-developing technique. The increasing number of X-ray free electron laser facilities not only enables tackling long-standing fundamental scientific problems, but also promises to unleash novel inelastic X-ray scattering spectroscopies.

7. Direct Detection of Dimer Orbitals in Ba₅AlIr₂O₁₁ Y Wang, Ruitang Wang, Jungho Kim, MH Upton, D Casa, T Gog, G Cao, G Kotliar, MPM Dean, and X Liu Phys. Rev. Lett. 122, 106401 (2019) [Abs] [PDF] [HTML]

   The electronic states of many Mott insulators, including iridates, are often conceptualized in terms of localized atomic states such as the famous “J_eff=1/2 state.” Although orbital hybridization can strongly modify such states and dramatically change the electronic properties of materials, probing this process is highly challenging. In this Letter, we directly detect and quantify the formation of dimer orbitals in an iridate material Ba₅AlIr₂O₁₁ using resonant inelastic x-ray scattering. Sharp peaks corresponding to the excitations of dimer orbitals are observed and analyzed by a combination of density functional theory calculations and theoretical simulations based on an Ir-Ir cluster model. Such partially delocalized dimer states lead to a redefinition of the angular momentum of the electrons and changes in the magnetic and electronic behaviors of the material. We use this to explain the reduction of the observed magnetic moment with respect to predictions based on atomic states. This study opens new directions to study dimerization in a large family of materials, including solids, heterostructures, molecules, and transient states.

8. EDRIXS: An open source toolkit for simulating spectra of resonant inelastic x-ray scattering YL Wang, G Fabbris, MPM Dean, and G Kotliar Computer Physics Communications 243, 151–165 (2019) [Abs] [HTML] [arXiv]

   Resonant inelastic x-ray scattering (RIXS) has become a very powerful experimental technique to probe a broad range of intrinsic elementary excitations, for example, from low energy phonons and (bi-)magnons to high energy -, charge-transfer and plasmon excitations in strongly correlated electronic systems. Due to the complexity of the RIXS cross-section and strong core-hole effects, theoretical simulation of the experimental RIXS spectra is still a difficult task which hampers the understanding of RIXS spectra and the development of the RIXS technique. In this paper, we present an open source toolkit (dubbed EDRIXS) to facilitate the simulations of RIXS spectra of strongly correlated materials based on exact diagonalization (ED) of certain model Hamiltonians. The model Hamiltonian can be from a single atom, small cluster or Anderson impurity model, with model parameters from density functional theory plus Wannier90 or dynamical mean-field theory calculations. The spectra of x-ray absorption spectroscopy (XAS) and RIXS are then calculated using Krylov subspace techniques. This toolkit contains highly efficient ED, XAS and RIXS solvers written in modern Fortran 90 language and a convenient Python library used to prepare inputs and set up calculations. We first give a short introduction to RIXS spectroscopy, and then we discuss the implementation details of this toolkit. Finally, we show three examples to demonstrate its usage.

9. Charge density wave memory in a cuprate superconductor XM Chen, C Mazzoli, Y Cao, V Thampy, AM Barbour, W Hu, M Lu, TA Assefa, H Miao, G Fabbris, G. D. Gu, J. M. Tranquada, M. P. M. Dean, S. B. Wilkins, and I. K. Robinson Nature communications 10, 1435 (2019) [Abs] [PDF] [HTML] [arXiv] [Supp]

   Although CDW correlations are a ubiquitous feature of the superconducting cuprates, their disparate properties suggest a crucial role for pinning the CDW to the lattice. Here, we report coherent resonant X-ray speckle correlation analysis, which directly determines the reproducibility of CDW domain patterns in La_1.875Ba_0.125CuO₄ (LBCO 1/8) with thermal cycling. While CDW order is only observed below 54 K, where a structural phase transition creates inequivalent Cu-O bonds, we discover remarkably reproducible CDW domain memory upon repeated cycling to far higher temperatures. That memory is only lost on cycling to 240(3) K, which recovers the four-fold symmetry of the CuO₂ planes. We infer that the structural features that develop below 240 K determine the CDW pinning landscape below 54 K. This opens a view into the complex coupling between charge and lattice degrees of freedom in superconducting cuprates.

10. Epitaxial stabilization of Sr₃Ir₂O₇ thin films Junyi Yang, Lin Hao, Peyton Nanney, Kyle Noordhoek, Derek Meyers, Lukas Horak, Joshua Sanchez, Jiun-Haw Chu, Christie Nelson, Mark PM Dean, and Jian Liu Applied Physics Letters 114, 182401 (2019) [Abs] [PDF] [HTML]

    Ruddlesden-Popper type Sr_n+1Ir_nO_3n+1 compounds are a major focus of condensed matter physics, where the subtle balance between electron-electron correlation, spin–orbit interaction, and crystal field effect brings a host of emergent phenomena. While it is understandable that a canted antiferromagnetic insulating state with an easy-plane anisotropy is developed in Sr₂IrO₄ as the two-dimensional limit of the series, it is intriguing that bilayer Sr₃Ir₂O₇, with slightly higher effective dimensionality, stabilizes c-axis collinear antiferromagnetism. This also renders Sr3Ir2O7 a unique playground to study exotic physics near a critical spin transition point. However, the epitaxial growth of Sr3Ir2O7 is still a challenging task because of the narrow growth window. In our research, we have studied the thermodynamic process during the synthesis of Sr₃Ir₂O₇ thin films. We expanded the synthesis window by mapping out the relationship between the thin film crystal structure and the gas pressure. Our work thus provides a more accessible avenue to stabilize metastable materials.

11. Control of dopant crystallinity in electrochemically treated cuprate thin films Alex Frano, Martin Bluschke, Zhijun Xu, Benjamin Frandsen, Yi Lu, Ming Yi, Ronald Marks, Apurva Mehta, Valery Borzenets, Derek Meyers, M. P. M. Dean, F. Baiutti, J. Maier, G. Kim, G. Christiani, G. Logvenov, Benckiser, B. Keimer, and R. J. Birgeneau Phys. Rev. Mat. 3, 063803 (2019) [Abs] [PDF] [HTML]

    We present a methodology based on ex situ (postgrowth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide La₂CuO_4+y grown epitaxially on substrates of isostructural LaSrAlO₄. The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and we describe the resulting methodology as a platform to be explored in thin films of other transition-metal oxides.

12. Momentum-resolved lattice dynamics of parent and electron-doped Sr₂IrO₄ C. D. Dashwood, H. Miao, J. G. Vale, D. Ishikawa, D. A. Prishchenko, V. V. Mazurenko, V. G. Mazurenko, R. S. Perry, G. Cao, A. Torre, F. Baumberger, A. Q. R. Baron, D. F. McMorrow, and M. P. M. Dean Phys. Rev. B 100, 085131 (2019) [Abs] [PDF] [HTML]

    The mixing of orbital and spin character in the wave functions of the 5d iridates has led to predictions of strong couplings among their lattice, electronic, and magnetic degrees of freedom. As well as realizing a novel spin-orbit assisted Mott-insulating ground state, the perovskite iridate Sr₂IrO₄ has strong similarities with the cuprate La₂CuO₄, which on doping hosts a charge-density wave that appears intimately connected to high-temperature superconductivity. These phenomena can be sensitively probed through momentum-resolved measurements of the lattice dynamics, made possible by meV-resolution inelastic x-ray scattering. Here we report the first such measurements for both parent and electron-doped Sr₂IrO₄. We find that the low-energy phonon dispersions and intensities in both compounds are well described by the same nonmagnetic density functional theory calculation. In the parent compound, no changes of the phonons on magnetic ordering are discernible within the experimental resolution, and in the doped compound no anomalies are apparent due to charge-density waves. These measurements extend our knowledge of the lattice properties of (Sr_1−xLa_x)₂IrO₄ and constrain the couplings of the phonons to magnetic and charge order.

13. Formation of Incommensurate Charge Density Waves in Cuprates H. Miao, R. Fumagalli, M. Rossi, J. Lorenzana, G. Seibold, F. Yakhou-Harris, K. Kummer, N. B. Brookes, G. D. Gu, L. Braicovich, G. Ghiringhelli, and M. P. M. Dean Phys. Rev. X 9, 031042 (2019) [Abs] [PDF] [HTML]

    Although charge density waves (CDWs) are omnipresent in cuprate high-temperature superconductors, they occur at significantly different wave vectors, confounding efforts to understand their formation mechanism. Here, we use resonant inelastic x-ray scattering to investigate the doping- and temperature-dependent CDW evolution in La_2-xBa_xCuO₄ (x=0.115–0.155). We discover that the CDW develops in two stages with decreasing temperature. A precursor CDW with a quasicommensurate wave vector emerges first at high temperature. This doping-independent precursor CDW correlation originates from the CDW phase mode coupled with a phonon and “seeds” the low-temperature CDW with a strongly doping-dependent wave vector. Our observation reveals the precursor CDW and its phase mode as the building blocks of the highly intertwined electronic ground state in the cuprates.

14. Strong Orbital Polarization in a Cobaltate-Titanate Oxide Heterostructure Sangjae Lee, Alex Taekyung Lee, Alexandru B. Georgescu, Gilberto Fabbris, Myung-Geun Han, Yimei Zhu, John W. Freeland, Ankit S. Disa, Yichen Jia, Mark P. M. Dean, Frederick J. Walker, Sohrab Ismail-Beigi, and Charles H. Ahn Phys. Rev. Lett. 123, 117201 (2019) [Abs] [PDF] [HTML]

    Through a combination of experimental measurements and theoretical modeling, we describe a strongly orbital-polarized insulating ground state in an (LaTiO₃)₂/(LaCoO₃)₂ oxide heterostructure. X-ray absorption spectra and ab initio calculations show that an electron is transferred from the titanate to the cobaltate layers. The charge transfer, accompanied by a large octahedral distortion, induces a substantial orbital polarization in the cobaltate layer of a size unattainable via epitaxial strain alone. The asymmetry between in-plane and out-of-plane orbital occupancies in the high-spin cobaltate layer is predicted by theory and observed through x-ray linear dichroism experiments. Manipulating orbital configurations using interfacial coupling within heterostructures promises exciting ground-state engineering for realizing new emergent electronic phases in metal oxide superlattices.

15. Domain Texture of the Orthorhombic Phase of La_2−xBa_xCuO₄ Ian Robinson, Tadesse A. Assefa, Yue Cao, Genda Gu, Ross Harder, Evan Maxey, and Mark P. M. Dean Journal of Superconductivity and Novel Magnetism (2019) [Abs] [PDF] [HTML]

    Bragg coherent diffraction imaging (BCDI) experiments have been carried out at the Advanced Photon Source using a new cryostat system developed to achieve high mechanical stability and low vibrations. We measured the (012)_LTO Bragg peak which is unique to the low-temperature orthorhombic (LTO) phase of micron-sized crystals of the high-temperature superconductor La_2−xBa_xCuO₄ (LBCO) to study the formation of structural domains. Each time the sample was cooled into the orthorhombic phase, the diffraction pattern of domains was different. This confirms the interpretation of pinning of the lower-temperature charge density Wave domains observed in coherent resonant X-ray speckle correlation analysis experiments.

2018

1. Imaging antiferromagnetic antiphase domain boundaries using magnetic Bragg diffraction phase contrast Min Gyu Kim, Hu Miao, Bin Gao, S-W Cheong, C Mazzoli, A Barbour, Wen Hu, SB Wilkins, Ian K Robinson, MPM Dean, and V. Kiryukhin Nature communications 9, 5013 (2018) [Abs] [PDF] [HTML] [BNL Press Release]

   Manipulating magnetic domains is essential for many technological applications. Recent breakthroughs in Antiferromagnetic Spintronics brought up novel concepts for electronic device development. Imaging antiferromagnetic domains is of key importance to this field. Unfortunately, some of the basic domain types, such as antiphase domains, cannot be imaged by conventional techniques. Herein, we present a new domain projection imaging technique based on the localization of domain boundaries by resonant magnetic diffraction of coherent X rays. Contrast arises from reduction of the scattered intensity at the domain boundaries due to destructive interference effects. We demonstrate this approach by imaging antiphase domains in a collinear antiferromagnet Fe₂Mo₃O₈, and observe evidence of domain wall interaction with a structural defect. This technique does not involve any numerical algorithms. It is fast, sensitive, produces large-scale images in a single-exposure measurement, and is applicable to a variety of magnetic domain types.

2. Giant magnetic response of a two-dimensional antiferromagnet Lin Hao, D Meyers, Hidemaro Suwa, Junyi Yang, Clayton Frederick, Tamene R Dasa, Gilberto Fabbris, Lukas Horak, Dominik Kriegner, Yongseong Choi, Jong-Woo Kim, Daniel Haskel, Philip J. Ryan, Haixuan Xu, Cristian D. Batista, M. P. M. Dean, and Jian Liu Nature Physics 14, 806 (2018) [Abs] [HTML] [arXiv] [BNL Press Release] [UTK Press Release] [ANL Highlight]

   A fundamental difference between antiferromagnets and ferromagnets is the lack of linear coupling to a uniform magnetic field due to the staggered order parameter1. Such coupling is possible via the Dzyaloshinskii–Moriya (DM) interaction2,3, but at the expense of reduced antiferromagnetic (AFM) susceptibility due to the canting-induced spin anisotropy4. We solve this long-standing problem with a top-down approach that utilizes spin–orbit coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM responses to sub-tesla external fields by exploiting the extremely strong two-dimensional critical fluctuations preserved under a symmetry-invariant exchange anisotropy, which is built into a square lattice artificially synthesized as a superlattice of SrIrO₃ and SrTiO₃. The observed field-induced logarithmic increase of the ordering temperature enables highly efficient control of the AFM order. Our results demonstrate that symmetry can be exploited in spin–orbit-coupled magnets to develop functional AFM materials for fast and secured spintronic devices5,6,7,8,9.

3. Observation of double Weyl phonons in parity-breaking FeSi Hu Miao, TT Zhang, Le Wang, Derek Meyers, AH Said, YL Wang, YG Shi, HM Weng, Zhong Fang, and MPM Dean Phys. Rev. Lett. 121, 035302 (2018) [Abs] [PDF] [HTML] [ANL Highlight]

   Condensed matter systems have now become a fertile ground to discover emerging topological quasiparticles with symmetry protected modes. While many studies have focused on fermionic excitations, the same conceptual framework can also be applied to bosons yielding new types of topological states. Motivated by Zhang et al.’s recent theoretical prediction of double Weyl phonons in transition metal monosilicides [Phys. Rev. Lett. 120, 016401 (2018)], we directly measure the phonon dispersion in parity-breaking FeSi using inelastic x-ray scattering. By comparing the experimental data with theoretical calculations, we make the first observation of double Weyl points in FeSi, which will be an ideal material to explore emerging bosonic excitations and its topologically nontrivial properties.

4. Emergent c-axis magnetic helix in manganite-nickelate superlattices Gilberto Fabbris, N Jaouen, D Meyers, J Feng, JD Hoffman, R Sutarto, SG Chiuzbăian, A Bhattacharya, and MPM Dean Phys. Rev. B 98, 180401 (2018) [Abs] [PDF] [HTML]

   The nature of the magnetic order in (La_2/3Sr_1/3MnO₃)₉/(LaNiO₃)₃ superlattices is investigated using x-ray resonant magnetic reflectometry. We observe a new c-axis magnetic helix state in the (LaNiO₃)₃ layers that had never been reported in nickelates, and which mediates the  130deg magnetic coupling between the ferromagnetic (La_2/3Sr_1/3MnO₃)₉ layers, illustrating the power of x-rays for discovering the magnetic state of complex oxide interfaces. Resonant inelastic x-ray scattering and x-ray absorption spectroscopy show that Ni-O ligand hole states from bulk LaNiO₃ are mostly filled due to interfacial electron transfer from Mn, driving the Ni orbitals closer to an atomic-like 3d⁸ configuration. We discuss the constraints imposed by this electronic configuration to the microscopic origin of the observed magnetic structure. The presence of a magnetic helix in (La_2/3Sr_1/3MnO₃)₉/(LaNiO₃)₃ is crucial for modeling the potential spintronic functionality of this system and may be important for designing emergent magnetism in novel devices in general.

5. Universal 2 Δ_max/k_BT_c scaling decoupled from the electronic coherence in iron-based superconductors Hu Miao, WH Brito, ZP Yin, RD Zhong, GD Gu, PD Johnson, MPM Dean, S Choi, G Kotliar, W Ku, X. C. Wang, C. Q. Jin, S.-F. Wu, T. Qian, and H. Ding Phys. Rev. B 98, 020502 (2018) [Abs] [PDF] [HTML]

   Here, we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi-liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe_0.55Se_0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum-superconducting-gap and 2Δ_max/k_BT_c, which shows a universal scaling that is valid for all iron-based superconductors (FeSCs). Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong-coupling pairing mechanism for the FeSCs.

6. Decoupling carrier concentration and electron-phonon coupling in oxide heterostructures observed with resonant inelastic x-ray scattering Derek Meyers, Ken Nakatsukasa, Sai Mu, Lin Hao, Junyi Yang, Yue Cao, G Fabbris, Hu Miao, J Pelliciari, D McNally, M. Dantz, E. Paris, E. Karapetrova, Yongseong Choi, D. Haskel, P. Shafer, E. Arenholz, Thorsten Schmitt, Tom Berlijn, S. Johnston, Jian Liu, and M. P. M. Dean Phys. Rev. Lett. 121, 236802 (2018) [Abs] [PDF] [HTML]

   We report the observation of multiple phonon satellite features in ultrathin superlattices of the form nSrIrO₃/_mSrTiO₃ using resonant inelastic x-ray scattering (RIXS). As the values of n and m vary, the energy loss spectra show a systematic evolution in the relative intensity of the phonon satellites. Using a closed-form solution for the RIXS cross section, we extract the variation in the electron-phonon coupling strength as a function of n and m. Combined with the negligible carrier doping into the SrTiO₃ layers, these results indicate that the tuning of the electron-phonon coupling can be effectively decoupled from doping. This work both showcases a feasible method to extract the electron-phonon coupling in superlattices and unveils a potential route for tuning this coupling, which is often associated with superconductivity in SrTiO₃-based systems.

7. Inverted orbital polarization in strained correlated oxide films Paul C Rogge, Robert J Green, Padraic Shafer, Gilberto Fabbris, Andi M Barbour, Benjamin M Lefler, Elke Arenholz, Mark PM Dean, and Steven J May Phys. Rev. B 98, 201115 (2018) [Abs] [PDF] [HTML]

   Manipulating the orbital occupation of valence electrons via epitaxial strain in an effort to induce new functional properties requires considerations of how changes in the local bonding environment affect the band structure at the Fermi level. Using synchrotron radiation to measure the x-ray linear dichroism of epitaxially strained films of the correlated oxide CaFeO3, we demonstrate that the orbital polarization of the Fe valence electrons is opposite from conventional understanding. Although the energetic ordering of the Fe $3d$ orbitals is confirmed by multiplet ligand field theory analysis to be consistent with previously reported strain-induced behavior, we find that the nominally higher energy orbital is more populated than the lower. We ascribe this inverted orbital polarization to an anisotropic bandwidth response to strain in a compound with nearly filled bands. These findings provide an important counterexample to the traditional understanding of strain-induced orbital polarization and reveal a method to engineer otherwise unachievable orbital occupations in correlated oxides.

8. Incommensurate phonon anomaly and the nature of charge density waves in cuprates Hu Miao, D Ishikawa, Rolf Heid, Matthieu Le Tacon, Gilberto Fabbris, Derek Meyers, GD Gu, AQR Baron, and MPM Dean Phys. Rev. X 8, 011008 (2018) [Abs] [PDF] [HTML] [Spring8 Research Highlight]

   While charge density wave (CDW) instabilities are ubiquitous to superconducting cuprates, the different ordering wave vectors in various cuprate families have hampered a unified description of the CDW formation mechanism. Here, we investigate the temperature dependence of the low-energy phonons in the canonical CDW-ordered cuprate La_1.875Ba_0.125CuO₄. We discover that the phonon softening wave vector associated with CDW correlations becomes temperature dependent in the high-temperature precursor phase and changes from a wave vector of 0.238 reciprocal lattice units (r.l.u.) below the ordering transition temperature to 0.3 r.l.u. at 300 K. This high-temperature behavior shows that “214”-type cuprates can host CDW correlations at a similar wave vector to previously reported CDW correlations in non-214-type cuprates such as YBa₂Cu₃O_6+δ. This indicates that cuprate CDWs may arise from the same underlying instability despite their apparently different low-temperature ordering wave vectors.

2017

1. Doping dependence of collective spin and orbital excitations in the Spin-1 quantum antiferromagnet La_2-xSr_xNiO4 Observed by X rays G Fabbris, D Meyers, L Xu, VM Katukuri, L Hozoi, X Liu, ZY Chen, J Okamoto, T Schmitt, AC Uldry, B. Delley, G. D. Gu, D. Prabhakaran, A. T. Boothroyd, J. Brink, D. J. Huang, and M. P. M. Dean Phys. Rev. Lett. 118, (2017) [Abs] [PDF] [HTML]

   We report the first empirical demonstration that resonant inelastic x-ray scattering (RIXS) is sensitive to collective magnetic excitations in S=1 systems by probing the Ni L₃ edge of La_2−xSr_xNiO₄ (x=0, 0.33, 0.45). The magnetic excitation peak is asymmetric, indicating the presence of single and multi-spin-flip excitations. As the hole doping level is increased, the zone boundary magnon energy is suppressed at a much larger rate than that in hole doped cuprates. Based on the analysis of the orbital and charge excitations observed by RIXS, we argue that this difference is related to the orbital character of the doped holes in these two families. This work establishes RIXS as a probe of fundamental magnetic interactions in nickelates opening the way towards studies of heterostructures and ultrafast pump-probe experiments.

2. Resonant inelastic X-ray scattering study of spin-wave excitations in the cuprate parent compound Ca₂CuO₂Cl₂ BW Lebert, MPM Dean, Alessandro Nicolaou, Jonathan Pelliciari, Marcus Dantz, Thorsten Schmitt, Runze Yu, Masaki Azuma, J-P Castellan, Hu Miao, A. Gauzzi, B. Baptiste, and M. d’Astuto Phys. Rev. B 95, 155110 (2017) [Abs] [PDF] [HTML]

   By means of resonant inelastic x-ray scattering at the Cu L3 edge, we measured the spin-wave dispersion along <100> and <110> in the undoped cuprate Ca₂CuO₂Cl₂. The data yield a reliable estimate of the superexchange parameter J = 135 ± 4meV using a classical spin-1/2 two-dimensional Heisenberg modelwith nearest-neighbor interactions and including quantum fluctuations. Including further exchange interactions increases the estimate to J = 141 meV. The 40 meV dispersion between the magnetic Brillouin zone boundary points (1/2, 0) and (1/4, 1/4) indicates that next-nearest-neighbor interactions in this compound are intermediate between the values found in La₂CuO₄ and Sr₂CuO₂Cl₂. Due to the low-Z elements composing Ca₂CuO₂Cl₂, the present results may enable a reliable comparison with the predictions of quantum many-body calculations, which would improve our understanding of the role of magnetic excitations and of electronic correlations in cuprates.

3. Doping dependence of the magnetic excitations in La_2-xSr_xCuO₄ D Meyers, H Miao, AC Walters, V Bisogni, RS Springell, Matteo d’Astuto, M Dantz, J Pelliciari, HY Huang, J Okamoto, D. J. Huang, J. P. Hill, X. He, I. Bozovic, T. Schmitt, and M. P. M. Dean Phys. Rev. B 95, 075139 (2017) [Abs] [PDF] [HTML]

   The magnetic correlations within the cuprates have undergone intense scrutiny as part of efforts to understand high-temperature superconductivity. We explore the evolution of the magnetic correlations along the nodal direction of the Brillouin zone in La_2−xSr_xCuO₄, spanning the doping phase diagram from the antiferromagnetic Mott insulator at x=0 to the metallic phase at x=0.26. Magnetic excitations along this direction are found to be systematically softened and broadened with doping, at a higher rate than the excitations along the antinodal direction. This phenomenology is discussed in terms of the nature of the magnetism in the doped cuprates. Survival of the high-energy magnetic excitations, even in the overdoped regime, indicates that these excitations are marginal to pairing, while the influence of the low-energy excitations remains ambiguous.

4. Two-dimensional j_eff= 1/2 antiferromagnetic insulator unraveled from interlayer exchange coupling in artificial perovskite iridate superlattices Lin Hao, Derek Meyers, Clayton Frederick, Gilberto Fabbris, Junyi Yang, Nathan Traynor, Lukas Horak, Dominik Kriegner, Yongseong Choi, Jong-Woo Kim, Daniel Haskel, Phil J. Ryan, M. P. M. Dean, and Jian Liu Phys. Rev. Lett. 119, 027204 (2017) [Abs] [PDF] [HTML]

   We report an experimental investigation of the two-dimensional Jeff=1/ antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [(SrIrO₃)₁, (SrTiO₃)_m] (m = 1, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the m=2 and m=3 samples than the m=1 sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive SrTiO₃ layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the Néel temperature from 150 K for m=1 to 40 K for m=2 demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the SrTiO₃ layer number m, for maintaining canting-induced weak ferromagnetism. The nearly unaltered transition temperature between the m=2 and the m=3 demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.

5. Static charge-density-wave order in the superconducting state of La_2-xBa_xCuO₄ V Thampy, XM Chen, Y Cao, C Mazzoli, AM Barbour, W Hu, H Miao, G Fabbris, RD Zhong, GD Gu, J. M. Tranquada, I. K. Robinson, S. B. Wilkins, and M. P. M. Dean Phys. Rev. B 95, 241111 (2017) [Abs] [PDF] [HTML]

   Charge-density-wave (CDW) correlations feature prominently in the phase diagram of the cuprates, motivating competing theories of whether fluctuating CDW correlations aid superconductivity or whether static CDW order coexists with superconductivity in inhomogeneous or spatially modulated states. Here we report Cu L-edge resonant x-ray photon correlation spectroscopy measurements of CDW correlations in superconducting La_2−xBa_xCuO₄, x = 0.11. Static CDW order is shown to exist in the superconducting state at low temperatures and to persist up to at least 85% of the CDW transition temperature. We discuss the implications of our observations for how nominally competing order parameters can coexist in the cuprates.

6. On the possibility to detect multipolar order in URu₂Si₂ by the electric quadrupolar transition of resonant elastic x-ray scattering YL Wang, G Fabbris, D Meyers, NH Sung, RE Baumbach, Eric Dietzgen Bauer, PJ Ryan, J-W Kim, X Liu, MPM Dean, G. Kotliar, and X. Dai Phys. Rev. B 96, 085146 (2017) [Abs] [PDF] [HTML]

   Resonant elastic x-ray scattering is a powerful technique for measuring multipolar order parameters. In this paper, we theoretically and experimentally study the possibility of using this technique to detect the proposed multipolar order parameters in URu₂Si₂ at the U-L3 edge with the electric quadrupolar transition. Based on an atomic model, we calculate the azimuthal dependence of the quadrupolar transition at the U-L3 edge. The results illustrate the potential of this technique for distinguishing different multipolar order parameters. We then perform experiments on ultraclean single crystals of URu₂Si₂ at the U-L3 edge to search for the predicted signal, but do not detect any indications of multipolar moments within the experimental uncertainty. We theoretically estimate the orders of magnitude of the cross section and the expected count rate of the quadrupolar transition and compare them to the dipolar transitions at the U-M4 and U-L3 edges, clarifying the difficulty in detecting higher order multipolar order parameters in URu2Si2 in the current experimental setup.

7. High-temperature charge density wave correlations in La_1.875Ba_0.125CuO₄ without spin–charge locking H Miao, José Lorenzana, Götz Seibold, YY Peng, A Amorese, F Yakhou-Harris, K Kummer, NB Brookes, RM Konik, V Thampy, G. D. Gu, G. Ghiringhelli, L. Braicovich, and M. P. M. Dean Proceedings of the National Academy of Sciences 114, 12430–12435 (2017) [Abs] [PDF] [HTML] [BNL Press Release]

   Although all superconducting cuprates display charge-ordering tendencies, their low-temperature properties are distinct, imped- ing efforts to understand the phenomena within a single con- ceptual framework. While some systems exhibit stripes of charge and spin, with a locked periodicity, others host charge den- sity waves (CDWs) without any obviously related spin order. Here we use resonant inelastic X-ray scattering to follow the evolution of charge correlations in the canonical stripe-ordered cuprate La_1.875Ba_0.125CuO₄ across its ordering transition. We find that high-temperature charge correlations are unlocked from the wavevector of the spin correlations, signaling analogies to CDW phases in various other cuprates. This indicates that stripe order at low temperatures is stabilized by the coupling of otherwise inde- pendent charge and spin density waves, with important implica- tions for the relation between charge and spin correlations in the cuprates. charge

8. Novel spin-orbit coupling driven emergent states in iridate-based heterostructures Lin Hao, D Meyers, MPM Dean, and Jian Liu Journal of Physics and Chemistry of Solids (2017) [Abs] [HTML] [arXiv]

   Recent years have seen many examples of how the strong spin-orbit coupling present in iridates can stabilize new emergent states that are difficult or impossible to realize in more conventional materials. In this review we outline a representative set of studies detailing how heterostructures based on Ruddlesden-Popper (RP) iridates can be used to access yet more novel physics. Beginning with a short synopsis of iridate thin film growth, the effects of the heterostructure morphology on the RP iridates including Sr₂IrO₄ and SrIrO₃ are discussed. Example studies explore the effects of epitaxial strain, laser-excitation to access transient states, topological semimetallicity in SrIrO₃, 2D magnetism in artificial RP iridates, and interfacial magnetic coupling between iridate and neighboring layers. Taken together, these works show the fantastic potential for controlled engineering of novel quantum phenomena in iridate heterostructures.

9. Giant spin gap and magnon localization in the disordered Heisenberg antiferromagnet Sr₂Ir_1-xRu_xO₄ Yue Cao, Xuerong Liu, Wenhu Xu, Wei-Guo Yin, Derek Meyers, Jungho Kim, Diego Casa, MH Upton, Thomas Gog, Tom Berlijn, Gonzalo Alvarez, Shujuan Yuan, Jasminka Terzic, J. M. Tranquada, John P. Hill, Gang Cao, Robert M. Konik, and M. P. M. Dean Phys. Rev. B 95, 121103 (2017) [Abs] [PDF] [HTML]

   We study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet Sr₂Ir_1-xRu_xO₄. The maximum energy of the magnetic excitation remains robust up to x = 0.77, with a gap opening at low dopings and increasing to over 150 meV at x = 0.77. At these higher Ru concentrations, the dispersive magnetic excitations in Sr₂IrO₄ are rendered essentially momentum independent. Up to a Ru concentration of x = 0.77, both experiments and first-principles calculations show the Ir J_eff = 1/2 state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder. Under the coherent potential approximation, we reproduce the experimental magnetic excitations using the disordered Heisenberg antiferromagnetic model with suppressed next-nearest-neighbor ferromagnetic coupling.

2016

1. Anisotropic softening of magnetic excitations in lightly electron-doped Sr₂IrO₄ Xuerong Liu, MPM Dean, ZY Meng, MH Upton, T Qi, T Gog, Y Cao, JQ Lin, D Meyers, H Ding, G. Cao, and J. P. Hill Phys. Rev. B 93, 241102 (2016) [Abs] [PDF] [HTML]

   The magnetic excitations in electron-doped (Sr_1−xLa_x)₂IrO₄ with x = 0.03 were measured using resonant inelastic x-ray scattering at the Ir L3 edge.Although much broadened, well defined dispersivemagnetic excitations were observed. Comparing with the magnetic dispersion from the undoped compound, the evolution of the magnetic excitations upon doping is highly anisotropic. Along the antinodal direction, the dispersion is almost intact. On the other hand, the magnetic excitations along the nodal direction show significant softening. These results establish the presence of strong magnetic correlations in electron-doped (Sr_1−xLa_x)₂IrO₄ with close analogies to the hole-doped cuprates, further motivating the search for high temperature superconductivity in this system.

2. Orbital engineering in nickelate heterostructures driven by anisotropic oxygen hybridization rather than orbital energy levels G Fabbris, D Meyers, J Okamoto, J Pelliciari, AS Disa, Y Huang, Z-Y Chen, WB Wu, CT Chen, S Ismail-Beigi, C. H. Ahn, F. J. Walker, D. J. Huang, T. Schmitt, and M. P. M. Dean Phys. Rev. Lett. 117, 147401 (2016) [Abs] [PDF] [HTML]

   Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking LaTiO₃−LaNiO₃−3×LaAlO₃, a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni 3d orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O K-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena.

3. Ultrafast energy and momentum resolved dynamics of magnetic correlations in photo-doped Mott insulator Sr₂IrO₄ MPM Dean, Yue Cao, X Liu, S Wall, D Zhu, Roman Mankowsky, V Thampy, XM Chen, JG Vale, D Casa, Jungho Kim, A. H. Said, P. Juhas, R. Alonso-Mori, J. M. Glownia, A. Robert, J. Robinson, M. Sikorski, S. Song, M. Kozina, H. Lemke, L. Patthey, S. Owada, T. Katayama, M. Yabashi, Yoshikazu Tanaka, T. Togashi, J. Liu, C. Rayan Serrao, B. J. Kim, L. Huber, C.-L. Chang, D. F. McMorrow, M. Först, and J. P. Hill Nature Materials 15, 601–605 (2016) [Abs] [HTML] [arXiv] [Featured in News and Views] [BNL Press Release]

   Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and higherature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr₂IrO₄. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Neel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.

4. Remarkable stability of charge density wave order in La_2-xBa_xCuO₄ XM Chen, V Thampy, C Mazzoli, AM Barbour, H Miao, GD Gu, Y Cao, JM Tranquada, MPM Dean, and SB Wilkins Phys. Rev. Lett. 117, 167001 (2016) [Abs] [PDF] [HTML] [BNL Press Release]

   The occurrence of charge-density-wave (CDW) order in underdoped cuprates is now well established, although the precise nature of the CDW and its relationship with superconductivity is not. Theoretical proposals include contrasting ideas such as that pairing may be driven by CDW fluctuations or that static CDWs may intertwine with a spatially modulated superconducting wave function. We test the dynamics of CDW order in La_2-xBa_xCuO₄ by using x-ray photon correlation spectroscopy at the CDW wave vector, detected resonantly at the Cu L3 edge. We find that the CDW domains are strikingly static, with no evidence of significant fluctuations up to 2/3/4 h. We discuss the implications of these results for some of the competing theories.

5. Oscillatory noncollinear magnetism induced by interfacial charge transfer in superlattices composed of metallic oxides Jason D Hoffman, Brian J Kirby, Jihwan Kwon, Gilberto Fabbris, D Meyers, John W Freeland, Ivar Martin, Olle G Heinonen, Paul Steadman, Hua Zhou, Christian M. Schlepütz, Mark P. M. Dean, Suzanne G. E. Velthuis, Jian-Min Zuo, and Anand Bhattacharya Phys. Rev. X 6, 041038 (2016) [Abs] [PDF] [HTML]

   Interfaces between correlated complex oxides are promising avenues to realize new forms of magnetism that arise as a result of charge transfer, proximity effects, and locally broken symmetries. We report on the discovery of a noncollinear magnetic structure in superlattices of the ferromagnetic metallic oxide La_2/3Sr_1/3MnO₃ (LSMO) and the correlated metal LaNiO₃ (LNO). The exchange interaction between LSMO layers is mediated by the intervening LNO, such that the angle between the magnetization of neighboring LSMO layers varies in an oscillatory manner with the thickness of the LNO layer. The magnetic field, temperature, and spacer thickness dependence of the noncollinear structure are inconsistent with the bilinear and biquadratic interactions that are used to model the magnetic structure in conventional metallic multilayers. A model that couples the LSMO layers to a helical spin state within the LNO fits the observed behavior. We propose that the spin-helix results from the interaction between a spatially varying spin susceptibility within the LNO and interfacial charge transfer that creates localized Ni2+ states. Our work suggests a new approach to engineering noncollinear spin textures in metallic oxide heterostructures.

2015

1. Insights into the high temperature superconducting cuprates from resonant inelastic X-ray scattering MPM Dean Journal of Magnetism and Magnetic Materials 376, 3–13 (2015) [Abs] [PDF] [HTML]

   Recent improvements in instrumentation have established resonant inelastic X-ray scattering (RIXS) as a valuable new probe of the magnetic excitations in the cuprates. This paper introduces RIXS, focusing on the Cu L₃ resonance, and reviews recent experiments using this technique. These are discussed in light of other experimental probes such as inelastic neutron scattering and Raman scattering. The success of these studies has motivated the development of several new RIXS spectrometers at synchrotrons around the world that promise, among other improvements, 5–10 times better energy resolution. We finish by outlining several key areas which hold promise for further important discoveries in this emerging field.

2. Probing single magnon excitations in Sr₂IrO₄ using O K-edge resonant inelastic X-ray scattering X Liu, MPM Dean, J Liu, SG Chiuzbaian, N Jaouen, A Nicolaou, WG Yin, C Rayan Serrao, R Ramesh, H Ding, and JP Hill Journal of Physics: Condensed Matter 27, 202202 (2015) [Abs] [HTML] [arXiv]

   Resonant inelastic x-ray scattering (RIXS) at the L-edge of transition metal elements is now commonly used to probe single magnon excitations. Here we show that single magnon excitations can also be measured with RIXS at the K-edge of the surrounding ligand atoms when the center heavy metal elements have strong spin–orbit coupling. This is demonstrated with oxygen K-edge RIXS experiments on the perovskite Sr₂IrO₄, where low energy peaks from single magnon excitations were observed. This new application of RIXS has excellent potential to be applied to a wide range of magnetic systems based on heavy elements, for which the L-edge RIXS energy resolution in the hard x-ray region is usually poor.

3. Superconductivity in graphite intercalation compounds Robert P Smith, Thomas E Weller, Christopher A Howard, Mark PM Dean, Kaveh C Rahnejat, Siddharth S Saxena, and Mark Ellerby Physica C: Superconductivity and its Applications 514, 50–58 (2015) [Abs] [HTML] [arXiv]

   The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s (Dresselhaus and Dresselhaus, 1981; Enoki et al., 2003). This paper recontextualizes the field in light of the discovery of superconductivity in CaC₆ and YbC₆ in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how these relate to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity, and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition.

4. First-order melting of a weak spin-orbit Mott insulator into a correlated metal Tom Hogan, Z. Yamani, D. Walkup, Xiang Chen, Rebecca Dally, Thomas Z. Ward, M. P. M. Dean, John Hill, Z. Islam, Vidya Madhavan, and Stephen D. Wilson Phys. Rev. Lett. 114, 257203 (2015) [Abs] [PDF] [HTML]

   The electronic phase diagram of the weak spin-orbit Mott insulator (Sr_1−xLa_x)₃Ir₂O₇ is determined via an exhaustive experimental study. Upon doping electrons via La substitution, an immediate collapse in resistivity occurs along with a narrow regime of nanoscale phase separation comprised of antiferromag- netic, insulating regions and paramagnetic, metallic puddles persisting until x ≈ 0.04. Continued electron doping results in an abrupt, first-order phase boundary where the Néel state is suppressed and a homogenous, correlated, metallic state appears with an enhanced spin susceptibility and local moments. As the metallic state is stabilized, a weak structural distortion develops and suggests a competing instability with the parent spin-orbit Mott state.

2014

1. Ferroelectric quantum criticality SE Rowley, LJ Spalek, RP Smith, MPM Dean, M Itoh, JF Scott, GG Lonzarich, and SS Saxena Nature Physics 10, 367 (2014) [Abs] [HTML] [arXiv]

   Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new effects, like the breakdown of the conventional Fermi-liquid theory of metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure or chemical or isotopic substitution. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the quantum fluctuations of the electrical polarization. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state as the difficulties found in metals due to a high density of gapless excitations on the Fermi surface are avoided. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO₃ and KTaO₃. We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2K in SrTiO₃ and 3K in KTaO₃ believed to arise from coupling to acoustic phonons. Looking ahead, we suggest that in ferroelectric materials supporting mobile charge carriers, quantum paraelectric fluctuations may mediate new effective electron-electron interactions giving rise to a number of possible states such as superconductivity.

2. Rotated stripe order and its competition with superconductivity in La_1.88Sr_0.12CuO₄ V Thampy, MPM Dean, Niels Bech Christensen, L Steinke, Z Islam, M Oda, M Ido, N Momono, SB Wilkins, and JP Hill Phys. Rev. B 90, 100510 (2014) [Abs] [PDF] [HTML]

   We report the observation of a bulk charge modulation in La_1.88Sr_0.12CuO₄ (LSCO) with a characteristic in-plane wave vector of (0.236, ±δ), with δ = 0.011 r.l.u. The transverse shift of the ordering wave vector indicates the presence of rotated charge-stripe ordering, demonstrating that the charge ordering is not pinned to the Cu-O bond direction. On cooling through the superconducting transition, we find an abrupt change in the growth of the charge correlations and a suppression of the charge order parameter indicating competition between the two orderings. Orthorhombic LSCO thus helps bridge the apparent disparities between the behavior previously observed in the tetragonal “214” cuprates and the orthorhombic yttrium and bismuth-based cuprates and thus lends strong support to the idea that there is a common motif to charge order in all cuprate families.

3. Itinerant effects and enhanced magnetic interactions in Bi-based multilayer cuprates M Dean, A James, AC Walters, V Bisogni, I Jarrige, M Hucker, E Giannini, M Fujita, J Pelliciari, Y Huang, R. M. Konik, T. Schmitt, and J. P. Hill Phys. Rev. B 90, 220506 (2014) [Abs] [PDF] [HTML]

   The cuprate high temperature superconductors exhibit a pronounced trend in which the superconducting transition temperature Tc increases with the number of CuO2 planes n in the crystal structure. We compare the magnetic excitation spectrum of Bi2+xSr2−xCuO6+d (Bi-2201) and Bi2Sr2Ca2Cu3O10+d (Bi-2223), with n = 1 and 3, respectively, using Cu L3-edge resonant inelastic x-ray scattering. Near the antinodal zone boundary we find the paramagnon energy in Bi-2223 is substantially higher than that in Bi-2201, indicating that multilayer cuprates host stronger effective magnetic exchange interactions, providing a possible explanation for the Tc vs n scaling. In contrast, the nodal direction exhibits very strongly damped, almost nondispersive excitations. We argue that this implies that the magnetism in the doped cuprates is partially itinerant in nature.

2013

1. Ferromagnetic Exchange Anisotropy from Antiferromagnetic Superexchange in the Mixed 3 d- 5 d Transition-Metal Compound Sr₃CuIrO₆ Wei-Guo Yin, X Liu, AM Tsvelik, MPM Dean, MH Upton, Jungho Kim, D Casa, A Said, T Gog, TF Qi, G. Cao, and J. P. Hill Phys. Rev. Lett. 111, 057202 (2013) [Abs] [PDF] [HTML]

   We report a combined experimental and theoretical study of the unusual ferromagnetism in the one- dimensional copper-iridium oxide Sr₃CuIrO₆. Utilizing Ir L3 edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromag- netic superexchange, driven by the alternating strong and weak spin-orbit coupling on the 5d Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu²⁺O₄ plaquettes and Ir⁴⁺O₆ octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.

2. High-energy magnetic excitations in the cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ: towards a unified description of its electronic and magnetic degrees of freedom MPM Dean, AJA James, RS Springell, X Liu, C Monney, KJ Zhou, RM Konik, JS Wen, ZJ Xu, GD Gu, V. N. Strocov, T. Schmitt, and J. P. Hill Phys. Rev. Lett. 110, 147001 (2013) [Abs] [PDF] [HTML]

   We investigate the high-energy magnetic excitation spectrum of the high-T(c) cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) using Cu L(3) edge resonant inelastic x-ray scattering. Broad, dispersive magnetic excitations are observed, with a zone boundary energy of ∼ 300 meV and a weak dependence on doping. These excitations are strikingly similar to the bosons proposed to explain the high-energy "kink" observed in photoemission. A phenomenological calculation of the spin response, based on a parametrization of the the angle-resolved photoemission spectroscopy derived electronic structure and Yang-Rice-Zhang quasiparticles, provides a reasonable prediction of the energy dispersion of the observed magnetic excitations. These results indicate a possible unified framework to reconcile the magnetic and electronic properties of the cuprates and we discuss the advantages and disadvantages of such an approach.

3. Comparison of charge modulations in La_1.875Ba_0.125CuO₄ and YBa₂Cu₃O_6.6 V. Thampy, S. Blanco-Canosa, M. Garcı́a-Fernández, M. P. M. Dean, G. D. Gu, M. Först, T. Loew, B. Keimer, M. Le Tacon, S. B. Wilkins, and J. P. Hill Phys. Rev. B 88, 024505 (2013) [Abs] [PDF] [HTML]

   A charge modulation has recently been reported in (Nd,Y)Ba₂Cu₃O_6+x [G. Ghiringhelli et al., Science 337, 821 (2012)]. Here we report Cu L₃ edge soft x-ray scattering studies comparing the lattice modulation associated with the charge modulation in YBa₂Cu₃O_6.6 with that associated with the well-known charge and spin stripe order in La_1.875Ba_0.125CuO₄. We find that the correlation length in the CuO₂ plane is isotropic in both cases, and is 259±9 Å for La_1.875Ba_0.125CuO₄ and 55±15 Å for YBa₂Cu₃O_6.6. Assuming weak interplanar correlations of the charge ordering in both compounds, we conclude that the order parameters of the lattice modulations in La_1.875Ba_0.125CuO₄ and YBa₂Cu₃O_6.6 are of the same order of magnitude.

4. Spin polarization of Ru in superconducting Ba(Fe_0.795Ru_0.205)₂As₂ studied by x-ray resonant magnetic scattering MG Kim, J Soh, J Lang, MPM Dean, A Thaler, SL Bud’ko, PC Canfield, E Bourret-Courchesne, A Kreyssig, AI Goldman, and RJ Birgeneau Phys. Rev. B 88, 014424 (2013) [Abs] [PDF] [HTML]

   We have employed the x-ray resonant magnetic scattering (XRMS) technique at the Ru L2 edge of the Ba(Fe_0.795Ru_0.205)₂As₂ (x = 0.205) superconductor. We show that pronounced resonance enhancements at the Ru L2 edge are observed at the wave vector which is consistent with the antiferromagnetic propagation vector of the Fe in the undoped BaFe2As2. We also demonstrate that the XRMS signals at the Ru L2 edge follow the magnetic ordering of the Fe with a long correlation length, ξ_ab > 2850 ± 400 ˚A. Our experimental observation shows that the Ru is spin polarized in a(Fe_0.795Ru_0.205)₂As₂ compounds.

5. Magnetic excitations in stripe-ordered La_2-xBa_xCuO₄ studied using resonant inelastic x-ray scattering MPM Dean, Greta Dellea, Matteo Minola, SB Wilkins, RM Konik, GD Gu, M Le Tacon, NB Brookes, F Yakhou-Harris, K Kummer, J. P. Hill, L. Braicovich, and G. Ghiringhelli Phys. Rev. B 88, 020403 (2013) [Abs] [PDF] [HTML]

   The charge and spin correlations in La_2-xBa_xCuO₄ (LBCO 1/8) are studied using Cu L₃ edge resonant inelastic x-ray scattering (RIXS). The static charge order (CO) is observed at a wave vector of (0.24,0) and its charge nature confirmed by measuring the dependence of this peak on the incident x-ray polarization. The damped spin excitation or “paramagnon” in LBCO 1/8 is then measured as it disperses through the CO wave vector. Within the experimental uncertainty no changes are observed in the paramagnon at that wave vector and the paramagnon seems to be similar to that measured in other cuprates, which have no static CO. Given that the stripe correlation modulates both the charge and spin degrees of freedom, it is likely that subtle changes do occur in the paramagnon due to CO. Consequently, we propose that future RIXS measurements, realized with higher-energy resolution and sensitivity, should be performed to test for these effects.

6. Persistence of magnetic excitations in La_2-xSr_xCuO₄ from the undoped insulator to the heavily overdoped non-superconducting metal M. P. M. Dean, G. Dellea, R. S. Springell, F. Yakhou-Harris, K. Kummer, N. B. Brookes, X. Liu, Y.-J. Sun, J. Strle, T. Schmitt, and J. P. Hill Nature Materials 12 12, 1019–1023 (2013) [Abs] [HTML] [arXiv] [BNL Press Release]

   One of the most intensely studied scenarios of high- temperature superconductivity (HTS) postulates pairing by exchange of magnetic excitations1. Indeed, such excitations have been observed up to optimal doping in the cuprates2–7. In the heavily overdoped regime, neutron scattering mea- surements indicate that magnetic excitations have effectively disappeared8–10, and this has been argued to cause the demise of HTS with overdoping1,8,10. Here we use resonant inelastic X-ray scattering, which is sensitive to complementary parts of reciprocal space, to measure the evolution of the magnetic excitations in La_2−xSr_xCuO₄ across the entire phase diagram, from a strongly correlated insulator (x = 0) to a non-superconducting metal (x=0.40). For x=0, well-defined magnon excitations are observed11. These magnons broaden with doping, but they persist with a similar dispersion and comparable intensity all the way to the non-superconducting, heavily overdoped metallic phase. The destruction of HTS with overdoping is therefore caused neither by the general disappearance nor by the overall softening of magnetic excitations. Other factors, such as the redistribution of spectral weight, must be considered.

2012

1. Magnetically polarized Ir dopant atoms in superconducting B(Fe_{1- x} Ir_x)₂ As₂ MPM Dean, MG Kim, A Kreyssig, JW Kim, X Liu, PJ Ryan, A Thaler, SL Bud’ko, W Strassheim, PC Canfield, J. P. Hill, and A. I. Goldman Phys. Rev. B 85, 140514 (2012) [Abs] [PDF] [HTML]

   We investigate the magnetic polarization of the Ir 5d dopant states in the pnictide superconductor B(Fe_{1- x} Ir_x)₂ As₂ with x=0.027(2) using Ir L3 edge x-ray resonant magnetic scattering (XRMS). Despite the fact that doping partially suppresses the antiferromagnetic transition, we find that magnetic order survives around the Ir dopant sites. The Ir states are magnetically polarized with commensurate stripe-like antiferromagnetic order and long correlations lengths, ξmag>2800 and >850 Å, in the ab plane and along the c axis, respectively, driven by their interaction with the Fe spins. This Ir magnetic order persists up to the Néel transition of the majority Fe spins at T_N=74(2) K. At 5 K we find that magnetic order coexists microscopically with superconductivity in B(Fe_{1- x} Ir_x)₂ As₂. The energy dependence of the XRMS through the Ir L3 edge shows a non-Lorentzian line shape, which we explain in terms of interference between Ir resonant scattering and Fe nonresonant magnetic scattering.

2. Spin excitations in a single La₂CuO₄ layer MPM Dean, RS Springell, C Monney, KJ Zhou, J Pereiro, I Božović, Bastien Dalla Piazza, HM Rønnow, E Morenzoni, J Van Den Brink, T. Schmitt, and J. P. Hill Nature Materials 11, 850–854 (2012) [Abs] [HTML] [arXiv] [BNL Press Release]

   Cuprates and other high-temperature superconductors consist of two-dimensional layers that are crucial to their properties. The dynamics of the quantum spins in these layers lie at the heart of the mystery of the cuprates. In bulk cuprates such as La₂CuO₄, the presence of a weak coupling between the two-dimensional layers stabilizes a three-dimensional magnetic order up to high temperatures. In a truly two-dimensional system however, thermal spin fluctuations melt long-range order at any finite temperature8. Here, we measure the spin response of isolated layers of La₂CuO₄ that are only one-unit-cell-thick. We show that coherent magnetic excitations, magnons, known from the bulk order, persist even in a single layer of La2CuO4, with no evidence for more complex correlations such as resonating valence bond correlations9,10,11. These magnons are, therefore, well described by spin-wave theory (SWT). On the other hand, we also observe a high-energy magnetic continuum in the isotropic magnetic response that is not well described by two-magnon SWT, or indeed any existing theories.

3. Testing the validity of the strong spin-orbit-coupling limit for octahedrally coordinated iridates in a model system Sr₃CuIrO₆ X Liu, Vamshi M Katukuri, L Hozoi, Wei-Guo Yin, MPM Dean, MH Upton, Jungho Kim, D Casa, A Said, T Gog, T. F. Qi, G. Cao A. M. Tsvelik, Jeroen Brink, and J. P. Hill Phys. Rev. Lett. 109, 157401 (2012) [Abs] [PDF] [HTML]

   The electronic structure of Sr₃CuIrO₆, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering and theoretical calculations. Resonant inelastic x-ray scattering spectra at the Ir L3 edge reveal an Ir t2g manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and ab inito quantum chemistry calculations find a strikingly large noncubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the j_eff=1/2 and j_eff=3/2 states and modifies the isotropic wave functions on which many theoretical models are based.

2011

1. Comparison of stripe modulations in La_1.875Ba_0.125CuO₄ and La_1.48Nd_0.4 Sr_0.12CuO₄ SB Wilkins, MPM Dean, Jörg Fink, Markus Hücker, J Geck, V Soltwisch, E Schierle, E Weschke, G Gu, S Uchida, N. Ichikawa, J. M. Tranquada, and J. P. Hill Phys. Rev. B 84, 195101 (2011) [Abs] [PDF] [HTML]

   We report combined soft and hard x-ray scattering studies of the electronic and lattice modulations associatedwith stripe order in La_1.875Ba_0.125CuO₄ and La_1.48Nd_0.4 Sr_0.12CuO₄. We find that the amplitude of both the electronic modulation of the hole density and the strain modulation of the lattice is significantly larger in La_1.875Ba_0.125CuO₄ than in La_1.48Nd_0.4Sr_0.12CuO₄ and is also better correlated. The in-plane correlation lengths are isotropic in each case; for La1.875Ba0.125CuO4, ξ hole = 255 ± 5 A, whereas for La_1.48Nd_0.4Sr_0.12CuO₄, ξ hole = 111 ± 7 A. We find that the modulations are temperature independent in La1.875Ba0.125CuO4 in the low temperature tetragonal phase. In contrast, in La1.48Nd0.4Sr0.12CuO4, the amplitude grows smoothly from zero, beginning 13 K below the LTT phase transition. We speculate that the reduced average tilt angle in La_1.875Ba_0.125CuO₄ results in reduced charge localization and incoherent pinning, leading to the longer correlation length and enhanced periodic modulation amplitude.

2. Phonons in potassium-doped graphene: the effects of electron-phonon interactions, dimensionality, and adatom ordering CA Howard, MPM Dean, and F Withers Phys. Rev. B 84, 241404 (2011) [Abs] [PDF] [HTML]

   Graphene phonons are measured as a function of electron doping via the addition of potassium adatoms. In the low doping regime, the in-plane carbon G peak hardens and narrows with increasing doping, analogous to the trend seen in graphene doped via the field effect. At high dopings, beyond those accessible by the field effect, the G peak strongly softens and broadens. This is interpreted as a dynamic, nonadiabatic renormalization of the phonon self-energy. At dopings between the light and heavily doped regimes, we find a robust inhomogeneous phase where the potassium coverage is segregated into regions of high and low density. The phonon energies, linewidths, and tunability are notably very similar for one- to four-layer potassium-doped graphene, but significantly different to bulk potassium-doped graphite.

3. Comparative study of the phonons in nonsuperconducting BaC₆ and superconducting CaC₆ using inelastic x-ray scattering AC Walters, CA Howard, MH Upton, MPM Dean, A Alatas, BM Leu, M Ellerby, DF McMorrow, JP Hill, M Calandra, and F. Mauri Phys. Rev. B 84, 014511 (2011) [Abs] [PDF] [HTML]

   The low-energy phonons of two different graphite intercalation compounds (GICs) have been measured as a function of temperature using inelastic x-ray scattering (IXS). In the case of the non-superconductor BaC₆, the phonons observed are significantly higher (up to 20%) in energy than those predicted by theory, in contrast to the reasonable agreement found in superconducting CaC₆. Additional IXS intensity is observed below 15 meV in both BaC₆ and CaC₆. It has been previously suggested that this additional inelastic intensity may arise from defect or vacancy modes not predicted by theory [d’Astuto et al., Phys. Rev. B 81, 104519 (2010)]. Here it is shown that this additional intensity can arise directly from the polycrystalline nature of the available samples. Our results show that future theoretical work is required to understand the relationship between the crystal structure, the phonons, and the superconductivity in GICs.

4. Novel metallic states at low temperatures SE Rowley, RP Smith, Noelia Marcano, MPM Dean, A Kusmartseva, LJ Spalek, ECT O’Farrell, David A Tompsett, ML Sutherland, PL Alireza, C. Ko, C. Liu, E. Pugh, S. S. Saxena, and G. G. Lonzarich Low Temperature Physics 37, 2–7 (2011) [Abs] [HTML]

   We present an overview of unconventional metallic states arising close to magnetic quantum critical points with a focus on d-electron systems. The applicability and potential breakdowns of traditional self-consistent field theories of such materials are discussed as well as related phenomena in other systems.

2010

1. Nonadiabatic phonons within the doped graphene layers of XC₆ compounds Mark PM Dean, Christopher A Howard, Siddharth S Saxena, and Mark Ellerby Phys. Rev. B 81, 045405 (2010) [Abs] [PDF] [HTML]

   We report Raman-scattering measurements of BaC₆, SrC₆, YbC₆, and CaC₆, which permit a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC₆, SrC₆, YbC₆, and CaC₆. This is due to increasing charge in the
   
   π electronic band. Electron-phonon interaction effects between the in-plane carbon modes at 1500 cm⁻¹ and the
   
   electrons cause a strong nonadiabatic renormalization. As charge is transferred into the
   
   band, these nonadiabatic effects are found to increase concurrent with a reduction in the phonon lifetime.

2. Neutron scattering study of the high-energy graphitic phonons in superconducting CaC₆ MPM Dean, AC Walters, CA Howard, TE Weller, M Calandra, F Mauri, M Ellerby, SS Saxena, A Ivanov, and DF McMorrow Phys. Rev. B 82, 014533 (2010) [Abs] [PDF] [HTML]

   We present the results of a neutron scattering study of the high-energy phonons in the superconducting graphite intercalation compound CaC₆. The study was designed to address hitherto unexplored aspects of the lattice dynamics in CaC₆, and, in particular, any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density-functional theory DFT. A description is given of the analysis methods developed to account for the highly textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experi- mental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on CaC₆. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies.

3. Ferromagnetic and ferroelectric quantum phase transitions Stephen Rowley, Robert Smith, Mark Dean, Leszek Spalek, Michael Sutherland, Montu Saxena, Patricia Alireza, Chris Ko, Cheng Liu, Emma Pugh, Suchitra Sebastian, and Gilbert Lonzarich physica status solidi (b) 247, 469–475 (2010) [Abs] [PDF] [HTML]

   The applicability of mean field models of ferroelectric and ferromagnetic quantum critical points is examined for a selection of d‐electron systems. Crucially, we find that the tendency of the effective interaction between critical fluctuation modes to become attractive and anomalous as the ordering temperatures tend to absolute zero results in particularly complex and striking phenomena. The multiplicity of quantum critical fields at the border of metallic ferromagnetism, in particular, is discussed here.

2006

1. Crystallization on heating and complex phase behavior of α-cyclodextrin solutions M Plazanet, M Dean, M Merlini, A Hüller, H Emerich, C Meneghini, MR Johnson, and HP Trommsdorff The Journal of chemical physics 125, 154504 (2006) [Abs] [PDF] [HTML]

   Solutions composed of alpha-cyclodextrin, water, and various methylpyridines, in particular, 4-methylpyridine (4MP), undergo reversible liquid-solid transitions upon heating, the crystalline solid phases undergoing further phase transformations at higher temperatures. This unusual behavior has been characterized by an ensemble of measurements, including solubility, differential scanning calorimetry, quasielastic neutron scattering, as well as x-ray powder diffraction. For the alpha-CD/4MP system five crystalline phases have been identified. The unit cell parameters and corresponding changes with temperature indicate a scenario for the crystallization process. A simple model is proposed that mimics the observed disorder-order transition.