Heterostructures

Modifying electronic states at the nanoscale

Oxide heterostructures hold great potential for realizing new electronic states and functionalities, but probing these electronic states is often very challenging. We work to extend the applicability of RIXS to probe magnetic interactions, electronic orbitals and electron-phonon coupling in heterostructures.

Figure 1: Artwork illustrating our work measuring magnetic excitation in single layers of La₂CuO₄. See M. P. M. Dean et al., Nature Materials 11, 850–854 (2012).

References

2020

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)

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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.

2019

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)

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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.
Novel spin-orbit coupling driven emergent states in iridate-based heterostructures

Lin Hao, D. Meyers, M.P.M. Dean, and Jian Liu

Journal of Physics and Chemistry of Solids 128, 39 - 53 (2019)

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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.
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)

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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.
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)

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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.
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)

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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.

2018

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)

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[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.
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)

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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.
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)

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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.

2017

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)

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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.

2016

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)

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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.
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)

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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.