Heterostructures

Modifying electronic states at the nanoscale

Oxide heterostructures and van der Waals layered materials hold great potential for realizing new electronic states and functionalities by engineering interfaces, proximity effects, and atomic-scale confinement. Probing the resulting electronic states is challenging given the small sample volumes involved. We use RIXS to probe magnetic interactions, electronic orbital ordering, and electron-phonon coupling in artificial heterostructures and atomically thin layers. RIXS is particularly well suited for these measurements: it is element-selective, enabling layer-specific spectroscopy, and can be focused into small spots compatible with thin-film and exfoliated-flake geometries.

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

2022

Quasi-Two-Dimensional Anomalous Hall Mott Insulator of Topologically Engineered J_eff=1/2 Electrons

Junyi Yang, Hidemaro Suwa, Derek Meyers, Han Zhang, Lukas Horak, Zhaosheng Wang, Gilberto Fabbris, Yongseong Choi, Jenia Karapetrova, Jong-Woo Kim, Daniel Haskel, Philip J. Ryan, M. P. M. Dean, Lin Hao, and Jian Liu

Phys. Rev. X 12, 031015 (2022)

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We investigate an experimental toy-model system of a pseudospin-half square-lattice Hubbard Hamiltonian in [(SrIrO3)1/(CaTiO3)1] to include both nontrivial complex hopping and moderate electron correlation. While the former induces electronic Berry phases as anticipated from the weak-coupling limit, the latter stabilizes an antiferromagnetic Mott insulator ground state analogous to the strong-coupling limit. Their combined results in the real system are found to be an anomalous Hall effect with a nonmonotonic temperature dependence due to the competition of the antiferromagnetic order and charge excitations in the Mott state, and an exceptionally large Ising anisotropy that is captured as a giant magnon gap beyond the superexchange approach. The unusual phenomena highlight the rich interplay of electronic topology and electron correlation in the intermediate-coupling regime that is largely unexplored and challenging in theoretical modeling.
@article{yang2022quasi, dimensions = {true}, title = {Quasi-Two-Dimensional Anomalous Hall Mott Insulator of Topologically Engineered Jeff=1/2 Electrons}, author = {Yang, Junyi and Suwa, Hidemaro and Meyers, Derek and Zhang, Han and Horak, Lukas and Wang, Zhaosheng and Fabbris, Gilberto and Choi, Yongseong and Karapetrova, Jenia and Kim, Jong-Woo and Haskel, Daniel and Ryan, Philip J. and Dean, M. P. M. and Hao, Lin and Liu, Jian}, journal = {Phys. Rev. X}, volume = {12}, issue = {3}, pages = {031015}, numpages = {11}, year = {2022}, month = jul, publisher = {American Physical Society}, doi = {10.1103/PhysRevX.12.031015}, url = {https://link.aps.org/doi/10.1103/PhysRevX.12.031015} }

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.
@article{yang2020strain, dimensions = {true}, title = {Strain-Modulated Slater-Mott Crossover of Pseudospin-Half Square-Lattice in (SrIrO3)1/(SrTiO3)1 Superlattices}, author = {Yang, Junyi and Hao, Lin and Meyers, Derek and Dasa, Tamene and Xu, Liubin and Horak, Lukas and Shafer, Padraic and Arenholz, Elke and Fabbris, Gilberto and Choi, Yongseong and Haskel, Daniel and Karapetrova, Jenia and Kim, Jong-Woo and Ryan, Philip J. and Xu, Haixuan and Batista, Cristian D. and Dean, Mark P. M. and Liu, Jian}, journal = {Phys. Rev. Lett.}, volume = {124}, issue = {17}, pages = {177601}, numpages = {7}, year = {2020}, month = apr, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.124.177601}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.124.177601} }

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.
@article{hao2019anomalous, dimensions = {true}, author = {Hao, Lin and Wang, Zhentao and Yang, Junyi and Meyers, D. and Sanchez, Joshua and Fabbris, Gilberto and Choi, Yongseong and Kim, Jong-Woo and Haskel, Daniel and Ryan, Philip J. and Barros, Kipton and Chu, Jiun-Haw and Dean, M. P. M. and Batista, Cristian D. and Liu, Jian}, title = {Anomalous magnetoresistance due to longitudinal spin fluctuations in a Jeff = 1/2 Mott semiconductor}, journal = {Nature Communications}, month = nov, year = {2019}, volume = {10}, number = {1}, pages = {5301}, issn = {2041-1723}, doi = {10.1038/s41467-019-13271-6}, url = {https://doi.org/10.1038/s41467-019-13271-6} }
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.
@article{lee2019strong, dimensions = {true}, title = {Strong Orbital Polarization in a Cobaltate-Titanate Oxide Heterostructure}, author = {Lee, Sangjae and Lee, Alex Taekyung and Georgescu, Alexandru B. and Fabbris, Gilberto and Han, Myung-Geun and Zhu, Yimei and Freeland, John W. and Disa, Ankit S. and Jia, Yichen and Dean, Mark P. M. and Walker, Frederick J. and Ismail-Beigi, Sohrab and Ahn, Charles H.}, journal = {Phys. Rev. Lett.}, volume = {123}, issue = {11}, pages = {117201}, numpages = {7}, year = {2019}, month = sep, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.123.117201}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.123.117201} }

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)

[BNL Press Release]

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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.
@article{hao2018giant, dimensions = {true}, title = {Giant magnetic response of a two-dimensional antiferromagnet}, author = {Hao, Lin and Meyers, D and Suwa, Hidemaro and Yang, Junyi and Frederick, Clayton and Dasa, Tamene R and Fabbris, Gilberto and Horak, Lukas and Kriegner, Dominik and Choi, Yongseong and Kim, Jong-Woo and Haskel, Daniel and Ryan, Philip J. and Xu, Haixuan and Batista, Cristian D. and Dean, M. P. M. and Liu, Jian}, journal = {Nature Physics}, volume = {14}, number = {8}, pages = {806}, note = {[<a href="https://www.bnl.gov/newsroom/news.php?a=213028" target="_blank">BNL Press Release</a>]}, note1 = {[<a href="http://www.phys.utk.edu/news/2018/hidden-symmetry.html" target="_blank">UTK Press Release</a>]}, year = {2018}, note2 = {[<a href="https://www.aps.anl.gov/APS-Science-Highlight/2018-06-11/hidden-magnetism-appears-under-hidden-symmetry" target="_blank">ANL Highlight</a>]}, month = jun, doi = {10.1038/s41567-018-0152-6}, publisher = {Nature Publishing Group} }
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.
@article{meyers2018decoupling, dimensions = {true}, title = {Decoupling carrier concentration and electron-phonon coupling in oxide heterostructures observed with resonant inelastic x-ray scattering}, author = {Meyers, Derek and Nakatsukasa, Ken and Mu, Sai and Hao, Lin and Yang, Junyi and Cao, Yue and Fabbris, G and Miao, Hu and Pelliciari, J and McNally, D and Dantz, M. and Paris, E. and Karapetrova, E. and Choi, Yongseong and Haskel, D. and Shafer, P. and Arenholz, E. and Schmitt, Thorsten and Berlijn, Tom and Johnston, S. and Liu, Jian and Dean, M. P. M.}, journal = {Phys. Rev. Lett.}, volume = {121}, number = {23}, pages = {236802}, year = {2018}, month = dec, doi = {10.1103/PhysRevLett.121.236802}, publisher = {American Physical Society} }

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.
@article{hao2017two, dimensions = {true}, title = {Two-dimensional jeff= 1/2 antiferromagnetic insulator unraveled from interlayer exchange coupling in artificial perovskite iridate superlattices}, author = {Hao, Lin and Meyers, Derek and Frederick, Clayton and Fabbris, Gilberto and Yang, Junyi and Traynor, Nathan and Horak, Lukas and Kriegner, Dominik and Choi, Yongseong and Kim, Jong-Woo and Haskel, Daniel and Ryan, Phil J. and Dean, M. P. M. and Liu, Jian}, journal = {Phys. Rev. Lett.}, volume = {119}, number = {2}, pages = {027204}, year = {2017}, month = jul, doi = {10.1103/PhysRevLett.119.027204}, publisher = {American Physical Society} }

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.
@article{fabbris2016orbital, dimensions = {true}, title = {Orbital engineering in nickelate heterostructures driven by anisotropic oxygen hybridization rather than orbital energy levels}, author = {Fabbris, G and Meyers, D and Okamoto, J and Pelliciari, J and Disa, AS and Huang, Y and Chen, Z-Y and Wu, WB and Chen, CT and Ismail-Beigi, S and Ahn, C. H. and Walker, F. J. and Huang, D. J. and Schmitt, T. and Dean, M. P. M.}, journal = {Phys. Rev. Lett.}, volume = {117}, number = {14}, pages = {147401}, year = {2016}, month = sep, doi = {10.1103/PhysRevLett.117.147401}, publisher = {American Physical Society} }
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.
@article{hoffman2016oscillatory, dimensions = {true}, nosupp = {}, title = {Oscillatory noncollinear magnetism induced by interfacial charge transfer in superlattices composed of metallic oxides}, author = {Hoffman, Jason D and Kirby, Brian J and Kwon, Jihwan and Fabbris, Gilberto and Meyers, D and Freeland, John W and Martin, Ivar and Heinonen, Olle G and Steadman, Paul and Zhou, Hua and Schlep\"utz, Christian M. and Dean, Mark P. M. and te Velthuis, Suzanne G. E. and Zuo, Jian-Min and Bhattacharya, Anand}, journal = {Phys. Rev. X}, volume = {6}, number = {4}, pages = {041038}, year = {2016}, month = nov, doi = {10.1103/PhysRevX.6.041038}, publisher = {American Physical Society} }

2012

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)

[BNL Press Release]

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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.
@article{dean2012spin, dimensions = {true}, title = {Spin excitations in a single La2CuO4 layer}, author = {Dean, MPM and Springell, RS and Monney, C and Zhou, KJ and Pereiro, J and Bo{\v{z}}ovi{\'c}, I and Dalla Piazza, Bastien and R{\o}nnow, HM and Morenzoni, E and Van Den Brink, J and Schmitt, T. and Hill, J. P.}, journal = {Nature Materials}, volume = {11}, number = {10}, pages = {850--854}, note = {[<a href="https://www.bnl.gov/newsroom/news.php?a=111447" target="_blank">BNL Press Release</a>]}, year = {2012}, month = sep, doi = {10.1038/nmat3409}, publisher = {Nature Publishing Group} }