Showing 1–50 of 106 results for author: Shen, K M

Incommensurate spin fluctuations in one-dimensional Kondo metal CeCo2Ga8

Authors: Yixuan Huang, P. Murgatroyd, Yi Wu, M. Cook, P. F. S. Rosa, E. D. Bauer, Asish K. Kundu, A. Rajapitamahuni, E. Vescovo, S. Zhang, V. Anil, P. Piyawongwatthana, N. Murai, M. Kofu, K. M. Shen, F. Ronning, Jian-Xin Zhu, A. Scheie

Abstract: We present an experimental and numerical study of the spin fluctuations in 1D Kondo metal CeCo$_2$Ga$_8$. Using inelastic neutron spectroscopy, we measure highly one-dimensional magnetism with low-energy incommensurate short-ranged magnetic fluctuations. ARPES similarly shows a highly one-dimensional electronic band structure, confirming the one-dimensional nature of the system. We use density mat… ▽ More We present an experimental and numerical study of the spin fluctuations in 1D Kondo metal CeCo$_2$Ga$_8$. Using inelastic neutron spectroscopy, we measure highly one-dimensional magnetism with low-energy incommensurate short-ranged magnetic fluctuations. ARPES similarly shows a highly one-dimensional electronic band structure, confirming the one-dimensional nature of the system. We use density matrix renormalization group (DMRG) simulations of the 1D Kondo lattice model to interpret the measured spectrum, which successfully reproduce the neutron scattering features. We are thus able to place CeCo$_2$Ga$_8$ within the emergent incommensurate phase of the 1D Kondo lattice phase diagram, and demonstrate that the Kondo lattice simulated non-perturbatively is an accurate microscopic model for heavy fermion physics. This shows CeCo$_2$Ga$_8$ to be one-dimensional despite its complexities, and reveals a coexistence of low-energy Kondo and magnetic features in its inelastic spectrum. △ Less

Submitted 26 August, 2026; originally announced August 2026.

Comments: 6 pages, 6 figures, plus 9 pages supplemental information

Crystallography of periodic nanotextures in a strained Mott insulator

Authors: Benjamin Z. Gregory, Yorick A. Birkhölzer, Noah Schnitzer, Ziming Shao, Jeff Hodgson, Suchismita Sarker, Jacob P. Ruff, Berit H. Goodge, David A. Muller, Kyle M. Shen, Darrell G. Schlom, Andrej Singer

Abstract: Here we investigate stripes of alternating structural phases spontaneously forming in epitaxially strained $Ca_2RuO_4$ thin films below the metal-insulator transition. Using large-volume X-ray reciprocal-space mapping, we show that satellite-pattern intensities across 24 symmetry-inequivalent Bragg reflections collapse onto a single parameter-free curve. The collapse identifies a coherent martensi… ▽ More Here we investigate stripes of alternating structural phases spontaneously forming in epitaxially strained $Ca_2RuO_4$ thin films below the metal-insulator transition. Using large-volume X-ray reciprocal-space mapping, we show that satellite-pattern intensities across 24 symmetry-inequivalent Bragg reflections collapse onto a single parameter-free curve. The collapse identifies a coherent martensitic laminate of few-nm-wide domains separated by ${012}$ interfaces, with displacements along $\left\langle01\bar{2}\right\rangle$. Satellite-extinction analysis demonstrates that both coexisting phases retain the bulk orthorhombic space group despite the pseudocubic $LaAlO_3$ substrate, biaxial epitaxial strain, and intrinsic strain at the interfaces. Classical invariant-plane-strain crystallography thus governs the nanoscale domain geometry of a Mott insulator with intertwined magnetic, electronic, and lattice order. △ Less

Submitted 8 June, 2026; originally announced June 2026.

What controls the superconducting dome of electron-doped FeSe?

Authors: Paul T. Malinowski, Chad J. Mowers, Yaoju Tarn, Darrell G. Schlom, Brendan D. Faeth, Kyle M. Shen

Abstract: Superconducting domes are conspicuous features of the phase diagrams of most unconventional and high-temperature superconductors. The superconducting transition temperature ($T_{c}$) of FeSe can be dramatically enhanced with electron doping, but unlike all other high-temperature and unconventional superconductors, its full phase diagram and superconducting dome have yet to be fully explored. Here,… ▽ More Superconducting domes are conspicuous features of the phase diagrams of most unconventional and high-temperature superconductors. The superconducting transition temperature ($T_{c}$) of FeSe can be dramatically enhanced with electron doping, but unlike all other high-temperature and unconventional superconductors, its full phase diagram and superconducting dome have yet to be fully explored. Here, we employ a combination of molecular beam epitaxy synthesis, alkali surface doping, in-vacuum electrical transport, and angle-resolved photoemission spectroscopy to investigate the entire superconducting dome of electron-doped FeSe, achieving a fully metallic state where superconductivity is suppressed in the heavily overdoped regime. We discover a robust scaling between $T_{c}$ and the residual resistivity ($ρ_{0}$) which holds across the entire superconducting dome, suggesting that the evolution of $T_{c}$ is heavily influenced by the evolution of the elastic scattering rate in the high-$T_{c}$ electron-doped phase. This in turn suggests that the superconducting dome in electron-doped FeSe appears to be fundamentally different than that of other unconventional superconductors where doping plays the primary role, and may be driven primarily by the sensitivity of the superconductivity to disorder. △ Less

Submitted 29 May, 2026; originally announced May 2026.

Comments: 20 pages, 4 figures

Reconciling strange metal transport in CeCoIn$_5$ through the difference of optical and cyclotron effective masses

Authors: Jingyuan Wang, Zhenisbek Tagay, Liyu Shi, Jiahao Liang, Nghiep Khoan Duong, Yi Wu, P. M. T. Vianez, F. Ronning, D. G. Rickel, Darrell G. Schlom, K. M. Shen, S. A. Crooker, N. P. Armitage

Abstract: The strange metal behavior in cuprate superconductors - characterized by linear in temperature resistivity and anomalous Hall transport - stands in stark contrast to the expectation of conventional Fermi liquid (FL) theory. Remarkably, the similar transport behavior has also been observed in the heavy fermion metal CeCoIn$_5$, whose d-wave superconducting ground state and strong antiferromagnetic… ▽ More The strange metal behavior in cuprate superconductors - characterized by linear in temperature resistivity and anomalous Hall transport - stands in stark contrast to the expectation of conventional Fermi liquid (FL) theory. Remarkably, the similar transport behavior has also been observed in the heavy fermion metal CeCoIn$_5$, whose d-wave superconducting ground state and strong antiferromagnetic fluctuations draw parallels to the cuprates. Here we have investigated the optical conductivity of the strange metal state of CeCoIn$_5$ over a wide magnetic field range using time-domain THz spectroscopy (TDTS). Using unique high-field THz spectroscopy we have shown that the current relaxation rate scales approximately as T$^2$, giving evidence for a hidden Fermi liquid state over a large field range. This result can be reconciled with linear in T resistivity with the realization that heavy quasiparticles have an optical mass that scales roughly like 1/T. This optical mass contrasts with the mass that characterizes cyclotron motion, which does not suffer the same large temperature dependent renormalization. Although by itself anomalous, this allows one to understand a number of other phenomena in CeCoIn$_5$ that have been taken to be signatures of strange metals, including the coexistence of a conventional T$^2$ dependence of the cotangent of the Hall angle with the linear in T resistivity, which with our observation also reflects FL-like physics. △ Less

Submitted 24 March, 2026; originally announced March 2026.

Tracking the local order parameter through the Hubbard exciton decoherence time in the Mott-Hubbard insulator LaVO3

Authors: Alessandra Milloch, Paolo Franceschini, Pablo Villar-Arribi, Sandeep Kumar Chaluvadi, Pasquale Orgiani, Giancarlo Panaccione, Giorgio Rossi, Yang Liu, Darrell G. Schlom, Kyle M. Shen, Massimo Capone, Claudio Giannetti

Abstract: The prototypical Mott-Hubbard insulator LaVO3 undergoes a structural phase transition accompanied by the onset of spin and orbital ordering below 140 K. By combining ultrafast optical pump-probe spectroscopy and two-dimensional electronic spectroscopy, we investigate the interplay between fluctuations of the local spin and orbital order parameter and the lifetime of high-energy electron-hole excit… ▽ More The prototypical Mott-Hubbard insulator LaVO3 undergoes a structural phase transition accompanied by the onset of spin and orbital ordering below 140 K. By combining ultrafast optical pump-probe spectroscopy and two-dimensional electronic spectroscopy, we investigate the interplay between fluctuations of the local spin and orbital order parameter and the lifetime of high-energy electron-hole excitations. Specifically, we demonstrate that the pump-induced perturbation of the order parameter leads to a change of the Hubbard exciton decoherence time and, consequently, of its homogeneous linewidth. Dynamical mean-field theory calculations confirm that the exciton scattering rate is crucially affected by the degree of order of the spin and orbital lattices in LaVO3. Our results demonstrate that multi-dimensional ultrafast optical spectroscopy can be used to track the dynamics of the order parameter, thus opening new routes in the study of correlated quantum materials characterized by intertwined orders. △ Less

Submitted 21 March, 2026; originally announced March 2026.

Journal ref: Phys. Rev. B 113, 205146 (2026)

Impact of Electron Correlations on Infinite-Layer Cuprates and Nickelates

Authors: Xunyang Hong, Yuetong Wu, Ying Chan, Sze Tung Li, I. Biało, L. Martinelli, A. Drewanowski, Qiang Gao, Xiaolin Ren, Xingjiang Zhou, Zhihai Zhu, A. Galdi, D. G. Schlom, K. M. Shen, J. Choi, M. Garcia Fernandez, Ke-Jin Zhou, N. B. Brookes, H. M. Rønnow, Qisi Wang, J. Chang

Abstract: Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their inte… ▽ More Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their interaction strengths. For both compounds, we study the orbital and magnetic excitations and extract their dispersions along high-symmetry directions. Using a single-band Hubbard model and including higher-order exchange interactions, we derive the correlation factor $U/t$ for both compounds. A key finding is that despite a smaller Coulomb repulsion $U$, PrNiO$_2$ exhibits a correlation strength that is 20% stronger than that of its isostructural cuprate counterpart SrCuO$_2$. This indicates that a moderation of the correlation strength may further optimize superconductivity in nickelates. △ Less

Submitted 12 November, 2025; originally announced November 2025.

Comments: Supplementary information available upon request

Journal ref: Communications Materials 7, 182 (2026)

Resonant diffraction and photoemission inconsistent with altermagnetism in epitaxial RuO$_2$ films

Authors: Benjamin Z. Gregory, Neha Wadehra, Shuyuan Zhang, Yi Wu, Samuel Poage, Jörg Strempfer, Asish K. Kundu, Anil Rajapitamahuni, Elio Vescovo, Anita Verma, Betül Pamuk, Jacob Ruf, Hari Nair, Nathaniel J. Schreiber, Kaveh Ahadi, Kyle M. Shen, Darrell G. Schlom, Andrej Singer

Abstract: Excitement about the magnetic and electronic properties of RuO$_2$ is growing, fueled by reports of antiferromagnetism, strain-induced superconductivity, and its recent classification as a member of a newly proposed magnetic class, altermagnets, with RuO$_2$ widely regarded as the paradigmatic example. Nevertheless, the magnetic ground state of RuO$_2$ remains contentious, as several recent experi… ▽ More Excitement about the magnetic and electronic properties of RuO$_2$ is growing, fueled by reports of antiferromagnetism, strain-induced superconductivity, and its recent classification as a member of a newly proposed magnetic class, altermagnets, with RuO$_2$ widely regarded as the paradigmatic example. Nevertheless, the magnetic ground state of RuO$_2$ remains contentious, as several recent experiments report no evidence of magnetic order. To address this discrepancy, we performed resonant elastic scattering measurements on a series of epitaxial RuO$_2$ thin films grown on the (100)-plane of TiO$_2$ substrates across a range of strain states. Leveraging full polarization control and azimuthal scans of the structurally forbidden 100 Bragg reflection, we systematically tested for signatures of colinear antiferromagnetic order. We found that the resonant elastic scattering signal in RuO$_2$ thin films likely originates from anisotropic charge scattering, not long-range antiferromagnetic order. Using angle-resolved photoemission spectroscopy we uncover a band structure without altermagnetic band splitting that is consistent with a nonmagnetic phase. Similarly, anisotropic magnetoresistance results show no evidence of magnetism. The combination of three independent measurements suggests the absence of altermagnetism in RuO$_2$. △ Less

Submitted 15 October, 2025; originally announced October 2025.

Time-domain decoding of unconventional charge order mechanisms in nonmagnetic and magnetic kagome metals

Authors: Seongyong Lee, Byungjune Lee, Hoyoung Jang, Xueliang Wu, Jimin Kim, Gyeongbo Kang, Choongjae Won, Hyeongi Choi, Sang-Youn Park, Kyle M. Shen, Federico Cilento, Aifeng Wang, Jae-Hoon Park, Mingu Kang

Abstract: In kagome lattice materials, quantum interplay between charge, spin, orbital, and lattice degrees of freedom gives rise to a remarkably rich set of emergent phenomena, ranging from unconventional charge order and superconductivity to topological magnetism. While the exact nature of these exotic orders is often challenging to comprehend in static experiments, time-resolved techniques can offer crit… ▽ More In kagome lattice materials, quantum interplay between charge, spin, orbital, and lattice degrees of freedom gives rise to a remarkably rich set of emergent phenomena, ranging from unconventional charge order and superconductivity to topological magnetism. While the exact nature of these exotic orders is often challenging to comprehend in static experiments, time-resolved techniques can offer critical insights by disentangling coupled degrees of freedom on the time-axis. In this work, we demonstrate that the nature of charge orders in two representative kagome metals - nonmagnetic ScV6Sn6 and magnetic FeGe - which has been highly controversial in static studies, can be directly deciphered in the time-domain through their fundamentally distinct order parameter dynamics measured via time-resolved X-ray scattering at an X-ray free electron laser. In nonmagnetic ScV6Sn6, the dynamics are characterized by ultrafast melting and coherent amplitudon oscillations, typical of a phonon-coupled charge order. In stark contrast, magnetic FeGe exhibits resilient metastable charge order dynamics, hitherto unobserved in any other charge-ordered system - this unique time-domain behavior directly signifies an unconventional magnetism-interlocked charge order state realized in this kagome magnet. Our results not only provide a model case where unconventional nature of electronic order, hidden in equilibrium, is directly unraveled in the time-domain, but also pave the way for future out-of-equilibrium engineering of novel quantum orders in kagome lattice platforms. △ Less

Submitted 17 June, 2025; originally announced June 2025.

Comments: 4 figures

Superconductivity in the parent infinite-layer nickelate NdNiO$_2$

Authors: C. T. Parzyck, Y. Wu, L. Bhatt, M. Kang, Z. Arthur, T. M. Pedersen, R. Sutarto, S. Fan, J. Pelliciari, V. Bisogni, G. Herranz, A. B. Georgescu, D. G. Hawthorn, L. F. Kourkoutis, D. A. Muller, D. G. Schlom, K. M. Shen

Abstract: We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO$_2$. A combination of oxide molecular-beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples… ▽ More We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO$_2$. A combination of oxide molecular-beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to its nodal order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity. △ Less

Submitted 2 October, 2024; originally announced October 2024.

Comments: Main: 10 pages, 6 figures. Supplementary: 9 pages, 10 figures

Journal ref: Phys. Rev. X 15, 021048 (2025)

Anisotropic Spin Stripe Domains in Bilayer La$_3$Ni$_2$O$_7$

Authors: N. K Gupta, R. Gong, Y. Wu, M. Kang, C. T. Parzyck, B. Z. Gregory, N. Costa, R. Sutarto, S. Sarker, A. Singer, D. G. Schlom, K. M. Shen, D. G. Hawthorn

Abstract: The discovery of superconductivity in La$_3$Ni$_2$O$_7$ under pressure has motivated the investigation of a parent spin density wave (SDW) state, which could provide the underlying pairing interaction. Here, we employ resonant soft x-ray scattering and polarimetry on thin films of bilayer La$_3$Ni$_2$O$_7$ to determine that the magnetic structure of the SDW forms unidirectional diagonal spin strip… ▽ More The discovery of superconductivity in La$_3$Ni$_2$O$_7$ under pressure has motivated the investigation of a parent spin density wave (SDW) state, which could provide the underlying pairing interaction. Here, we employ resonant soft x-ray scattering and polarimetry on thin films of bilayer La$_3$Ni$_2$O$_7$ to determine that the magnetic structure of the SDW forms unidirectional diagonal spin stripes with moments lying within the NiO$_2$ plane and perpendicular to $\mathbf{Q}_{SDW}$, but without evidence of the strong charge disproportionation typically associated with other nickelates. These stripes form anisotropic domains with shorter correlation lengths perpendicular versus parallel to $\mathbf{Q}_{SDW}$, revealing nanoscale rotational and translational symmetry breaking analogous to the cuprate and Fe-based superconductors, with possible Bloch-like antiferromagnetic domain walls separating orthogonal domains. △ Less

Submitted 2 September, 2025; v1 submitted 4 September, 2024; originally announced September 2024.

Comments: 18 pages, 5 figures; replaced with published version. Supplementary information available at https://doi.org/10.1038/s41467-025-61653-w

Journal ref: Nature Communications 16, 6560 (2025)

Local magnetic response of superconducting Sr$\mathrm{_2}$RuO$\mathrm{_4}$ thin films and rings

Authors: G. M. Ferguson, Hari P. Nair, Nathaniel J. Schreiber, Ludi Miao, Kyle M. Shen, Darrell G. Schlom, Katja C. Nowack

Abstract: We conduct local magnetic measurements on superconducting thin-film samples of Sr$\mathrm{_2}$RuO$\mathrm{_4}$ using scanning Superconducting Quantum Interference Device (SQUID) susceptometry. From the diamagnetic response, we extract the magnetic penetration depth, $λ$, which exhibits a quadratic temperature dependence at low temperatures. Although a quadratic dependence in high-purity bulk sampl… ▽ More We conduct local magnetic measurements on superconducting thin-film samples of Sr$\mathrm{_2}$RuO$\mathrm{_4}$ using scanning Superconducting Quantum Interference Device (SQUID) susceptometry. From the diamagnetic response, we extract the magnetic penetration depth, $λ$, which exhibits a quadratic temperature dependence at low temperatures. Although a quadratic dependence in high-purity bulk samples has been attributed to non-local electrodynamics, our analysis suggests that in our thin-film samples the presence of scattering is the origin of the quadratic dependence. While we observe micron-scale variations in the diamagnetic response and superconducting transition temperature, the form of the temperature dependence of $λ$ is independent of position. Finally, we characterize flux trapping in superconducting rings lithographically fabricated from the thin films, paving the way to systematic device-based tests of the superconducting order parameter in Sr$\mathrm{_2}$RuO$\mathrm{_4}$. △ Less

Submitted 25 March, 2024; originally announced March 2024.

Controllable suppression of the unconventional superconductivity in bulk and thin-film Sr$_{2}$RuO$_{4}$ via high-energy electron irradiation

Authors: Jacob P. Ruf, Hilary M. L. Noad, Romain Grasset, Ludi Miao, Elina Zhakina, Philippa H. McGuinness, Hari P. Nair, Nathaniel J. Schreiber, Naoki Kikugawa, Dmitry Sokolov, Marcin Konczykowski, Darrell G. Schlom, Kyle M. Shen, Andrew P. Mackenzie

Abstract: In bulk Sr$_{2}$RuO$_{4}$, the strong sensitivity of the superconducting transition temperature $T_{\text{c}}$ to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr$_{2}$RuO$_{4}$, the relationship between $T_{\text{c}}$ and the residual resistivity $ρ_0$, which in bulk samples… ▽ More In bulk Sr$_{2}$RuO$_{4}$, the strong sensitivity of the superconducting transition temperature $T_{\text{c}}$ to nonmagnetic impurities provides robust evidence for a superconducting order parameter that changes sign around the Fermi surface. In superconducting epitaxial thin-film Sr$_{2}$RuO$_{4}$, the relationship between $T_{\text{c}}$ and the residual resistivity $ρ_0$, which in bulk samples is taken to be a proxy for the low-temperature elastic scattering rate, is far less clear. Using high-energy electron irradiation to controllably introduce point disorder into bulk single-crystal and thin-film Sr$_{2}$RuO$_{4}$, we show that $T_{\text{c}}$ is suppressed in both systems at nearly identical rates. This suggests that part of $ρ_0$ in films comes from defects that do not contribute to superconducting pairbreaking, and establishes a quantitative link between the superconductivity of bulk and thin-film samples. △ Less

Submitted 29 February, 2024; originally announced February 2024.

Synthesis of thin film infinite-layer nickelates by atomic hydrogen reduction: clarifying the role of the capping layer

Authors: Christopher T. Parzyck, Vivek Anil, Yi Wu, Berit H. Goodge, Matthew Roddy, Lena F. Kourkoutis, Darrell G. Schlom, Kyle M. Shen

Abstract: We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO$_3$/SrTiO$_3$, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO$_3$ films to the infinite-la… ▽ More We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO$_3$/SrTiO$_3$, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO$_3$ films to the infinite-layer phase, NdNiO$_2$, using atomic hydrogen; the resulting films display excellent structural quality, smooth surfaces, and lower residual resistivities than films reduced by other methods. We utilize the in situ nature of this technique to investigate of the role that SrTiO$_3$ capping layers play in the reduction process, illustrating their importance in preventing the formation of secondary phases at the exposed nickelate surface. A comparative bulk- and surface-sensitive study indicates formation of a polycrystalline crust on the film surface serves to limit the reduction process. △ Less

Submitted 13 January, 2024; originally announced January 2024.

Comments: Main text: 12 pages, 7 figures. Supplemental Materials: 11 pages, 11 figures

Energy Relaxation and dynamics in the correlated metal Sr$_2$RuO$_4$ via THz two-dimensional coherent spectroscopy

Authors: David Barbalas, Ralph Romero III, Dipanjan Chaudhuri, Fahad Mahmood, Hari P. Nair, Nathaniel J. Schreiber, Darrel G. Schlom, K. M. Shen, N. P. Armitage

Abstract: Separating out the contributions of different scattering channels in strongly interacting metals is crucial in identifying the mechanisms that govern their properties. While momentum or current relaxation rates can be readily probed via \textit{dc} resistivity or optical/THz spectroscopy, distinguishing different kinds of inelastic scattering can be more challenging. Using nonlinear THz 2D coheren… ▽ More Separating out the contributions of different scattering channels in strongly interacting metals is crucial in identifying the mechanisms that govern their properties. While momentum or current relaxation rates can be readily probed via \textit{dc} resistivity or optical/THz spectroscopy, distinguishing different kinds of inelastic scattering can be more challenging. Using nonlinear THz 2D coherent spectroscopy, we measure the rates of energy relaxation after THz excitation in the strongly interacting Fermi liquid, Sr$_2$RuO$_4$. Energy relaxation is a bound on the total scattering and specifically a measure of contributions to the electron self-energy that arise from {\it inelastic} coupling to a bath. We observe two distinct energy relaxation channels: a fast process that we interpret as energy loss to the phonon system and a much slower relaxation that we interpret as arising from a non-equilibrium phonon effects and subsequent heat loss through diffusion. Interestingly, even the faster energy relaxation rate is at least an order of magnitude slower than the overall momentum relaxation rate, consistent with strong electron interactions and the dominance of energy-conserving umklapp or interband electron-electron scattering in momentum relaxation. The slowest energy relaxation rate decays on a sub-GHz scale, consistent with the relaxation dynamics of non-equilibrium phonons. Our observations reveal the versatility of nonlinear THz spectroscopy to measure the energy relaxation dynamics in correlated metals. Our work also highlights the need for improved theoretical understanding of such processes in interacting metals. △ Less

Submitted 20 December, 2023; originally announced December 2023.

Comments: 6 pages, 3 figures. SM included

Magnon interactions in a moderately correlated Mott insulator

Authors: Qisi Wang, S. Mustafi, E. Fogh, N. Astrakhantsev, Z. He, I. Biało, Ying Chan, L. Martinelli, M. Horio, O. Ivashko, N. E. Shaik, K. von Arx, Y. Sassa, E. Paris, M. H. Fischer, Y. Tseng, N. B. Christensen, A. Galdi, D. G. Schlom, K. M. Shen, T. Schmitt, H. M. Rønnow, J. Chang

Abstract: Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO$_2$, single- and bi-magnon d… ▽ More Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO$_2$, single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La$_2$CuO$_4$ indicate that quantum fluctuations are much stronger in SrCuO$_2$ suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic Néel order as the ground state. Our results indicate that SrCuO$_2$ - due to strong quantum fluctuations - is a unique starting point for the exploration of novel magnetic ground states. △ Less

Submitted 26 June, 2024; v1 submitted 28 November, 2023; originally announced November 2023.

Journal ref: Nature Communications 15, 5348 (2024)

Spontaneous supercrystal formation during a strain-engineered metal-insulator transition

Authors: O. Yu. Gorobtsov, L. Miao, Z. Shao, Y. Tan, N. I. Schnitzer, B. H. Goodge, J. Ruf, D. Weinstock, M. Cherukara, M. V. Holt, H. Nair, L. -Q. Chen, L. F. Kourkoutis, D. G. Schlom, K. M. Shen, A. Singer

Abstract: Mott metal-insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next generation energy-efficient electronics. We report a previously unknown, hierarchically ordered state during a Mott transition and demonstrate correlated switching of functional electronic properties. We elucidate in-situ formation of an intrinsic supercrystal in a Ca2RuO4 thin film. Mac… ▽ More Mott metal-insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next generation energy-efficient electronics. We report a previously unknown, hierarchically ordered state during a Mott transition and demonstrate correlated switching of functional electronic properties. We elucidate in-situ formation of an intrinsic supercrystal in a Ca2RuO4 thin film. Machine learning-assisted X-ray nanodiffraction together with electron microscopy reveal multi-scale periodic domain formation at and below the film transition temperature (TFilm ~ 200-250 K) and a separate anisotropic spatial structure at and above TFilm. Local resistivity measurements imply an intrinsic coupling of the supercrystal orientation to the material's anisotropic conductivity. Our findings add an additional degree of complexity to the physical understanding of Mott transitions, opening opportunities for designing materials with tunable electronic properties. △ Less

Submitted 20 November, 2023; originally announced November 2023.

Low energy electrodynamics and a hidden Fermi liquid in the heavy-fermion CeCoIn$_5$

Authors: L. Y. Shi, Zhenisbek Tagay, Jiahao Liang, Khoan Duong, Yi Wu, F. Ronning, Darrell G. Schlom, K. M. Shen, N. P. Armitage

Abstract: We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn$_5$. Below the $\approx$ 40 K Kondo coherence temperature, a narrow Drude-like peak forms, as the result of the $f$ orbital - conduction electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analy… ▽ More We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor CeCoIn$_5$. Below the $\approx$ 40 K Kondo coherence temperature, a narrow Drude-like peak forms, as the result of the $f$ orbital - conduction electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Via the extended Drude model analysis, we measure the frequency-dependent scattering rate ($1/ τ$) and effective mass ($m^*/m_b$). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nevertheless, the width of the low-frequency Drude peak itself that is set by the {\it renormalized} quasiparticle scattering rate ($1 / τ^* = m_b/ m^* τ$) shows a $T^2$ dependence. This is the scattering rate that characterizes the relaxation time of the renormalized quasiparticles. This gives evidence for Fermi liquid state, which in conventional transport experiments is hidden by the strong temperature dependent mass. △ Less

Submitted 7 April, 2025; v1 submitted 16 October, 2023; originally announced October 2023.

Comments: 5 pages, 4 figures

Absence of $3a_0$ Charge Density Wave Order in the Infinite Layer Nickelates

Authors: C. T. Parzyck, N. K. Gupta, Y. Wu, V. Anil, L. Bhatt, M. Bouliane, R. Gong, B. Z. Gregory, A. Luo, R. Sutarto, F. He, Y. -D. Chuang, T. Zhou, G. Herranz, L. F. Kourkoutis, A. Singer, D. G. Schlom, D. G. Hawthorn, K. M. Shen

Abstract: A hallmark of many unconventional superconductors is the presence of many-body interactions which give rise to broken symmetry states intertwined with superconductivity. Recent resonant soft x-ray scattering experiments report commensurate $3a_0$ charge density wave order in the infinite layer nickelates, which has important implications regarding the universal interplay between charge order and s… ▽ More A hallmark of many unconventional superconductors is the presence of many-body interactions which give rise to broken symmetry states intertwined with superconductivity. Recent resonant soft x-ray scattering experiments report commensurate $3a_0$ charge density wave order in the infinite layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both the cuprates and nickelates. Here, we present x-ray scattering and spectroscopy measurements on a series of NdNiO$_{2+x}$ samples which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO$_2$. The $3a_0$ superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with 3 unit cell periodicity. The absence of any observable charge density wave order in NdNiO$_2$ highlights a crucial difference between the phase diagrams of the cuprate and nickelate superconductors. △ Less

Submitted 12 July, 2023; originally announced July 2023.

Comments: Main Text: 8 pages, 4 figures. Supplemental: 12 pages, 12 figures

Atomically smooth films of CsSb: a chemically robust visible light photocathode

Authors: C. T. Parzyck, C. A. Pennington, W. J. I. DeBenedetti, J. Balajka, E. Echeverria, H. Paik, L. Moreschini, B. D. Faeth, C. Hu, J. K. Nangoi, V. Anil, T. A. Arias, M. A. Hines, D. G. Schlom, A. Galdi, K. M. Shen, J. M. Maxson

Abstract: Alkali antimonide semiconductor photocathodes provide a promising platform for the generation of high brightness electron beams, which are necessary for the development of cutting-edge probes including x-ray free electron lasers and ultrafast electron diffraction. However, to harness the intrinsic brightness limits in these compounds, extrinsic degrading factors, including surface roughness and co… ▽ More Alkali antimonide semiconductor photocathodes provide a promising platform for the generation of high brightness electron beams, which are necessary for the development of cutting-edge probes including x-ray free electron lasers and ultrafast electron diffraction. However, to harness the intrinsic brightness limits in these compounds, extrinsic degrading factors, including surface roughness and contamination, must be overcome. By exploring the growth of CsxSb thin films monitored by in situ electron diffraction, the conditions to reproducibly synthesize atomically smooth films of CsSb on 3C-SiC (100) and graphene coated TiO2 (110) substrates are identified, and detailed structural, morphological, and electronic characterization is presented. These films combine high quantum efficiency in the visible (up to 1.2% at 400 nm), an easily accessible photoemission threshold of 550 nm, low surface roughness (down to 600 pm on a 1 um scale), and a robustness against oxidation up to 15 times greater then Cs3Sb. These properties suggest that CsSb has the potential to operate as an alternative to Cs$_3$Sb in electron source applications where the demands of the vacuum environment might otherwise preclude the use of traditional alkali antimonides. △ Less

Submitted 31 May, 2023; originally announced May 2023.

Comments: 11 pages, 6 figures, 1 table

Journal ref: APL Mater. 11, 101125 (2023)

Electronic nematic order in the normal state of strontium ruthenate

Authors: Ryan Russell, Hari P. Nair, Kyle M. Shen, Darrell G. Schlom, John W. Harter

Abstract: Despite significant achievements in characterizing the properties of Sr$_2$RuO$_4$ over the last three decades, the precise nature of its electronic ground state is still unresolved. In this work, we provide a missing piece of the puzzle by uncovering evidence of electronic nematic order in the normal state of Sr$_2$RuO$_4$, revealed by ultrafast time-resolved optical dichroism measurements of uni… ▽ More Despite significant achievements in characterizing the properties of Sr$_2$RuO$_4$ over the last three decades, the precise nature of its electronic ground state is still unresolved. In this work, we provide a missing piece of the puzzle by uncovering evidence of electronic nematic order in the normal state of Sr$_2$RuO$_4$, revealed by ultrafast time-resolved optical dichroism measurements of uniaxially strained thin films. This nematic order, whose domains are aligned by the strain, spontaneously breaks the four-fold rotational symmetry of the crystal. The temperature dependence of the dichroism resembles an Ising-like order parameter, and optical pumping induces a coherent oscillation of its amplitude mode. A microscopic model of intra-unit-cell nematic order is presented, highlighting the importance of Coulomb repulsion between neighboring oxygen $p$-orbitals. The existence of electronic nematic order in the normal state of Sr$_2$RuO$_4$ may have consequences for the form and mechanism of superconductivity in this material. △ Less

Submitted 5 April, 2023; originally announced April 2023.

Journal ref: Physical Review B 108, L081105 (2023)

Picosecond volume expansion drives a later-time insulator-metal transition in a nano-textured Mott Insulator

Authors: Anita Verma, Denis Golež, Oleg Yu. Gorobtsov, Kelson Kaj, Ryan Russell, Jeffrey Z. Kaaret, Erik Lamb, Guru Khalsa, Hari P Nair, Yifei Sun, Ryan Bouck, Nathaniel Schreiber, Jacob P. Ruf, Varun Ramaprasad, Yuya Kubota, Tadashi Togashi, Vladimir A. Stoica, Hari Padmanabhan, John W. Freeland, Nicole A. Benedek, Oleg Shpyrko, John W. Harter, Richard D. Averitt, Darrell G. Schlom, Kyle M. Shen , et al. (2 additional authors not shown)

Abstract: Technology moves towards ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of electrons (femtoseconds) and associated phonons (10s of femtoseconds to few picoseconds), which is possible with short-pulse photoexcitation. Yet, in many Mott insulators, the electronic transition is ac… ▽ More Technology moves towards ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of electrons (femtoseconds) and associated phonons (10s of femtoseconds to few picoseconds), which is possible with short-pulse photoexcitation. Yet, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical time scales dominated by slow coarsening dynamics. Here, we use time-resolved X-ray diffraction and reflectivity measurements to investigate the photoinduced insulator-to-metal transition in an epitaxially strained thin film Mott insulator Ca2RuO4. The dynamical transition occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation drives a fast lattice rearrangement, followed by a slower electronic evolution into a metastable non-equilibrium state. Microscopic calculations based on time-dependent dynamical mean-field theory and semiclassical lattice dynamics within a recently published equilibrium energy landscape picture explain the threshold-behavior and elucidate the delayed onset of the electronic phase transition in terms of kinematic constraints on recombination. Analysis of satellite scattering peaks indicates the persistence of a strain-induced nano-texture in the photoexcited film. This work highlights the importance of combined electronic and structural studies to unravel the physics of dynamic transitions and elucidates the role of strain in tuning the timescales of photoinduced processes. △ Less

Submitted 6 April, 2023; v1 submitted 4 April, 2023; originally announced April 2023.

An Algorithm for Subtraction of Doublet Emission Lines in Angle-Resolved Photoemission Spectroscopy

Authors: Yaoju Tarn, Mekhola Sinha, Christopher Pasco, Darrell G. Schlom, Tyrel M. McQueen, Kyle M. Shen, Brendan D. Faeth

Abstract: Plasma discharge lamps are widely utilized in the practice of angle-resolved photoemission spectroscopy (ARPES) experiments as narrow-linewidth ultraviolet photon sources. However, many emission lines such as Ar-I, Ne-I, and Ne-II have closely spaced doublet emission lines, which result in superimposed replica on the measured ARPES spectra. Here, we present a simple method for subtracting the cont… ▽ More Plasma discharge lamps are widely utilized in the practice of angle-resolved photoemission spectroscopy (ARPES) experiments as narrow-linewidth ultraviolet photon sources. However, many emission lines such as Ar-I, Ne-I, and Ne-II have closely spaced doublet emission lines, which result in superimposed replica on the measured ARPES spectra. Here, we present a simple method for subtracting the contribution of these doublet emission lines from photoemission spectra. Benchmarking against ARPES spectra of well-characterized 2D materials, we demonstrate that this algorithm manages to subtract the doublet signal and reproduce the key features of the monochromated He-I$α$ spectra in a physically sound manner that reliably reproduces quantifiable dispersion relations and quasiparticle lifetimes. △ Less

Submitted 13 March, 2023; originally announced March 2023.

Comments: 8 pages, 4 main figures, 3 appendix figures

Real-space imaging of polar and elastic nano-textures in thin films via inversion of diffraction data

Authors: Ziming Shao, Noah Schnitzer, Jacob Ruf, Oleg Y. Gorobtsov, Cheng Dai, Berit H. Goodge, Tiannan Yang, Hari Nair, Vlad A. Stoica, John W. Freeland, Jacob Ruff, Long-Qing Chen, Darrell G. Schlom, Kyle M. Shen, Lena F. Kourkoutis, Andrej Singer

Abstract: Exploiting the emerging nanoscale periodicities in epitaxial, single-crystal thin films is an exciting direction in quantum materials science: confinement and periodic distortions induce novel properties. The structural motifs of interest are ferroelastic, ferroelectric, multiferroic, and, more recently, topologically protected magnetization and polarization textures. A critical step towards heter… ▽ More Exploiting the emerging nanoscale periodicities in epitaxial, single-crystal thin films is an exciting direction in quantum materials science: confinement and periodic distortions induce novel properties. The structural motifs of interest are ferroelastic, ferroelectric, multiferroic, and, more recently, topologically protected magnetization and polarization textures. A critical step towards heterostructure engineering is understanding their nanoscale structure, best achieved through real-space imaging. X-ray Bragg coherent diffractive imaging visualizes sub-picometer crystalline displacements with tens of nanometers spatial resolution. Yet, it is limited to objects spatially confined in all three dimensions and requires highly coherent, laser-like x-rays. Here we lift the confinement restriction by developing real-space imaging of periodic lattice distortions: we combine an iterative phase retrieval algorithm with unsupervised machine learning to invert the diffuse scattering in conventional x-ray reciprocal-space mapping into real-space images of polar and elastic textures in thin epitaxial films. We first demonstrate our imaging in PbTiO3/SrTiO3 superlattices to be consistent with published phase-field model calculations. We then visualize strain-induced ferroelastic domains emerging during the metal-insulator transition in Ca2RuO4 thin films. Instead of homogeneously transforming into a low-temperature structure (like in bulk), the strained Mott insulator splits into nanodomains with alternating lattice constants, as confirmed by cryogenic scanning transmission electron microscopy. Our study reveals the type, size, orientation, and crystal displacement field of the nano-textures. The non-destructive imaging of textures promises to improve models for their dynamics and enable advances in quantum materials and microelectronics. △ Less

Submitted 2 November, 2022; originally announced November 2022.

Gapped collective charge excitations and interlayer hopping in cuprate superconductors

Authors: M. Hepting, M. Bejas, A. Nag, H. Yamase, N. Coppola, D. Betto, C. Falter, M. Garcia-Fernandez, S. Agrestini, K. -J. Zhou, M. Minola, C. Sacco, L. Maritato, P. Orgiani, H. I. Wei, K. M. Shen, D. G. Schlom, A. Galdi, A. Greco, B. Keimer

Abstract: We use resonant inelastic x-ray scattering (RIXS) to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr$_{0.9}$La$_{0.1}$CuO$_2$ (SLCO). We detect a plasmon gap of $\sim$~120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in SLCO are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by laye… ▽ More We use resonant inelastic x-ray scattering (RIXS) to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr$_{0.9}$La$_{0.1}$CuO$_2$ (SLCO). We detect a plasmon gap of $\sim$~120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in SLCO are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered $t$-$J$-$V$ model calculations. A similar analysis performed on recent RIXS data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping $t_z$. Our work signifies the three-dimensionality of the charge dynamics in layered cuprates and provides a new method to determine $t_z$. △ Less

Submitted 28 June, 2022; originally announced June 2022.

Comments: 17 pages, 10 figures, includes Supplemental Material. Accepted for publication in Physical Review Letters

Journal ref: Phys. Rev. Lett. 129, 047001 (2022)

Magnetic Excitations in Square Lattice Iridates: Contrast between Ba$_2$IrO$_4$ and Sr$_2$IrO$_4$

Authors: J. P. Clancy, H. Gretarsson, A. Lupascu, J. A. Sears, Z. Nie, M. H. Upton, Jungho Kim, Z. Islam, M. Uchida, D. G. Schlom, K. M. Shen, Young-June Kim

Abstract: We report a resonant inelastic x-ray scattering (RIXS) investigation of ultra-thin epitaxial films of Ba$_2$IrO$_4$, and compare their low energy magnetic and spin-orbit excitations to those of their sister compound Sr$_2$IrO$_4$. Due to the 180$^\circ$ Ir-O-Ir bond, the bandwidth of the magnon and spin-orbiton is significantly larger in Ba$_2$IrO$_4$, making it difficult to describe these two typ… ▽ More We report a resonant inelastic x-ray scattering (RIXS) investigation of ultra-thin epitaxial films of Ba$_2$IrO$_4$, and compare their low energy magnetic and spin-orbit excitations to those of their sister compound Sr$_2$IrO$_4$. Due to the 180$^\circ$ Ir-O-Ir bond, the bandwidth of the magnon and spin-orbiton is significantly larger in Ba$_2$IrO$_4$, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion observed in Sr$_2$IrO$_4$ is absent in Ba$_2$IrO$_4$, as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba$_2$IrO$_4$ are substantially different from those of Sr$_2$IrO$_4$, suggesting that these materials need to be examined more carefully with electron itinerancy taken into account. △ Less

Submitted 24 March, 2022; originally announced March 2022.

Comments: 7 pages, 4 figures

X-ray Nano-imaging of Defects in Thin Film Catalysts via Cluster Analysis

Authors: Aileen Luo, Oleg Yu. Gorobtsov, Jocienne N. Nelson, Ding-Yuan Kuo, Ziming Shao, Ryan Bouck, Mathew Cherukara, Martin V. Holt, Kyle M. Shen, Darrell G. Schlom, Jin Suntivich, Andrej Singer

Abstract: Functional properties of transition-metal oxides strongly depend on crystallographic defects. In transition-metal-oxide electrocatalysts such as SrIrO3 (SIO), crystallographic lattice deviations can affect ionic diffusion and adsorbate binding energies. Scanning x-ray nanodiffraction enables imaging of local structural distortions across an extended spatial region of thin samples. Line defects rem… ▽ More Functional properties of transition-metal oxides strongly depend on crystallographic defects. In transition-metal-oxide electrocatalysts such as SrIrO3 (SIO), crystallographic lattice deviations can affect ionic diffusion and adsorbate binding energies. Scanning x-ray nanodiffraction enables imaging of local structural distortions across an extended spatial region of thin samples. Line defects remain challenging to detect and localize using nanodiffraction, due to their weak diffuse scattering. Here we apply an unsupervised machine learning clustering algorithm to isolate the low-intensity diffuse scattering in as-grown and alkaline-treated thin epitaxially strained SIO films. We pinpoint the defect locations, find additional strain variation in the morphology of electrochemically cycled SIO, and interpret the defect type by analyzing the diffraction profile through clustering. Our findings demonstrate the use of a machine learning clustering algorithm for identifying and characterizing hard-to-find crystallographic defects in thin films of electrocatalysts and highlight the potential to study electrochemical reactions at defect sites in operando experiments. △ Less

Submitted 9 January, 2023; v1 submitted 18 March, 2022; originally announced March 2022.

Comments: 11 pages and 4 figures in main text, supporting information included

Strain-induced orbital energy shift in antiferromagnetic RuO2 revealed by resonant elastic x-ray scattering

Authors: Benjamin Gregory, Jörg Strempfer, Daniel Weinstock, Jacob Ruf, Yifei Sun, Hari Nair, Nathaniel J. Schreiber, Darrell G. Schlom, Kyle M. Shen, Andrej Singer

Abstract: In its ground state, RuO2 was long thought to be an ordinary metallic paramagnet. Recent neutron and x-ray diffraction revealed that bulk RuO2 is an antiferromagnet (AFM) with TN above 300 K. Furthermore, epitaxial strain induces novel superconductivity in thin films of RuO2 below 2 K. Here, we present a resonant elastic x-ray scattering (REXS) study at the Ru L2 edge of the strained RuO2 films ex… ▽ More In its ground state, RuO2 was long thought to be an ordinary metallic paramagnet. Recent neutron and x-ray diffraction revealed that bulk RuO2 is an antiferromagnet (AFM) with TN above 300 K. Furthermore, epitaxial strain induces novel superconductivity in thin films of RuO2 below 2 K. Here, we present a resonant elastic x-ray scattering (REXS) study at the Ru L2 edge of the strained RuO2 films exhibiting the strain-induced superconductivity. We observe an azimuthal modulation of the 100 Bragg peak consistent with canted AFM found in bulk. Most notably, in the strained films displaying novel superconductivity, we observe a ~1 eV shift of the Ru eg orbitals to a higher energy. The energy shift is smaller in thicker, relaxed films and films with a different strain direction. Our results provide further evidence of the utility of epitaxial strain as a tuning parameter in complex oxides. △ Less

Submitted 18 January, 2022; originally announced January 2022.

Comments: 20 pages, 3 main figures, 3 supplementary figures

A single-crystal alkali antimonide photocathode: high efficiency in the ultra-thin limit

Authors: C. T. Parzyck, A. Galdi, J. K. Nangoi, W. J. I. DeBenedetti, J. Balajka, B. D. Faeth, H. Paik, C. Hu, T. A. Arias, M. A. Hines, D. G. Schlom, K. M. Shen, J. M. Maxson

Abstract: The properties of photoemission electron sources determine the ultimate performance of a wide class of electron accelerators and photon detectors. To date, all high-efficiency visible-light photocathode materials are either polycrystalline or exhibit intrinsic surface disorder, both of which limit emitted electron beam brightness. In this letter we demonstrate the synthesis of epitaxial thin films… ▽ More The properties of photoemission electron sources determine the ultimate performance of a wide class of electron accelerators and photon detectors. To date, all high-efficiency visible-light photocathode materials are either polycrystalline or exhibit intrinsic surface disorder, both of which limit emitted electron beam brightness. In this letter we demonstrate the synthesis of epitaxial thin films of Cs$_3$Sb on 3C-SiC (001) using molecular-beam epitaxy. Films as thin as 4 nm have quantum efficiencies exceeding 2\% at 532 nm. We also find that epitaxial films have an order of magnitude larger quantum efficiency at 650 nm than comparable polycrystalline films on Si. Additionally, these films permit angle-resolved photoemission spectroscopy measurements of the electronic structure, which are found to be in good agreement with theory. Epitaxial films open the door to dramatic brightness enhancements via increased efficiency near threshold, reduced surface disorder, and the possibility of engineering new photoemission functionality at the level of single atomic layers. △ Less

Submitted 28 December, 2021; originally announced December 2021.

Comments: Submitted Manuscript: 8 pages, 4 figures

Enhanced Surface Superconductivity in Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$

Authors: Christopher T. Parzyck, Brendan D. Faeth, Gordon N. Tam, Gregory R. Stewart, Kyle M. Shen

Abstract: We present direct evidence for an enhanced superconducting $T_c$ on the surface of cleaved single crystals of Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$. Transport measurements performed on samples cleaved in ultra high vacuum (UHV) show a significantly enhanced superconducting transition when compared to equivalent measurements performed in air. Deviations from the bulk resistivity appear at 21K, well… ▽ More We present direct evidence for an enhanced superconducting $T_c$ on the surface of cleaved single crystals of Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$. Transport measurements performed on samples cleaved in ultra high vacuum (UHV) show a significantly enhanced superconducting transition when compared to equivalent measurements performed in air. Deviations from the bulk resistivity appear at 21K, well above the 10K bulk $T_c$ of the underdoped compound. We demonstrate that the excess conductivity above the bulk $T_c$ can be controllably suppressed by application of potassium ions on the cleaved surface, indicating that the enhanced superconductivity is strongly localized to the sample surface. Additionally, we find that the effects of the potassium surface dosing are strongly influenced by the presence of residual gas absorbates on the sample surface, which may prevent effective charge transfer from the potassium atoms to the FeAs plane. This is further support for the conclusion that the effects of the dosing (and enhanced superconductivity) are localized within a few layers of the surface. △ Less

Submitted 22 December, 2021; originally announced December 2021.

Comments: 11 pages, 3 figures

Journal ref: Appl. Phys. Lett. 116, 062601 (2020)

Quantum Oscillations and the Quasiparticle Properties of Thin Film Sr$_2$RuO$_4$

Authors: Yawen Fang, Hari P. Nair, Ludi Miao, Berit Goodge, Nathaniel J. Schreiber, Jacob P. Ruf, Lena F. Kourkoutis, Kyle M. Shen, Darrell G. Schlom, B. J. Ramshaw

Abstract: We measure the Shubnikov-de Haas effect in thin-film Sr$_2$RuO$_4$ grown on an (LaAlO$_3$)$_{0.29}$-(SrAl$_{1/2}$Ta$_{1/2}$O$_3$)$_{0.71}$ (LSAT) substrate. We detect all three known Fermi surfaces and extract the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes. We show that the electronic structure is nearly identical to that of single-crystal Sr$_2$RuO$_4$, and that the… ▽ More We measure the Shubnikov-de Haas effect in thin-film Sr$_2$RuO$_4$ grown on an (LaAlO$_3$)$_{0.29}$-(SrAl$_{1/2}$Ta$_{1/2}$O$_3$)$_{0.71}$ (LSAT) substrate. We detect all three known Fermi surfaces and extract the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes. We show that the electronic structure is nearly identical to that of single-crystal Sr$_2$RuO$_4$, and that the quasiparticle lifetime is consistent with the Tc of comparably clean, single-crystal Sr$_2$RuO$_4$. Unlike single-crystal Sr$_2$RuO$_4$, where the quantum and transport lifetimes are roughly equal, we find that the transport lifetime is $1.3\pm0.1$ times longer than the quantum lifetime. This suggests that extended (rather than point) defects may be the dominant source of quasiparticle scattering in these films. To test this idea, we perform cross-sectional STEM and find that out-of-phase boundaries extending the entire thickness of the film occur with a density that is consistent with the quantum mean free path. The long quasiparticle lifetimes make these films ideal for studying the unconventional superconducting state in Sr$_2$RuO$_4$ through the fabrication of devices -- such as planar tunnel junctions and SQUIDs. △ Less

Submitted 2 August, 2021; v1 submitted 28 February, 2021; originally announced March 2021.

Comments: 12 pages, 8 figures

Journal ref: Phys. Rev. B 104, 045152 (2021)

Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO$_3$

Authors: Brendan D. Faeth, Saien Xie, Shuolong Yang, Jason K. Kawasaki, Jocienne N. Nelson, Shuyuan Zhang, Pramita Mishra, Chen Li, Christopher Jozwiak, Aaron Bostwick, Eli Rotenberg, Darrell G. Schlom, Kyle M. Shen

Abstract: The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO$_3$, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO$_3$ substrate might… ▽ More The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO$_3$, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO$_3$ substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO$_3$, where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings between the higher-order replicas, as well as other self-energy effects, allow us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of $λ= 0.19 \pm 0.02$. △ Less

Submitted 5 February, 2021; originally announced February 2021.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Lett. 127, 016803 (2021)

Separated transport relaxation scales and interband scattering in SrRuO$_3$, CaRuO$_3$, and Sr$_2$RuO$_4$ thin films

Authors: Youcheng Wang, H. P. Nair, N. J. Schreiber, J. P. Ruf, Bing Cheng, D. G. Schlom, K. M. Shen, N. P. Armitage

Abstract: The anomalous charge transport observed in some strongly correlated metals raises questions as to the universal applicability of Landau Fermi liquid theory. The coherence temperature $T_{FL}$ for normal metals is usually taken to be the temperature below which $T^2$ is observed in the resistivity. Below this temperature, a Fermi liquid with well-defined quasiparticles is expected. However, metalli… ▽ More The anomalous charge transport observed in some strongly correlated metals raises questions as to the universal applicability of Landau Fermi liquid theory. The coherence temperature $T_{FL}$ for normal metals is usually taken to be the temperature below which $T^2$ is observed in the resistivity. Below this temperature, a Fermi liquid with well-defined quasiparticles is expected. However, metallic ruthenates in the Ruddlesden-Popper family, frequently show non-Drude low-energy optical conductivity and unusual $ω/T$ scaling, despite the frequent observation of $T^2$ dc resistivity. Herein we report time-domain THz spectroscopy measurements of several different high-quality metallic ruthenate thin films and show that the optical conductivity can be interpreted in more conventional terms. In all materials, the conductivity has a two-Drude peak lineshape at low temperature and a crossover to a one-Drude peak lineshape at higher temperatures. The two-component low-temperature conductivity is indicative of two well-separated current relaxation rates for different conduction channels. We discuss three particular possibilities for the separation of rates: (a) Strongly energy-dependent inelastic scattering; (b) an almost-conserved pseudomomentum operator that overlaps with the current, giving rise to the narrower Drude peak; (c) the presence of multiple conduction channels that undergoes a crossover to stronger interband scattering at higher temperatures. None of these scenarios require the existence of exotic quasiparticles. The results may give insight into the possible significance of Hund's coupling in determining interband coupling in these materials. Our results also show a route towards understanding the violation of Matthiessen's rule in this class of materials and deviations from the "Gurzhi" scaling relations in Fermi liquids. △ Less

Submitted 23 December, 2020; originally announced December 2020.

Comments: 8 figures. No Supplemental Materials

Journal ref: Phys. Rev. B 103, 205109 (2021)

Vanishing nematic order beyond the pseudogap phase in overdoped cuprate superconductors

Authors: Naman K. Gupta, C. McMahon, R. Sutarto, T. Shi, R. Gong, Haofei I. Wei, K. M. Shen, F. He, Q. Ma, M. Dragomir, B. D. Gaulin, D. G. Hawthorn

Abstract: During the last decade, translational and rotational symmetry-breaking phases -- density wave order and electronic nematicity -- have been established as generic and distinct features of many correlated electron systems, including pnictide and cuprate superconductors. However, in cuprates, the relationship between these electronic symmetry-breaking phases and the enigmatic pseudogap phase remains… ▽ More During the last decade, translational and rotational symmetry-breaking phases -- density wave order and electronic nematicity -- have been established as generic and distinct features of many correlated electron systems, including pnictide and cuprate superconductors. However, in cuprates, the relationship between these electronic symmetry-breaking phases and the enigmatic pseudogap phase remains unclear. Here, we employ resonant x-ray scattering in a cuprate high-temperature superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ (Nd-LSCO) to navigate the cuprate phase diagram, probing the relationship between electronic nematicity of the Cu 3$d$ orbitals, charge order, and the pseudogap phase as a function of doping. We find evidence for a considerable decrease in electronic nematicity beyond the pseudogap phase, either by raising the temperature through the pseudogap onset temperature $T^{*}$ or increasing doping through the pseudogap critical point, $p^{*}$. These results establish a clear link between electronic nematicity, the pseudogap, and its associated quantum criticality in overdoped cuprates. Our findings anticipate that electronic nematicity may play a larger role in understanding the cuprate phase diagram than previously recognized, possibly having a crucial role in the phenomenology of the pseudogap phase. △ Less

Submitted 1 September, 2021; v1 submitted 15 December, 2020; originally announced December 2020.

Comments: 30 pages, 9 figures, Main text and Supplementary material

Journal ref: PNAS 118(34), e2106881118 (2021)

Terahertz Electrodynamics of Mixed-Valent YbAl$_3$ and LuAl$_3$ Thin Films

Authors: D. Barbalas, S. Chatterjee, D. G. Schlom, K. M. Shen, N. P. Armitage

Abstract: We present THz measurements of thin films of mixed-valent YbAl$_3$ and its structural analogue LuAl$_3$. Combined with traditional Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the low-frequency transport of these materials. We find that LuAl$_3$ demonstrates conventional Drude transport whereas at low temperatures YbAl$_3$ demonstrates a sharply… ▽ More We present THz measurements of thin films of mixed-valent YbAl$_3$ and its structural analogue LuAl$_3$. Combined with traditional Fourier transform infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study the low-frequency transport of these materials. We find that LuAl$_3$ demonstrates conventional Drude transport whereas at low temperatures YbAl$_3$ demonstrates a sharply renormalized Drude peak and a mid-infrared (MIR) peak in the conductivity, indicative of the formation of a heavy Fermi liquid. In YbAl$_3$ the extended Drude framework shows a consistency of the scattering rate with Fermi-liquid behavior below $T < 40$ K and a moderate mass enhancement. While a $ω^2$ Fermi liquid-like frequency dependence is not clearly exhibited, the temperature dependence of the Drude scattering rate and effective mass is consistent with the formation of a low-temperature moderately heavy Fermi liquid, albeit one with a smaller mass than observed in single crystals. The extended Drude analysis also supports a slow crossover between the Fermi liquid state and the normal state in YbAl$_3$. △ Less

Submitted 17 March, 2021; v1 submitted 4 December, 2020; originally announced December 2020.

Comments: 8 pages, 8 figures

Incoherent Cooper pairing and pseudogap behavior in single-layer FeSe/SrTiO$_3$

Authors: Brendan D. Faeth, Shuolong Yang, Jason K. Kawasaki, Jocienne N. Nelson, Pramita Mishra, Li Chen, Darrell G. Schlom, Kyle M. Shen

Abstract: In many unconventional superconductors, the presence of a pseudogap - a suppression in the electronic density of states extending above the critical temperature - has been a long-standing mystery. Here, we employ combined \textit{in situ} electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements to reveal an unprecedentedly large pseudogap regime in single-layer FeSe/… ▽ More In many unconventional superconductors, the presence of a pseudogap - a suppression in the electronic density of states extending above the critical temperature - has been a long-standing mystery. Here, we employ combined \textit{in situ} electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements to reveal an unprecedentedly large pseudogap regime in single-layer FeSe/SrTiO$_3$, an interfacial superconductor where incoherent Cooper pairs are initially formed above $T_Δ$ $\approx$ 60 K, but where a zero resistance state is only achieved below $T_{0}$ $<$ 30 K. We show that this behavior is accompanied by distinct transport signatures of two-dimensional phase fluctuating superconductivity, suggesting a mixed vortex state hosting incoherent Cooper pairs which persist well above the maximum clean limit $T_{c}$ of $\approx$ 40 K. Our work establishes the critical role of reduced dimensionality in driving the complex interplay between Cooper pairing and phase coherence in two-dimensional high-$T_c$ superconductors, providing a paradigm for understanding and engineering higher-$T_{c}$ interfacial superconductors. △ Less

Submitted 22 October, 2020; originally announced October 2020.

Journal ref: Phys. Rev. X 11, 021054 (2021)

Inhomogeneous ferromagnetism mimics signatures of the topological Hall effect in SrRuO$_3$ films

Authors: Gideok Kim, K. Son, Y. E. Suyolcu, L. Miao, N. J. Schreiber, H. P. Nair, D. Putzky, M. Minola, G. Christiani, P. A. van Aken, K. M. Shen, D. G. Schlom, G. Logvenov, B. Keimer

Abstract: Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the ``topological Hall effect'' (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epi… ▽ More Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the ``topological Hall effect'' (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal SrRuO$_3$ the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hall effect anomalies that closely resemble signatures of the THE, but a quantitative analysis demonstrates that they result from inhomogeneities in the ferromagnetic magnetization caused by a non-random distribution of Ru vacancies. As such inhomogeneities are difficult to avoid and are rarely characterized independently, our results call into question the identification of topological spin textures in numerous prior transport studies of quantum materials, heterostructures, and devices. Firm conclusions regarding the presence of such textures must meet stringent conditions such as probes that couple directly to the non-collinear magnetization on the atomic scale. △ Less

Submitted 19 October, 2020; originally announced October 2020.

Journal ref: Physical Review Materials 4, 104410 (2020)

Direct observation of distinct minibands in moiré superlattices

Authors: Saien Xie, Brendan D. Faeth, Yanhao Tang, Lizhong Li, Christopher T. Parzyck, Debanjan Chowdhury, Ya-Hui Zhang, Christopher Jozwiak, Aaron Bostwick, Eli Rotenberg, Jie Shan, Kin Fai Mak, Kyle M. Shen

Abstract: Moiré superlattices comprised of stacked two-dimensional materials present a versatile platform for engineering and investigating new emergent quantum states of matter. At present, the vast majority of investigated systems have long moiré wavelengths, but investigating these effects at shorter, incommensurate wavelengths, and at higher energy scales, remains a challenge. Here, we employ angle-reso… ▽ More Moiré superlattices comprised of stacked two-dimensional materials present a versatile platform for engineering and investigating new emergent quantum states of matter. At present, the vast majority of investigated systems have long moiré wavelengths, but investigating these effects at shorter, incommensurate wavelengths, and at higher energy scales, remains a challenge. Here, we employ angle-resolved photoemission spectroscopy (ARPES) with sub-micron spatial resolution to investigate a series of different moiré superlattices which span a wide range of wavelengths, from a short moiré wavelength of 0.5 nm for a graphene/WSe2 (g/WSe2) heterostructure, to a much longer wavelength of 8 nm for a WS2/WSe2 heterostructure. We observe the formation of minibands with distinct dispersions formed by the moiré potential in both systems. Finally, we discover that the WS2/WSe2 heterostructure can imprint a surprisingly large moiré potential on a third, separate layer of graphene (g/WS2/WSe2), suggesting a new avenue for engineering moiré superlattices in two-dimensional materials. △ Less

Submitted 15 October, 2020; originally announced October 2020.

Role of Dirac nodal lines and strain on the high spin Hall conductivity of epitaxial IrO2 thin films

Authors: Arnab Bose, Jocienne N. Nelson, Xiyue S. Zhang, Rakshit Jain, D. G. Schlom, D. C. Ralph, D. A. Muller, K. M. Shen, R. A. Buhrman

Abstract: Since the discovery of a 'giant' spin Hall effect (SHE) in certain heavy metal elements there has been an intense effort to identify and develop new and technologically viable, heavy-metal-based thin film materials that could generate spin currents with even greater efficiency to exert spin-orbit torques (SOT) on adjacent ferromagnetic nanostructures. In parallel, there have been wide ranging fund… ▽ More Since the discovery of a 'giant' spin Hall effect (SHE) in certain heavy metal elements there has been an intense effort to identify and develop new and technologically viable, heavy-metal-based thin film materials that could generate spin currents with even greater efficiency to exert spin-orbit torques (SOT) on adjacent ferromagnetic nanostructures. In parallel, there have been wide ranging fundamental studies of the spin currents that can arise from robust, intrinsic spin-orbit interaction (SOI) effects in more exotic systems including topological insulators, transition metal dichalcogenides with broken crystalline symmetry, Weyl and Dirac semimetals where gapless electronic excitations are protected by topology and symmetry. Here we experimentally study strong SOT from the topological semimetal IrO2 in (001) and (110) normal films, which exhibit distinctly different SHE strengths. Angle resolved photoemission spectroscopy studies have shown IrO2 exhibits Dirac nodal lines (DNL) in the band structure, which could enable a very high spin Hall conductivity (SHC). The (001) films exhibit exceptionally high damping like torque efficiency ranging from 0.45 at 293 K to 0.65 at 30 K which sets the lower bound of SHC that is ten times higher and of opposite sign than the theoretical prediction. We observe a substantial reduction of SHC in anisotropically strained (110) films, which suggests that the DNLs that are present in the (001) films and contribute to SHC, are disrupted and gapped due to the large anisotropic strain in (110) films, which in turn significantly lowers SHC. Very large value of SHC at room temperature of this Dirac semimetal could be very promising for the practical application. △ Less

Submitted 8 June, 2020; originally announced June 2020.

Journal ref: Applied Materials and Interfaces 2020

Strain-stabilized superconductivity

Authors: Jacob P. Ruf, Hanjong Paik, Nathaniel J. Schreiber, Hari P. Nair, Ludi Miao, Jason K. Kawasaki, Jocienne N. Nelson, Brendan D. Faeth, Yonghun Lee, Berit H. Goodge, Betül Pamuk, Craig J. Fennie, Lena F. Kourkoutis, Darrell G. Schlom, Kyle M. Shen

Abstract: Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendip… ▽ More Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the first instance of the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO$_{2}$ thin films on (110)-oriented TiO$_{2}$ substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of $d$ orbitals. △ Less

Submitted 13 May, 2020; originally announced May 2020.

Comments: 30 pages, 20 figures (including supplemental information)

Journal ref: Nature Communications 12, 59 (2021)

Strong, Temperature-Dependent Spin-Orbit Torques in Heavy Fermion YbAl$_3$

Authors: Neal D Reynolds, Shouvik Chatterjee, Gregory M. Stiehl, Joseph A. Mittelstaedt, Saba Karimeddiny, Alexander J. Buser, Darrell G. Schlom, Kyle M. Shen, Daniel C. Ralph

Abstract: The use of current-generated spin-orbit torques[1] to drive magnetization dynamics is under investigation to enable a new generation of non-volatile, low-power magnetic memory. Previous research has focused on spin-orbit torques generated by heavy metals[2-8], interfaces with strong Rashba interactions[9,10] and topological insulators [11-14]. These families of materials can all be well-described… ▽ More The use of current-generated spin-orbit torques[1] to drive magnetization dynamics is under investigation to enable a new generation of non-volatile, low-power magnetic memory. Previous research has focused on spin-orbit torques generated by heavy metals[2-8], interfaces with strong Rashba interactions[9,10] and topological insulators [11-14]. These families of materials can all be well-described using models with noninteracting-electron bandstructures. Here, we show that electronic interactions within a strongly correlated heavy fermion material, the Kondo lattice system YbAl$_{3}$, can provide a large enhancement in spin-orbit torque. The spin-torque conductivity increases by approximately a factor of 4 as a function of decreasing temperature from room temperature to the coherence temperature of YbAl$_{3}$ ($T^* \approx 37$ K), with a saturation at lower temperatures, achieving a maximum value greater than any heavy metal element. This temperature dependence mimics the increase and saturation at $T^*$ of the density of states at the Fermi level arising from the ytterbium 4$f$-derived heavy bands in the Kondo regime, as measured by angle-resolved photoemission spectroscopy[15]. We therefore identify the many-body Kondo resonance as the source of the large enhancement of spin-orbit torque in YbAl$_{3}$. Our observation reveals new opportunities in spin-orbit torque manipulation of magnetic memories by engineering quantum many-body states. △ Less

Submitted 30 January, 2022; v1 submitted 7 April, 2020; originally announced April 2020.

Comments: 7 pages, 3 figures

Sub-THz momentum drag and violation of Matthiessen's rule in an ultraclean ferromagnetic SrRuO$_3$ metallic thin film

Authors: Youcheng Wang, G. Bosse, H. P. Nair, N. J. Schreiber, J. P. Ruf, B. Cheng, C. Adamo, D. E. Shai, Y. Lubashevsky, D. G. Schlom, K. M. Shen, N. P. Armitage

Abstract: SrRuO$_3$, a ferromagnet with an approximately 160\,K Curie temperature, exhibits a $T^2$ dependent dc resistivity below $\approx$ 30 K. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures which seem to contradict a Fermi-liquid picture with long-lived quasiparticles. In this work, we measure the low-frequency THz range response of… ▽ More SrRuO$_3$, a ferromagnet with an approximately 160\,K Curie temperature, exhibits a $T^2$ dependent dc resistivity below $\approx$ 30 K. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures which seem to contradict a Fermi-liquid picture with long-lived quasiparticles. In this work, we measure the low-frequency THz range response of thin films with residual resistivity ratios, $ρ_{300K}/ ρ_{4K} \approx$ 74. Such low disorder samples allow an unprecedented look at the effects of electron-electron interactions on low-frequency transport. At temperatures below 30 K, we found both a very sharp zero-frequency mode which has a width narrower than $k_BT/\hbar$ as well as a broader zero frequency Lorentzian that has at least an order of magnitude larger scattering rate. Both features have temperature dependencies consistent with a Fermi-liquid with the wider feature explicitly showing a T$^2$ scaling. Such two -Drude transport sheds light on previous reports of the violation of Mathielssen's rule and extreme sensitivity to disorder in metallic ruthenates. We consider a number of possibilities for the origin of the two feature optical conductivity including multiband effects that arise from momentum conserving interband scattering and the approximate conservation of a pseudo-momentum that arises from quasi-1D Fermi surfaces. △ Less

Submitted 26 March, 2020; originally announced March 2020.

Comments: main text 5 pages, 3 figures

Journal ref: Phys. Rev. Lett. 125, 217401 (2020)

Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr$_3$SnO

Authors: Yanjun Ma, Anthony Edgeton, Hanjong Paik, Brendan Faeth, Chris Parzyck, Betül Pamuk, Shun-Li Shang, Zi-Kui Liu, Kyle M. Shen, Darrell G. Schlom, Chang-Beom Eom

Abstract: Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr$_3$SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this m… ▽ More Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr$_3$SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations we design and implement a deposition approach to achieve the adsorption-controlled growth of epitaxial Sr$_3$SnO single-crystal films by molecular-beam epitaxy (MBE). In-situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and non-trivial topological nature of the as-grown samples. Compared with conventional MBE, the synthesis route used results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of topological crystalline insulators opens opportunities to manipulate topological states by tuning symmetries via epitaxial strain and heterostructuring. △ Less

Submitted 31 December, 2019; originally announced December 2019.

Electronic and vibrational signatures of ruthenium vacancies in Sr$_2$RuO$_4$ thin films

Authors: Gideok Kim, Y. Eren Suyolcu, J. Herrero-Martin, D. Putzky, H. P. Nair, J. P. Ruf, N. J. Schreiber, C. Dietl, G. Christiani, G. Logvenov, M. Minola, P. A. van Aken, K. M. Shen, D. G. Schlom, B. Keimer

Abstract: The synthesis of stoichiometric Sr$_2$RuO$_4$ thin films has been a challenge because of the high volatility of ruthenium oxide precursors, which gives rise to ruthenium vacancies in the films. Ru vacancies greatly affect the transport properties and electronic phase behavior of Sr$_2$RuO$_4$, but their direct detection is difficult due to their atomic dimensions and low concentration. We applied… ▽ More The synthesis of stoichiometric Sr$_2$RuO$_4$ thin films has been a challenge because of the high volatility of ruthenium oxide precursors, which gives rise to ruthenium vacancies in the films. Ru vacancies greatly affect the transport properties and electronic phase behavior of Sr$_2$RuO$_4$, but their direct detection is difficult due to their atomic dimensions and low concentration. We applied polarized X-ray absorption spectroscopy at the oxygen K-edge and confocal Raman spectroscopy to Sr$_2$RuO$_4$ thin films synthesized under different conditions. The results show that these methods can serve as sensitive probes of the electronic and vibrational properties of Ru vacancies, respectively. The intensities of the vacancy-related spectroscopic features extracted from these measurements are well correlated with the transport properties of the films. The methodology introduced here can thus help to understand and control the stoichiometry and transport properties in films of Sr$_2$RuO$_4$ and other ruthenates. △ Less

Submitted 30 September, 2019; originally announced September 2019.

Journal ref: Physical Review Materials 3, 094802 (2019)

Revealing the Hidden Heavy Fermi Liquid in CaRuO3

Authors: Yang Liu, Hari P. Nair, Jacob P. Ruf, Darrell G. Schlom, Kyle M. Shen

Abstract: The perovskite ruthenate has attracted considerable interest due to reports of possible non-Fermi-liquid behavior and its proximity to a magnetic quantum critical point, yet its ground state and electronic structure remain enigmatic. Here we report the first measurements of the Fermi surface and quasiparticle dispersion in CaRuO3 through a combination of oxide molecular beam epitaxy and in situ an… ▽ More The perovskite ruthenate has attracted considerable interest due to reports of possible non-Fermi-liquid behavior and its proximity to a magnetic quantum critical point, yet its ground state and electronic structure remain enigmatic. Here we report the first measurements of the Fermi surface and quasiparticle dispersion in CaRuO3 through a combination of oxide molecular beam epitaxy and in situ angle-resolved photoemission spectroscopy. Our results reveal a complex and anisotropic Fermi surface consisting of small electron pockets and straight segments, consistent with the bulk orthorhombic crystal structure with large octahedral rotations. We observe a strongly band-dependent mass renormalization, with prominent heavy quasiparticle bands which lie close to the Fermi energy and exhibit strong temperature dependence. These results are consistent with a heavy Fermi liquid with a complex Fermiology and small hybridization gaps near the Fermi energy. Our results provide a unified framework for explaining previous experimental results on CaRuO3, such as its unusual optical conductivity, and demonstrate the importance of octahedral rotations in determining the quasiparticle band structure, and electron correlations in complex transition metal oxides. △ Less

Submitted 8 February, 2019; originally announced February 2019.

Comments: 4 figures

Journal ref: Physical Review B 98, 041110(R) (2018)

Strain-Engineering Mott-Insulating La$_2$CuO$_4$

Authors: O. Ivashko, M. Horio, W. Wan, N. B. Christensen, D. E. McNally, E. Paris, Y. Tseng, N. E. Shaik, H. M. Rønnow, H. I. Wei, C. Adamo, C. Lichtensteiger, M. Gibert, M. R. Beasley, K. M. Shen, J. M. Tomczak, T. Schmitt, J. Chang

Abstract: The transition temperature $T_\textrm{c}$ of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La$_{2-\mathrm{x}}$Sr$_\mathrm{x}$CuO$_4$ thin films, such substrates are sub-optimal and the highest $T_\textrm{c}$ is instead obtained using LaSrAlO$_4$. An outstanding challenge is thu… ▽ More The transition temperature $T_\textrm{c}$ of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La$_{2-\mathrm{x}}$Sr$_\mathrm{x}$CuO$_4$ thin films, such substrates are sub-optimal and the highest $T_\textrm{c}$ is instead obtained using LaSrAlO$_4$. An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in $T_\mathrm{c}$ and how can we tune them? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La$_2$CuO$_4$ thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest $T_\textrm{c}$ under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates. △ Less

Submitted 5 March, 2019; v1 submitted 18 May, 2018; originally announced May 2018.

Comments: 15 pages, 7 figures and 2 tables (including Supplementary Information)

Journal ref: Nature Communications 10, 786 (2019)

Adsorption-controlled growth of La-doped BaSnO3 by molecular-beam epitaxy

Authors: Hanjong Paik, Zhen Chen, Edward Lochocki, Ariel H. Seidner, Amit Verma, Nicholas Tanen, Jisung Park, Masaki Uchida, ShunLi Shang, Bi-Cheng Zhou, Mario Brützam, Reinhard Uecker, Zi-Kui Liu, Debdeep Jena, Kyle M. Shen, David A. Muller, Darrell G. Schlom

Abstract: Epitaxial La doped BaSnO3 films were grown in an adsorption controlled regime by molecular beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A film grown on a (001) DyScO3 substrate exhibited a mobility of 183 cm^2 V^-1 s^-1 at room temperature and 400 cm^2 V^-1 s^-1 at 10 K, despite the high concentration (1.2x10^11 cm^-2) of threading dislocations present. In comparison… ▽ More Epitaxial La doped BaSnO3 films were grown in an adsorption controlled regime by molecular beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A film grown on a (001) DyScO3 substrate exhibited a mobility of 183 cm^2 V^-1 s^-1 at room temperature and 400 cm^2 V^-1 s^-1 at 10 K, despite the high concentration (1.2x10^11 cm^-2) of threading dislocations present. In comparison to other reports, we observe a much lower concentration of (BaO)2 Ruddlesden Popper crystallographic shear faults. This suggests that in addition to threading dislocations that other defects possibly (BaO)2 crystallographic shear defects or point defects significantly reduce the electron mobility. △ Less

Submitted 1 November, 2017; originally announced November 2017.

Lifshitz transition from valence fluctuations in YbAl3

Authors: Shouvik Chatterjee, Jacob. P. Ruf, Haofei. I. Wei, Kenneth D. Finkelstein, Darrell G. Schlom, Kyle M. Shen

Abstract: In Kondo lattice systems with mixed valence, such as YbAl3, interactions between localized electrons in a partially filled f shell and delocalized conduction electrons can lead to fluctuations between two different valence configurations with changing temperature or pressure. The impact of this change on the momentum-space electronic structure and Fermi surface topology is essential for understand… ▽ More In Kondo lattice systems with mixed valence, such as YbAl3, interactions between localized electrons in a partially filled f shell and delocalized conduction electrons can lead to fluctuations between two different valence configurations with changing temperature or pressure. The impact of this change on the momentum-space electronic structure and Fermi surface topology is essential for understanding their emergent properties, but has remained enigmatic due to a lack of appropriate experimental probes. Here by employing a combination of molecular beam epitaxy (MBE) and in situ angle-resolved photoemission spectroscopy (ARPES) we show that valence fluctuations can lead to dramatic changes in the Fermi surface topology, even resulting in a Lifshitz transition. As the temperature is lowered, a small electron pocket in YbAl3 becomes completely unoccupied while the low-energy ytterbium (Yb) 4f states become increasingly itinerant, acquiring additional spectral weight, longer lifetimes, and well-defined dispersions. Our work presents the first unified picture of how local valence fluctuations connect to momentum space concepts including band filling and Fermi surface topology in the longstanding problem of mixed-valence systems. △ Less

Submitted 20 October, 2017; originally announced October 2017.

Comments: 18 pages, 11 figures

Journal ref: Nature Communications, 8, 852, 1-7 (2017)

A tunable low-energy photon source for high-resolution angle-resolved photoemission spectroscopy

Authors: John W. Harter, Philip D. C. King, Eric J. Monkman, Daniel E. Shai, Yuefeng Nie, Masaki Uchida, Bulat Burganov, Shouvik Chatterjee, Kyle M. Shen

Abstract: We describe a tunable low-energy photon source consisting of a laser-driven xenon plasma lamp coupled to a Czerny-Turner monochromator. The combined tunability, brightness, and narrow spectral bandwidth make this light source useful in laboratory-based high-resolution photoemission spectroscopy experiments. The source supplies photons with energies up to ~7 eV, delivering under typical conditions… ▽ More We describe a tunable low-energy photon source consisting of a laser-driven xenon plasma lamp coupled to a Czerny-Turner monochromator. The combined tunability, brightness, and narrow spectral bandwidth make this light source useful in laboratory-based high-resolution photoemission spectroscopy experiments. The source supplies photons with energies up to ~7 eV, delivering under typical conditions >10^12 ph/s within a 10 meV spectral bandwidth, which is comparable to helium plasma lamps and many synchrotron beamlines. We first describe the lamp and monochromator system and then characterize its output, with attention to those parameters which are of interest for photoemission experiments. Finally, we present angle-resolved photoemission spectroscopy data using the light source and compare its performance to a conventional helium plasma lamp. △ Less

Submitted 11 April, 2017; originally announced April 2017.

Journal ref: Review of Scientific Instruments 83, 113103 (2012)

Observation of semi-localized dispersive states in the strongly correlated electron-doped ferromagnet Eu$_{1-x}$Gd$_{x}$O

Authors: Daniel E. Shai, Mark H. Fischer, Alex J. Melville, Eric J. Monkman, John W. Harter, Dawei Shen, Darrell G. Schlom, Michael J. Lawler, Eun-Ah Kim, Kyle M. Shen

Abstract: Chemical substitution plays a key role in controlling the electronic and magnetic properties of complex materials. For instance, in EuO, carrier doping can induce a spin-polarized metallic state, colossal magnetoresistance, and significantly enhance the Curie temperature. Here, we employ a combination of molecular-beam epitaxy, angle-resolved photoemission spectroscopy, and an effective model calc… ▽ More Chemical substitution plays a key role in controlling the electronic and magnetic properties of complex materials. For instance, in EuO, carrier doping can induce a spin-polarized metallic state, colossal magnetoresistance, and significantly enhance the Curie temperature. Here, we employ a combination of molecular-beam epitaxy, angle-resolved photoemission spectroscopy, and an effective model calculation to investigate and understand how semi-localized states evolve in lightly electron doped Eu$_{1-x}$Gd$_{x}$O above the ferromagnetic Curie temperature. Our studies reveal a characteristic length scale for the spatial extent of the donor wavefunctions which remains constant as a function of doping, consistent with recent tunneling studies of doped EuO. Our work sheds light on the nature of the semiconductor-to-metal transition in Eu$_{1-x}$Gd$_{x}$O and should be generally applicable for doped complex oxides. △ Less

Submitted 17 October, 2016; originally announced October 2016.

Journal ref: Phys. Rev. B 94, 195102 (2016)

Epitaxial growth and electronic properties of mixed valence YbAl3 thin films

Authors: Shouvik Chatterjee, Suk Hyun Sung, David J. Baek, Lena F. Kourkoutis, Darrell G. Schlom, Kyle M. Shen

Abstract: We report the growth of thin films of the mixed valence compound YbAl$_{3}$ on MgO using molecular-beam epitaxy. Employing an aluminum buffer layer, epitaxial (001) films can be grown with sub-nm surface roughness. Using x-ray diffraction, in situ low-energy electron diffraction and aberration-corrected scanning transmission electron microscopy we establish that the films are ordered in the bulk a… ▽ More We report the growth of thin films of the mixed valence compound YbAl$_{3}$ on MgO using molecular-beam epitaxy. Employing an aluminum buffer layer, epitaxial (001) films can be grown with sub-nm surface roughness. Using x-ray diffraction, in situ low-energy electron diffraction and aberration-corrected scanning transmission electron microscopy we establish that the films are ordered in the bulk as well as at the surface. Our films show a coherence temperature of 37 K, comparable to that reported for bulk single crystals. Photoelectron spectroscopy reveals contributions from both $\textit{f}^{13}$ and $\textit{f}^{12}$ final states establishing that YbAl$_{3}$ is a mixed valence compound and shows the presence of a Kondo Resonance peak near the Fermi-level. △ Less

Submitted 2 August, 2016; originally announced August 2016.

Comments: 4 pages, 6 figures

Journal ref: J. Appl. Phys. 120, 035105 (2016