Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B45: Electronic Correlations in Complex Oxides |
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Sponsoring Units: DCMP Chair: Michael McGuire, Oak Ridge National Laboratory Room: LACC 505 |
Monday, March 5, 2018 11:15AM - 11:27AM |
B45.00001: Hints of Electronic Nematicity at the (111) LaAlO3/SrTiO3 Interface Samuel Davis, Zhen Huang, Kun Han, Ariando Ariando, Thirumalai Venkatesan, Venkat Chandrasekhar The two-dimensional conducting electron gas (2DCG) that forms at the interface between the two band insulators LaAlO3 (LAO) and SrTiO3 (STO) has been extensively studied due to the wealth of in-situ tunable phenomena that exist at the interface. We have shown that nearly every property of the 2DCG that forms at the interface of (111) oriented LAO/STO is strongly anisotropic with respect to in-plane crystal direction. This symmetry breaking points to the existence of an electronic nematic phase at the interface that can be modified by an in-situ electrostatic back gate potential. Here we show that the anisotropy turns on simultaneously in both the longitudinal resistance and Hall response at T ≈ 22 K, which does not match up with any known structural transition. Furthermore, below 22 K, charge transport is activated with different activation energies along the two in-plane crystal directions. This response implies that the band edges along the two directions are different giving further evidence of an electronic nematic state at the interface. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B45.00002: Transport and excitations in a negative-U quantum dot at the LaAlO3/SrTiO3 interface Guen Prawiroatmodjo, Martin Leijnse, Felix Trier, Yunzhong Chen, Dennis Christensen, Merlin von Soosten, Nini Pryds, Thomas Jespersen In a solid-state host, attractive electron–electron interactions can lead to the formation of local electron pairs which play an important role in the understanding of prominent phenomena such as high Tc superconductivity and the pseudogap phase. Recently, evidence of a paired ground state without superconductivity was demonstrated at the level of single electrons in quantum dots at the interface of LaAlO3 and SrTiO3. Here, we present a detailed study of the excitation spectrum and transport processes of a gate-defined LaAlO3/SrTiO3 quantum dot exhibiting pairing at low temperatures. For weak tunneling, the spectrum agrees with calculations based on the Anderson model with a negative effective charging energy U, and exhibits an energy gap corresponding to the Zeeman energy of the magnetic pair-breaking field. In contrast, for strong coupling, low-bias conductance is enhanced with a characteristic dependence on temperature, magnetic field and chemical potential consistent with the charge Kondo effect. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B45.00003: Polaronic T2 Resistivity in Strontium Titanate Alexander Edelman, Peter Littlewood Strontium titanate is a bulk insulator that upon doping becomes conducting at remarkably low carrier densities. We argue that this diluteness, which enhances the importance of collective modes of the electron gas, along with strong coupling to longitudinal optic phonons, produces a polaronic liquid of electrons coupled to hybrid plasmon-phonon modes. We argue that these unusual basic excitations of the system provide a microscopic basis for a swath of unusual transport phenomenology in this material, including T2 resistivity at low temperature. The latter, arising from electron-electron scattering but requiring a mechanism to dump energy to the lattice, is puzzling in this dilute limit of this material which contains only one species of electron and cannot Umklapp scatter. The experimentally measured scattering cross-section is also inconsistent with classical electron-electron scattering. Our scenario provides a mechanism for both of these observations. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B45.00004: Transport and Memory Behavior of Vanadium Pentoxide Nanoflakes Colin Kilcoyne, Sujay Singh, Justin Andrews, Sarbajit Banerjee, Sambandamurthy Ganapathy Vanadium pentoxide (V2O5) has interesting physical phenomena such as metal-insulator transitions, charge-ordering transitions and superconductivity. Recently, various phases of hydrated vanadium oxide bronzes (δ and σ phases of V2O5 with a metal) have been studied for their potential use in emerging memory applications. These compounds have layered structures allowing them to be exfoliated like transition metal chalcogenides and graphene, giving rise to a new class of 2D semiconductors down to the monolayer limit. The unique source of their memory properties, the migration of protons in the interlayer channels, allows for new physics to be explored. We demonstrate δ and σ phase V2O5 structures with nickel showing a pronounced hysteresis in their current-voltage characteristics at room temperature - at a singular voltage two different conductive states can be accessed. We modulate the shape and size of the hysteresis with factors such as temperature, sweep rate and the magnitude of write/erase voltage. The effects of these parameters and the contribution of electrostatic/electrochemical effects will be presented. In addition, an analysis of the device performance over many cycles and hours will demonstrate high endurance and retention for memory technologies. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B45.00005: Ultrafast Optical Spectroscopy of Epitaxial LaVO3 Thin Films: Evidence for Critical Dynamics Dylan Lovinger, Matthew Brahlek, Roman Engel-Herbert, Richard Averitt We investigate time-resolved dynamics in perovskite-type LaVO3 thin film samples using femtosecond pump-probe spectroscopy. LaVO3 is a Mott-Hubbard (MH) insulator that undergoes a magnetic transition at 143 K (paramagnetic to antiferromagnetic) and a structural phase transition at 141 K, below which orbital ordering occurs1. Excitation of carriers across the MH gap with ~50 fs pulses initiates dynamics that manifest as changes in the reflectivity. In the vicinity of the magnetic transition, we observe dramatic slowing down in the recovery dynamics, consistent with critical-like dynamics at a 2nd order phase transition2, likely related to spin-ordering dynamics3. Further, we observe coherent acoustic phonons which also exhibit an anomalous response near ~140K. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B45.00006: Direct Visualization of Fast Surface Ion Diffusion in Vanadium Dioxide Nanowires Yasen Hou, Rui Xiao, Xin Tong, Scott Dhuey, Dong Yu Diffusion of atoms in vanadium dioxide is at the heart of using ionic gating or chemical doping to modulate its metal insulator transition for potential Mott transistors and create unusual electronic phases. Here we report direct ion diffusion in vanadium dioxide (VO2) nanowires (NWs) by using photocurrent imaging. Alkali metal ions are injected into a NW segment via ionic liquid gating and are shown to diffuse along the NW axis. The visualization of ion diffusion is realized by spatially resolved photocurrent measurements, which detect the charge carrier density change associated with the ion incorporation. Diffusion constants are determined to be on the order of 10-10 cm2/s for both Li+ and Na+ ions at room temperature, while H+ diffuses much slower. The ion diffusion is also found to occur mainly at the surface of the NWs, as metal contacts can effectively block the ion diffusion. This novel method of visualizing ion distribution is expected to be applied to study ion diffusion in a broad range of materials, providing key insights on phase transition electronics and energy storage applications. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B45.00007: Revealing electron correlation effects in SrVO3 in the ultraclean limit Matthew Brahlek, Lei Zhang, Joseph Roth, Jason Lapano, Turan Birol, Roman Engel-Herbert The experimental challenge to engineer multicomponent ABO3 perovskite oxides as atomic scale thin films is to synthesize intrinsic defect-free materials. In this talk, I will focus on how the recently developed hybrid molecular beam epitaxy (hMBE), which combines traditional elemental sources with metal-organic sources, opens an adsorption-controlled growth-window, thus giving access to the cleanest complex oxide materials. In particular, I will discuss how temperature dependent resistivity and Hall effect measurements on ultraclean samples of the classic example of a “simple” correlated metal, SrVO3, reveal anomalous anisotropic correlation effects at low temperature. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B45.00008: Discovery of a New Quantum Dimer Magnet in a Strongly Spin-Orbit Coupled Material Gavin Hester, Hari Nair, Tim Reeder, Léo Berges, Djamel Ziat, Jeffrey Quilliam, James Neilson, Gabriele Sala, Kate Ross
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Monday, March 5, 2018 12:51PM - 1:03PM |
B45.00009: Cd2Re2O7: a Strong Spin-Orbit Coupled Hund's Metal Yilin Wang, Hu Miao, Run Yang, Youguo Shi, Christopher Homes, Gabriel Kotliar A correlation-driven hidden order phase transition is found in a spin-orbit coupled pyrochlore metal Cd2Re2O7 (5d2) at 200 K. In this work, we use both first principle DFT+DMFT calculations and optical conductivity measurements to study the strong correlation effects in this material. The calculated optical conductivity by DFT+DMFT in the normal state at large spin-orbit coupling (SOC) and Hund's coupling shows incoherent feature which is consistent with the experimental results, while it shows coherent feature at small Hund's coupling. The DFT+DMFT calculations at large Hund's coupling perfectly reproduce the complete saturation behavior of resistivity observed experimentally in the normal state down to very low temperature, which indicates that the coherent energy scale in this material is largely decreased by both SOC and Hund's coupling. We also find clear orbital-selective behavior between jeff=1/2 and jeff=3/2 orbitals, which is a result of strong SOC and Hund's coupling. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B45.00010: Orbital and spin ordering physics of the Mn3O4 spinel SANTANU PAL, Siddhartha Lal
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Monday, March 5, 2018 1:15PM - 1:27PM |
B45.00011: Microscopic states and the Verwey transition of magnetite nanocrystals investigated by nuclear magnetic resonance Sumin Lim, Baeksoon Choi, Sang Young Lee, Soonchil Lee, Ho-Hyun Nahm, Yong-Hyun Kim, Taehun Kim, Je-Guen Park, Jisoo Lee, Jaeyoung Hong, Soon Gu Kwon, Taeghwan Hyeon 57Fe Nuclear Magnetic Resonance (NMR) was measured for magnetite nanocrystals ranging in size from 7 nm to 7 μm. The linewidth of the NMR spectra changes drastically around 120 K, showing microscopic evidence of the Verwey transition. In the region above the transition temperature, the linewidth of the spectrum increases and the spin-spin relaxation time decreases as the nanocrystal size decreases. The linewidth broadening indicates that the magnetic structure of the nanocrystals is significantly deformed and the charge order is markedly reduced compared to bulk even when structural distortion is unobservable. The reduction of the spin-spin relaxation time is attributed to the enhanced electron-phonon coupling in the quantum-confinement regime and thus to the suppressed polaron hopping conductivity in ferromagnetic metals. The results show that the magnetic distortion occurs in the entire nanocrystal, contrary to the prediction of the frequently adopted core-shell binary model. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B45.00012: Spin-orbit and correlation effects in (Sr1−xLax)3Ir2O7 Gregory Affeldt, Tom Hogan, Stephen Wilson, Alessandra Lanzara Both spin-orbit coupling and electronic correlations are essential to the formation of the spin-orbit Mott state near half filling in the layered perovskite iridates. We explore the manifestations of these energy scales, as well as the bilayer splitting, in the electronic structure of (Sr1−xLax)3Ir2O7 across a wide range of doping. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B45.00013: Atomic scale impact of La doping on the electronic structure in bilayer iridate Sr3Ir2O7 Yulia Maximenko, Zhenyu Wang, Daniel Walkup, Wenwen Zhou, Tom Hogan, Stephen Wilson, Vidya Madhavan Strongly correlated electronic systems exhibit exotic states created by intricate interplay between strong spin-orbit coupling, Coulomb repulsion, and crystal field. Doped marginal Mott insulators are crucial to study at the nanoscale because of the inherent local nature of the interactions and disorder. Here, we use scanning tunneling microscopy (STM) to study La-doped bilayer strontium iridate Sr3Ir2O7 (Ir327), a member of Ruddlesden-Popper series of perovskite iridates. By inhomogeneously supplying electrons, La doping creates nanoscale phase separation and drives the crystal through the metal/Mott-insulator transition. The crystal structure of Ir327 is composed of stacked bilayers of corner-sharing IrO6 octahedra, while La atoms are introduced into Sr sites. By analyzing STM topography and spectroscopy, we conclude that (a) La-dopants preferentially dope the middle Sr-O plane in each bilayer and (b) La atoms in the middle plane are responsible for in-gap spectral weight transfer and impurity states. We report observation of charge or orbital order in the outer Sr-O planes. No similar order has been observed in the pristine or Ru-doped Ir327. We explain the results in terms of spatial interplay between strong local interactions and effect of the dopants. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B45.00014: Electronic structure studies on exotic insulating state in LaxSr2-xRhO4 Wonshik Kyung, Junyoung Kwon, Yoshiyuki Yoshida, Changyoung Kim So far, there have been a lot of researches on Sr2IrO4 due to its exotic insulating behavior. With the help of experiemental and theoretical studies, it is well known that J_eff, caused by strong spin-orbit coupling, make it Mott insulator. On the other hand, although Rh is isovalent of Ir, Sr2RhO4 is a metal because of the smaller SOC effect comparing to Sr2IrO4. Interestingly, however, we has been confirmed that it has an insulating state above specific electron doping number which was achieved through La substitution. We conducted electronic structure measurement studies to reveal the mechanism. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B45.00015: Coulomb correlations intertwined with spin and orbital excitations in LaCoO3 Keisuke Tomiyasu, Jun Okamoto, Hsiao-Yu Huang, Zhi-Ying Chen, Evelyn Pratami Sinaga, Wen-Bin Wu, Yen-Yi Chu, Amol Singh, Ru-Pan Wang, Frank de Groot, Ashish Chainani, Sumio Ishihara, C. Chen, Di-Jing Huang We carried out temperature-dependent (20–550 K) measurements of resonant inelastic x-ray scattering on LaCoO3 to investigate the evolution of its electronic structure across the spin-state crossover. In combination with charge-transfer multiplet calculations, we accurately quantified the renormalized crystal-field excitation energies and spin-state populations. We show that the screening of the effective on-site Coulomb interaction of 3d electrons is orbital selective and coupled to the spin-state crossover in LaCoO3. The results establish that the gradual spin-state crossover is associated with a relative change of Coulomb energy versus bandwidth, leading to a Mott-type insulator-to-metal transition. |
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