Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B14: Focus Session: Phase Transitions in Strongly Correlated Electron Systems |
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Sponsoring Units: DMP Chair: Divine Kumah, Yale University Room: 008A |
Monday, March 2, 2015 11:15AM - 11:51AM |
B14.00001: Atomic Scale Control of Competing Electronic Phases in Ultrathin Correlated Oxides Invited Speaker: Kyle Shen Ultrathin epitaxial thin films offer a number of unique advantages for engineering the electronic properties of correlated transition metal oxides. For example, atomically thin films can be synthesized to artificially confine electrons in two dimensions. Furthermore, using a substrate with a mismatched lattice constant can impose large biaxial strains of larger than 3\% ($\Delta a / a$), much larger than can achieved in bulk single crystals. Since these dimensionally confined or strained systems may necessarily be less than a few unit cells thick, investigating their properties and electronic structure can be particularly challenging. We employ a combination of reactive oxide molecular beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES) to investigate how dimensional confinement and epitaxial strain can be used to manipulate electronic properties and structure in correlated transition metal oxide thin films. We describe some of our recent work manipulating and studying the electronic structure of ultrathin LaNiO$_{3}$ through a thickness-driven metal-insulator transition between three and two unit cells (Nature Nanotechnology 9, 443, 2014), where coherent Fermi liquid-like quasiparticles are suppressed at the metal-insulator transition observed in transport. We also will describe some recent unpublished work using epitaxial strain to drive a Lifshitz transition in atomically thin films of the spin-triplet ruthenate superconductor Sr$_{2}$RuO$_{4}$, where we also can dramatically alter the quasiparticle scattering rates and drive the system towards non-Fermi liquid behavior near the critical point (B. Burganov, C. Adamo, in preparation). [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B14.00002: Extremely large electronic anisotropy caused by electronic phase separation in Ca$_{3}$(Ru$_{0.97}$Ti$_{0.03}$)$_{2}$O$_{7}$ single crystal Jing Peng, Xiaoshan Wu, Zhiqiang Mao Bilayered ruthenate $Ca_{3}Ru_{2}O_{7}$ exhibits rich electronic and magnetic properties. It orders at 56K, with FM bilayers antiferromagnetically coupled along c-axis (AFM-a). The AFM transition is closely followed by a first-order metal-insulator (MI) transition at 48K where spin directions switch to the b-axis (AFM-b). While this MI transition is accompanied by the opening of anisotropic charge gap; small Fermi pockets survive from the MI transition, thus resulting in quasi-2D metallic transport behavior for T<30K. We previously showed such a quasi-2D metal with the AFM-b order composed of FM bilayers can be tuned to a Mott-insulating state with a nearest-neighbor AFM order via Ti doping [Ke et al, PRB 84, 201102(11)]. $Ca_{3}(Ru_{0.97}Ti_{0.03})_{2}O_{7}$ is close to the critical composition for the AFM-b-to-G-AFM phase transition. Our recent studies show the sample with this composition is characterized by an electronic phase separation between the insulating G-AFM phase (major) and the localized AFM-b phase (minor). The minor AFM-b phase forms a conducting path through electronic percolation within the ab-plane, but not along the c-axis, thus resulting in extremely large electronic anisotropy with $\rho_{ab}/\rho_{c}\sim 10^9$, which may be the largest among bulk materials. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B14.00003: Separation of transport and Hall scattering rates in SrTiO$_{3}$/ReTiO$_{3}$ two-dimensional electron gases Evgeny Mikheev, Brandon Isaac, Tyler Cain, Christopher Freeze, Susanne Stemmer ReTiO$_{3}$/SrTiO$_{3}$/ReTiO$_{3}$ (Re$=$Gd, Sm) quantum well structures that contain a high-density, two-dimensional electron gas (2DEG) exhibit phenomena that are reminiscent of the normal state behavior of unconventional superconductors, including a pseudogap, proximity to two-dimensional antiferromagnetism, and non-Fermi liquid behavior. Here we will discuss another transport anomaly, namely that scattering rates measured in the longitudinal and Hall conductivities are distinct and have different temperature dependencies. We show that the two-scattering rate framework provides a remarkably simple, consistent, and accurate description for the dependencies of the Hall effect on temperature and quantum well thickness. This analysis reveals signatures of a spin density wave gap opening (Re$=$Sm) and a divergent Hall effect in the T$=$0 limit for an intermediate quantum well thickness (near 5 SrO layers), indicating a quantum critical point. Several theoretical proposals exist that may explain the two-lifetime separation. We discuss how the results in this system introduce a number of new, specific constraints and the need for a unifying microscopic theory. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B14.00004: Odd frequency Density Waves Yaron Kedem, Alexander Balatsky A new type of hidden order in many body systems is explored. This order appears in states which are analogues to charge density waves, or spin density waves, but involve anomalous particle hole correlations that are odd in time and frequency. These states are shown to be inherently different from the usual states of density waves. We discuss a method to experimentally observe the new type of pairing by measuring the density-density correlation, both in time and space, where a clear distinction between odd and even correlations can be detected. An order parameter for these states is defined and calculated for a simple model, eliminating the physical nature of this phenomenon. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 1:03PM |
B14.00005: Controlling Magnetism in Spin-Orbit-Driven Oxides with Epitaxial Strain Invited Speaker: Patrick Clancy The layered perovskite iridates Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ and Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ are the prototypical spin-orbital Mott insulators, displaying a novel j$_{\mathrm{eff}} $ = 1/2 ground state driven by strong 5d spin-orbit coupling effects. Efforts to understand, and ultimately control, this spin-orbit-induced ground state have led to a surge of interest in thin film iridates, which offer unique opportunities for the tuning of electronic and magnetic properties via epitaxial strain. We have performed complementary resonant magnetic x-ray scattering (RMXS) and resonant inelastic x-ray scattering (RIXS) measurements on epitaxial thin film samples of Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ and Ba$_{\mathrm{2}}$IrO$_{\mathrm{4}}$. By measuring 13 to 50 nm films grown on a variety of different substrates (PSO, GSO, STO, LSAT), we are able to investigate the impact of applied tensile and compressive strain on the magnetic structure, correlation lengths, and characteristic excitations of these materials. We find that the dispersion of the low-lying magnetic and orbital excitations is strongly affected by strain-induced structural changes, and show that epitaxial strain provides an effective method for tuning three distinct energy scales: the magnetic ordering temperature (T$_{\mathrm{N}})$, the magnetic exchange interactions (J), and the non-cubic crystal field splitting ($\Delta_{\mathrm{CEF}})$. Perhaps most strikingly, we demonstrate that hard x-ray RIXS can be used to perform detailed magnetic dispersion measurements on thin film samples of 13 nm ($\sim $5 unit cells) or less.\\[4pt] Work performed in collaboration with H. Gretarsson, A. Lupascu, J.A. Sears, Z. Nie, Y.-J. Kim (University of Toronto), Z. Islam, M.H. Upton, J. Kim, D. Casa, T. Gog, A.H. Said (Argonne National Laboratory), J. Nichols, J. Terzic, S.S.A. Seo, G. Cao (University of Kentucky), M. Uchida, D.G. Schlom, K.M. Shen (Cornell University), H. Stoll (University of Stuttgart), V.M. Katukuri, L.Hozoi, J. van den Brink (IFW Dresden).\\[4pt] [1] A. Lupascu et al, Phys. Rev. Lett. 112, 147201 (2014). [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B14.00006: Series of alternating states with unpolarized and spin-polarized bands in dimerized IrTe$_2$ V. Kiryukhin, G.L. Pascut, T. Birol, S.-W. Cheong, K. Haule, M.J. Gutmann, J.J. Yang A series of states with different densities of stripes of Ir dimers is investigated using x-ray diffraction and density functional theory in layered nonmagnetic metal IrTe$_2$. With decreasing temperature, structures with and without inversion symmetry alternate. In non-centrosymmetric states, spin-orbit coupling splits the electronic energy bands into spin-polarized pairs. Factors affecting the stability of the observed dimerized states are established, and it is conjectured that an infinite series of alternating states with and without polarized bands is realized in IrTe$_2$. Switching dimerized states with different symmetries by changing temperature or strain enables control of band polarization, adding a new tool for spintronics and valleytronics research. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B14.00007: Quantum Monte Carlo study of the nematic quantum critical point in a metal Yoni Schattner, Samuel Lederer, Erez Berg, Steven A. Kivelson The coupling of fermions to gapless collective modes can lead to interesting critical phenomena, non-Fermi-liquid behavior and/or superconductivity. As an example for such a system, we present a sign-problem free lattice model of quantum-critical Ising-nematic bosons coupled to fermions in two dimensions. Determinantal Quantum Monte-Carlo simulations show a second order nematic transition at low temperatures. As the transition is approached, we find evidence of non-Fermi-liquid behavior. At the temperature scales accessible to us, we find no traces of superconductivity. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B14.00008: Itinerant density instability at classical and quantum critical points Yejun Feng, Jasper van Wezel, Felix Flicker, Jiyang Wang, D.M. Silevitch, P. B. Littlewood, T. F. Rosenbaum Itinerant density waves are model systems for studying quantum critical behavior.~ In both~the model spin- and charge-density-wave systems Cr and NbSe$_{\mathrm{2}}$, it is possible to drive a continuous quantum phase transition with critical pressures below 10 GPa.~ Using x-ray diffraction techniques, we are able to directly track the evolution of the ordering wave vector Q across the pressure-temperature phase diagram. We find a non-monotonic dependence of $Q$ on pressure.~ Using a Landau-Ginsburg theoretical framework developed by McMillan for CDWs, we evaluate the importance of the physical terms in driving the formation of ordered states at both the thermal and quantum phase transitions. We find that the itinerant instability is the deciding factor for the emergent order, which is further influenced by the critical fluctuations in both the thermal and quantum limits. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B14.00009: Emergence of charge density wave domain walls above the superconducting dome in 1T-TiSe$_2$ Peter Abbamonte, Young Il Joe, Xiaoqian Chen, Pouyan Ghaemi, Ken Finkelstein, Gilberto de la Pena, Yu Gan, James Lee, Shi Yuan, Jochen Geck, Greg MacDougall, Tai Chiang, Lance Cooper, Eduardo Fradkin Superconductivity in so-called unconventional superconductors is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between superconductivity and fluctuations in some other order parameter. To better understand this connection, we used high-pressure X-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe$_2$, which was previously shown to exhibit superconductivity when the CDW is suppressed by pressure or intercalation of Cu atoms. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at P$_c$ = 5.1 $\pm$ 0.2 GPa, which is more than 1 GPa beyond the end of the superconducting region. Unexpectedly, at P $\sim$ 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that superconductivity in TiSe$_2$ may be connected to the formation of CDW domain walls [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B14.00010: Nanoscale Charge-order Dynamics in Stripe-phase Nickelates Probed via Ultrafast THz Spectroscopy Giacomo Coslovich, Sascha Behl, Bernhard Huber, Wei-Sheng Lee, Zhi-Xun Shen, Takao Sasagawa, Hans A. Bechtel, Michael C. Martin, Robert A. Kaindl Here we report ultrafast optical pump-THz probe spectroscopy of the model stripe-ordered system La$_{\mathrm{1.75}}$Sr$_{\mathrm{0.25}}$NiO$_{\mathrm{4}}$. Ultrafast experiments in the multi-THz spectral range show strong THz reflectivity variations around the phonon bending mode frequency ($\approx $11 THz). At low temperatures this phonon mode exhibits a splitting directly related to the formation of long-range stripe-order, while the background conductivity is reminiscent of the opening of the mid-IR pseudogap due to charge localization. The transient THz probe therefore captures both the electronic and structural dynamics in a single light pulse. The results reveal the dynamical interplay between charge localization and the bending mode folding, providing insight in the emergence of nanoscale charge-order in complex oxides. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B14.00011: Quasicrystalline Charge Order Jasper van Wezel, Felix Flicker Incommensurate charge density waves occur in a large variety of materials in one, two and even three dimensions. As a function of decreasing temperature or applied pressure, the propagation vector characterizing such charge order typically evolves smoothly towards a commensurate value, before discontinuously jumping to a fully commensurate phase. This so-called lock-in transition is often explained in terms of a proliferation of discommensurations, which separate local regions of commensurate CDW within a globally incommensurate structure. Here, we argue that in strongly incommensurate systems with a sharply peaked electronic susceptibility, a second possibility exists. Rather than forming a regular array of discommensurations, we show that within an extended region of parameter space, the system may lower its free energy further by forming a quasicrystalline charge ordered state. The characteristic properties of this novel implementation of a quasicrystal, as well as its effect on the phase diagram and wave vector evolution of typical incommensurate charge ordered materials will be discussed. [Preview Abstract] |
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