Correlated Electron Systems - Novel Developments Workshop 2018

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    Carrier Density Control of Magnetism and Hall Effects in EuTiO3 Films
    (2018-05) Steemer, Susanne
    The topological Hall effect is a hallmark of topologically nontrivial (chiral) spin textures and can be observed as a distinct, additional contribution in Hall measurements that is superposed on the ordinary and anomalous Hall effects. Oxide films and interfaces that support topologically nontrivial spin textures are interesting, because the potential for control by electric field effect and because proximity effects can be utilized to realize other exotic states within all-epitaxial heterostructures. In this talk, we discuss the role of carrier density and band structure in the topological and anomalous Hall effects in thin films of Eu1-xSmxTiO3 grown by molecular beam epitaxy. The carrier density controls the sign and strength of the topological and anomalous Hall effects, the spin textures, and other effects, such as metamagnetic transitions. We will discuss the results in terms of the interactions between electronic and spin structures in this material.
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    Normal and Superconducting state of doped Strontium Titanate
    (2018-05) Gastiasoro, Maria
    SrTiO is one of the most extensively studied perovskite oxides. Charge transport in the normal state and the origin of superconductivity are however still a source of debate in this material. In the first part of the talk, we present a combined theoretical and experimental study of specific heat and transport in the normal state, across a wide range of doping. On one hand, the Sommerfeld coefficients are consistent with a weakly correlated Fermi liquid across the entire doping range. On the other hand, combined with the T-square transport coefficient the Kadowaki-Woods scaling breaks down, implying an unusual mechanism for the T-square resistivity. Recent experimental work has led to new effort to understand the mechanism behind the most dilute superconducting semiconductor. In the second part of the talk, we consider the Bardeen-Pines interaction and explore how the instability in the superconducting channel is affected when the Fermi energy of the electrons is smaller than the characteristic phonon frequency.
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    The Quest for a Quantum Spin Liquid 
    (2018-05) Broholm, Collin
    In the ongoing quest for a robust realization of a quantum spin liquid, a range of interesting magnetic materials and phenomena have been discovered. I shall review experiments probing interacting quantum spins on kagome, honeycomb, and triangular lattices in 2D and on the pyrochlore lattice in 3D. These frustrated quantum magnets feature an intermediate energy and temperature regime with spin-liquid-like properties but also unique low temperature phases driven by quenched disorder or lattice instabilities. Such inevitable deviations from ideal spin liquid models are interesting in their own right and their elucidation may contribute to understanding age old puzzles such as the phase diagram of V2O3.
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    Spin-Orbit Coupling and Gapped Magnetic Excitations in Iron-Based Superconductors
    (2018-05) Li, Yuan
    Spin-orbit coupling in iron-based superconductors gives rise to anisotropy in spin space for the magnetism, which may in turn help establish a rich variety of electronic phases. In this talk, I will present our latest progress in using inelastic neutron scattering (INS) to determine low-energy spin excitations in Sr(1-x)Na(x)Fe2As2 and FeSe(1-x)S(x) superconductors. Upon cooling Sr(1-x)Na(x) Fe2As2 into its double-Q tetragonal magnetic phase, a relatively large spin excitation gap develops, which we attribute to pronounced spin-space anisotropy that echoes with the spin reorientation transition upon entering this phase. The existence of such a gap appears to preclude the development of any spin resonant mode in the superconducting phase at lower temperatures, hence it explains why the double-Q phase strongly suppresses the superconductivity. In FeSe(1-x)S(x) which exhibits nematicity but no magnetic order at low temperatures, we show that the spin excitations are also gapped at low energies, which by itself is consistent with the notion that the spins are in a "quantum paramagnetic" state with entanglement established along one of the in-plane Fe-Fe directions. However, our spin-polarized INS experiments further reveal that the excitations above the gap are strongly anisotropic in spin space. We expect our results to help unveil the intriguing interplay between the spin and the orbital physics in the iron-based superconductors.
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    What Do We Know About the Mechanism of Superconduc9vity at “Intermediate Coupling?”
    (2018-05) Kivelson, Steven
    Because a Fermi liquid is unstable to superconductivity in the presence of an arbitrarily weak effective attractive interaction in any Cooper channel, most theories of superconductivity effectively involve extrapolation from a weak coupling limit. To obtained theoretically well controlled results at intermediate coupling, one must either rely on (painfully time consuming) numerics or identify special model problems for which exact results can be extracted in one way or another, or be extremely clever. I will discuss some results that my collaborators and I have obtained - recently and less recently - on various aspects of this class of problems using all the tools available to us (i.e. without being clever). While these studies are motivated by the physics of various real materials - such as the cuprates, A3C60, magical graphene, and possibly even H3S - what I will discuss is the properties of simple model Hamiltonians.
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    AF Spin Gap Limits the Coherent SC Gap in Cuprates
    (2018-05) Tranquada, John
    We have performed inelastic neutron scattering measurements of the magnetic excitations in La(2-x)Sr(x)CuO(4) at doping levels on both sides of the putative quantum critical point, in both the superconducting and normal states. The low-energy excitations look similar at both dopings: there is a peak in the magnetic spectral weight at 15-20 meV, and the spin gap (~ 7 meV) in the superconducting state develops on the low-energy side of this peak. Past work in a magnetic field indicates that suppressing the superconducting order only causes the spin gap to fill in—there is no evidence of critical behavior of the peak in spectral weight. I will discuss this result in the broader context of magnetic correlations in the cuprates.
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    Optical effects in multiband conductors and superconductors
    (2018-05) Homes, Christopher
    Multiband materials display a wide variety of interesting transport phenomena, often with striking optical effects. Two examples are the colossal magnetoresistance in the semimetal WTe_2, and the emergence of superconductivity in the iron-based material FeTe_0.55Se_0.45 with a critical temperature (T_c) of ~14 K. The complex optical properties are determined from a Kramers-Kronig analysis of the reflectance, which is measured in the transition metal-chalcogenide (a-b) planes over a wide energy range. A poor metal at room temperature, WTe_2 undergoes a Lifschitz transition to become a perfectly compensated semimetal at low temperature, leading to the formation of a striking plasma edge in the far-infrared reflectance. By considering Drude components for the electron and hole pockets, and by examining both the real and imaginary parts of the optical conductivity, it can be demonstrated that one of the scattering rates collapses at low temperature [1]. Dirac and Weyl semimetals display very small scattering rates, and WTe_2 is thought to be a type-II Weyl semimetal. FeTe_0.55Se_0.45 is also a poor metal at room temperature, with a flat and almost frequency-independent optical conductivity. Just above T_c, a narrow Drude response emerges, superimposed on a broad, temperature-independent Drude component. Below T_c, dramatic changes in the in-plane reflectance signal the formation of multiple superconducting energy gaps which may be determined from the real part of the optical conductivity to be 2Δ_1 = 5.6 and 2Δ_2 = 11.2 meV on the broad and narrow bands, respectively. Interestingly, this material is simultaneously in both the clean and dirty limit [2]. *This work done in collaboration with A. Akrap, R. J. Cava, Y. M. Dai, and G. D. Gu. Supported by the Office of Science, U.S. Department of Energy, under Contract No. DE-SC0012704. [1] C. C. Homes, M. N. Ali, and R. J. Cava, Phys. Rev. B 92, 161109(R) (2015). [2] C. C. Homes, Y. M. Dai, J. S. Wen, Z. J. Xu, and G. D. Gu, Phys. Rev. B 91, 144503 (2015).
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    Nematic order and the superconducting gap in FeSe
    (2018-05) Watson, Matt
    The intense scrutiny of the electronic structure of FeSe over the last few years has been motivated by the opportunity to distil the essential physics of both the nematic and superconducting phases, and their interplay. Here I will address the evolution of the electronic structure in the nematic phase, below Ts = 90 K using high-resolution ARPES. The hole pocket undergoes elliptical distortions, but most dramatically, our `detwinned’ ARPES results show spectral weight on only one peanutshaped electron pocket. This unexpected result is also observed in the nematic phase of NaFeAs, and I will argue that this effect, rather than the 10-20 meV band shifts and distortions, is the critical ingredient of the electronic structure in the nematic phase. I will also present measurements of the highly anisotropic superconducting gap on both the hole and electron pockets of FeSe. The results are consistent with results from quasiparticle interference, but by considering the matrix element effects in ARPES we are explicitly able to show a scaling of the superconducting gap with the dyz orbital character. Furthermore we show that such a gap structure arises naturally from the solution to the linearized gap equation, starting from a tight-binding model with accuracy on both the band dispersions and their orbital characters, if we also take into account the one-peanut effect as observed in the nematic phase.
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    Second-order Topological Superconductors
    (2018-05) Wang, Yuxuan
    Topological superconductors are fully gapped in the bulk but host Majorana modes on their boundaries. We extend this notion to a new class of superconductors, second-order topological superconductors, that host Majorana modes instead on second-order boundaries, i.e., corners of a two-dimensional system and hinges for a three-dimensional system. Here we propose two general scenarios in which second-order topological superconductivity can be realized. First, we show that $p_x+ip_y$-wave pairing in a (doped) Dirac semimetal in two dimensions with four mirror symmetric Dirac nodes realizes second-order topological superconductivity. Second, we show that the four Dirac nodes can also come from the BdG spectrum of a $d$-wave superconductor. In this scenario, with an additional $p$-wave pairing that gaps out the Dirac nodes, the system realizes second-order topological superconductivity as well. We show that these exotic superconducting states can be intrinsically realized in a metallic system with electronic interactions, or induced by proximity effect.
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    Hund’s correlated Metals
    (2018-05) deMedici, Luca
    Metallic phases with Hund’s correlations (“Hund’s metals”) are presently the focus of intensive research, in particular in relation to unconventional Fe-based superconductors. Recent experiments validate this emerging theoretical picture of a metallic phase with evidences of large local paramagnetic moments, large and orbital-selective mass renormalizations and orbitallyselective pairing in the superconducting state. Further theoretical insight shows that Hund’s coupling can also alter the quasiparticle interactions in some regimes, thus potentially renormalizing the pairing strength. This is shown to correlate with experimental high-Tc superconductivity in Fe-based pnictides and FeSe. Additionally, specular many-body physics is predicted for Cr-based analogues of these materials, possibly indicating a new playground where high-Tc superconductors might be found. L. de’ Medici, Phys. Rev. Lett. 118, 167003 (2017) P. Villar-Arribi and L. de’ Medici, ArXiv:1803.01494 (2018) Edelmann et al. PRB 95, 205118 (2017)
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    Anomalous transport of SrTiO3 (STO) accumula.on layers
    (2018-05) Fu, Han
    In this talk, we study the low temperature conductivity of electron accumulation layers induced by the very strong electric field at the surface of STO sample. Due to the strongly nonlinear lattice dielectric response, the three-dimensional density of electrons n(z) in such a layer decays with the distance from the surface z very slowly as n(z)~1/z^{12/7}. We show that when the mobility is limited by the surface scattering, the contribution of such a tail to the conductivity diverges at large z because of growing time electrons need to reach the surface. We explore truncation of this divergence by the finite sample width, by the bulk scattering rate, or by the crossover to the bulk linear dielectric response with the dielectric constant \kappa. As a result we arrive at the anomalously large mobility, which depends not only on the rate of the surface scattering, but also on the physics of truncation. Similar anomalous behavior is found for the Hall factor, the magnetoresistance, and the thermopower.
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    Identifying mechanisms of quantum nematic transitions from the dynamic susceptibility
    (2018-05) Klein, Avi
    Several strongly correlated electronic materials, such as FeSe, are thought to manifest quantum critical transitions to an electronic nematic state. In my talk, I will discuss an interesting feature of the nematic QCP: its anisotropic (quadrupolar) nature implies the appearance of dynamical fluctuations at finite frequencies but very long spatial scales. These fluctuations encode information on the driving mechanism of the nematic transition: whether it involves an external degree of freedom, such as spin fluctuations, coupled to the electronic charge, or internal charge degrees of freedom (a Pomeranchuk instability). I will show that Raman data for FeSe1-xSx agrees with the hypothesis of a Pomeranchuk instability. Finally, I will also describe how these nematic critical fluctuations are ‘shaped’ by conservation laws (which are constraints on long spatial scales), implying a fascinating interplay between classical and quantum scales.
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    Elastic control of the Mott transition
    (2018-05) Littlewood, Peter2
    The metal-insulator transition driven by strong electronic correlations – generically called the “Mott” transition – is usually described entirely by electronic Hamiltonians, with models designed to exhibit related emergent phenomena such as magnetism and superconductivity. In real solids, the electronic localization also couples to the crystal lattice, and it turns out that these elastic degrees of freedom insert important new entropic phenomena more familiar in soft matter physics. The coupling to the lattice induces elastic strain fields, which have intrinsic long-range interactions that cannot be screened. When strain fields are produced as a secondary order parameter in phase transitions - as for example in ferroelectrics - this produces unexpected consequences for the dynamics of order parameter fluctuations, including the generation of a gap in what would otherwise have been expected to be Goldstone modes. A very important class of transition metal oxides – the perovskites – can be thought of as an array of tethered octahedra where the Mott transition produces a shape-change in the unit cell. Coupling of the fundamental order parameter to octahedral rotations gives rise to large entropic effects that can shift the transition temperature by hundreds of degrees K , essentially by exploiting the physics of jammed solids. The insight might offer ways to make better refrigerators by enhancing electro-caloric and magneto-electric effects.
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    Resonant X-ray Studies of Electronic Orders in Quantum Solids
    (2018-05) Comin, Riccardo
    Resonant X-ray scattering (RXS) has become one of the prime techniques to study of charge and spin order in reciprocal space, owing to its high sensitivity to electronic states near the Fermi energy. We have applied resonant X-ray methods to explore the emergent organization of the electronic degrees of freedom into periodically modulated patterns – a hallmark of strongly-correlated quantum solids. The consequent breaking of translational symmetry is often manifested in the form of charge- or spin-density-wave, two phenomena which are essential to the physics of two families of compounds in particular cuprates and nickelates. On the cuprate front, I will review the latest efforts to broadly chart out charge order across the extended doping-temperature phase diagram using RXS, as well as discuss recent implementations to extract symmetry information from the tensorial nature of the resonant scattering process. On the rare earth nickelates (RENiO3) front, I will focus on some of our recent efforts to use RXS at high spatial resolutions (50-75 nm) to visualize the nanoscale texture of spin-density-waves across the Neel and metal-insulator transition. I will discuss the experimental trends observed both in pristine NdNiO3 thin films and in electron-doped SmNiO3, where control of the oxygen stoichiometry leads to dramatic changes in the electronic transport. I will conclude with a few ideas and prospects on the use of coherent soft x-ray scattering methods (coherent diffractive imaging and ptychography) to map out the texture of electronic orders at even higher spatial resolutions, and with orders of magnitude improvements in imaging efficiency
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    Thermodynamic evidence for a nematic phase transition at the onset of pseudogap in YBCO and Hg1201
    (2018-05) Matsuda, Y
    A long-standing controversial issue in the quest to understand the superconductivity in cuprates is the nature of the enigmatic pseudogap region of the phase diagram. Especially important is whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature T*. Here we report torque-magnetometry measurements of anisotropic susceptibility within the ab planes in orthorhombic YBCO with exceptionally high precision. The in-plane anisotropy along [100] direction (Cu-O-Cu direction) displays a significant increase with a distinct kink at T*, showing a remarkable scaling behavior with respect to T/T* in a wide doping range. The analysis reveals that the rotational symmetry breaking (nematicity) sets in at T* in the limit where the effect of orthorhombicity is eliminated. We also performed the same measurements on simple tetragonal Hg1201 with single CuO2 layer. Two-fold susceptibility anisotropy emerges spontaneously at T*, providing direct evidence of the broken rotational symmetry. These firmly establish that the nematic phase transition is universal in high-Tc cuprates. Surprisingly, unlike YBCO, the diagonal nematicity along [110] direction develops in Hg1201. Furthermore, the nematicity in Hg1201 appears to compete with CDW.
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    Confinement and Superconductivity at the LaAlO3/SrTiO3 Interface
    (2018-05) Triscone, Jean-Marc
    The interface between LaAlO3 and SrTiO3, two good band insulators, was found to be conducting [1], and, in some doping range, superconducting with a maximum critical temperature of about 300 mK [2,3]. I will discuss in this presentation the electronic structure, superconductivity, and the Tc versus doping phase diagram of LaAlO3/SrTiO3 and ((LaAlO3)0.5-(SrTiO3)0.5)-SrTiO3 interfaces. I will also compare superconductivity at these interfaces with superconductivity in bulk doped SrTiO3 [4]. [1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004). [2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007). [3] A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008). [4] S. Gariglio, M. Gabay, and J.-M. Triscone, Review APL Materials, 4, 060701 (2016).