Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W62: Excitons and Other Collective Electronic Modes |
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Sponsoring Units: DCMP Chair: Jonathan Denlinger, Lawrence Berkeley National Laboratory Room: Mile High Ballroom 4C |
Friday, March 6, 2020 8:00AM - 8:12AM |
W62.00001: Exciton Transport in the Electron-hole System Ta2NiSe5 Akitoshi Nakano, Takayuki Nagai, Naoyuki Katayama, Hiroshi Sawa, Hiroki Taniguchi, Ichiro Terasaki A ternary transition metal chalcogenide, Ta2NiSe5 has been recently proposed as a candidate of the Excitonic insulator(EI). An extremely flattened band dispersion in a single particle excitation spectrum observed by an angle resolved photoemission spectroscopy is regarded as evidence of the EI phase in this compound[1]. However, macroscopic physical property to evidence the spontaneously generated electron-hole pairs in Ta2NiSe5 have not been found yet. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W62.00002: Time-resolved ARPES signatures of pump driven normal-to-excitonic insulator transition Enrico Perfetto, Davide Sangalli, Andrea Marini, Gianluca Stefanucci We consider a ground-state band insulator turning into a nonequilibrium excitonic insulator (EI) when pumping it at sub-gap frequencies slighlty larger than the excitonic energy. The macroscopic polarization in the non-equilibrium EI phase is characterized by self-sustained oscillations with a frequency depending on the absorbed energy. We show that during the pump-driving the excitonic ARPES structure undergoes a convex-to-concave shape transition and concomitantly the system goes through a BEC-BCS crossover. Furthermore attosecond pulses shone after the pump-driving at different times tdelay generate a photocurrent which oscillates in tdelay. This phenomenon is related to the AC response of an exotic Josephson junction. |
Friday, March 6, 2020 8:24AM - 8:36AM |
W62.00003: Dynamics of an order parameter coupled phonon in an excitonic insulator Honglie Ning, Omar Mehio, Michael Buchhold, Takashi Kurumaji, Joseph G Checkelsky, Gil Refael, David Hsieh An impulsively stimulated Raman active phonon can be suppressed or amplified using two optical pulses tailored to be period-matched with this certain mode. Ta2NiSe5 is an excitonic insulator candidate which exhibits multiple coherent phonons in transient reflectivity. We conducted double-pump coherent phonon spectroscopy measurements on Ta2NiSe5 to understand the interaction between the phonon and the excitonic order parameter. We further developed theoretical methods to simulate the dynamics of the order parameter coupled phonon. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W62.00004: Imaging and Spectroscopy of an Exciton Condensate Subhradeep Misra, Michael Stern, Vladimir Umansky, Israel Bar-Joseph We study the spatial and spectral properties of the photoluminescence of an exciton liquid at low temperatures and show that it evolves from a disordered liquid to a homogenous Bose-Einstein condensate, extending over a few hundred microns and covering the entire area of the mesa. The appearance of the condensate is marked by the emergence of a narrow PL peak, which gains strength as the temperature is reduced, and as the power is increased. We suggest that this peak is a result of the dark condensate formation and its interaction with the bright thermal exciton population. |
Friday, March 6, 2020 8:48AM - 9:00AM |
W62.00005: Bardasis-Schrieffer polaritons in excitonic insulators Zhiyuan Sun, Andrew Millis Bardasis-Schrieffer (BaSh) modes in superconductors are fluctuations in subdominant pairing channels, e.g., d-wave fluctuations in an s-wave superconductor. We generalize the notion to excitonic insulators and show that these modes generically occur. In s-wave excitonic insulators, a p-wave BaSh mode exists below the gap energy and has a non-vanishing optical matrix element with light. This mode hybridizes strongly with photons to form Bardasis-Schrieffer polaritons, which are observable in both far-field and near-field optical experiments. We also show that a strong photon pulse induces order parameter rotation on the $s+ip$ plane which results in Thouless charge pumping through the system. |
Friday, March 6, 2020 9:00AM - 9:12AM |
W62.00006: Discrete breaking of symmetry in the excitonic phase of Ta2NiSe5 Giacomo Mazza, Malte Roesner, Lukas Windgaetter, Simone Latini, Angel Rubio, Antoine Georges Ta2NiSe5 (TNS) is an excitonic insulator candidate material. Condensation of excitons is supposed to originate from particle-hole pairs formed across valence and conduction bands which weakly overlap at the Fermi level. Yet clear cut signatures of such a phase together with its possible origins remain open questions. Here we report on the understanding of the symmetry principles underlying an instability of the excitonic type in TNS. |
Friday, March 6, 2020 9:12AM - 9:24AM |
W62.00007: Cluster mean-field analysis of the finite-temperature properties of the one-dimensional extended Falicov-Kimball model Masahiro Kadosawa, Satoshi Nishimoto, Koudai Sugimoto, Yukinori Ohta Motivated by a recent finding that Ta2NiSe5 is in a strong-coupling excitonic insulator state [1], we study finite-temperature properties of the one-dimensional extended Falicov-Kimball model (EFKM), the simplest lattice model for the excitonic insulator state. Here, we use the cluster mean-field method with a sine-square deformation and calculate the finite-temperature phase diagrams of the model, which are compared with the results of a previous study at zero temperature [2]. Moreover, we calculate the optical conductivity spectra of the model at finite temperatures and find that the temperature dependence of the spectral features observed in experiment [3] are well reproduced by our calculations. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W62.00008: Global Spin Current in Excitonic Phases of the Two-Band Hubbard Model Shunsuke Yamamoto, Hisao Nishida, Koudai Sugimoto, Yukinori Ohta It has recently been pointed out that the k-space spin texture may emerge in the doped excitonic phase of a two-orbital Hubbard model containing the cross-hopping terms with certain symmetry [1-3]. Thanks to the spontaneous breaking of the inversion symmetry by the excitonic phase transition, the global spin current perpendicular to the applied electric field is induced by the spin-dependent off-diagonal component of the Drude weight. The mechanism is quite different from the conventional one originated from the spin Hall effect where a spin-orbit coupling plays an essential role, and thus our result gives a new route to generate a spin current. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W62.00009: Sign-Free Determinant Quantum Monte Carlo Study of the Bilayer Hubbard and Two-Orbital Hubbard-Kanamori Models Xuxin Huang, Martin Claassen, Brian Moritz, Thomas Devereaux Two-band Hubbard models, as prototypes for various strongly correlated systems, have attracted intense research interest over the past few decades. Determinant Quantum Monte Carlo (DQMC), an unbiased finite-temperature numerical technique well suited to study such models, generically suffers from the fermion sign problem. However, for some variations, e.g. the bilayer Hubbard model with symmetric electron-hole doping and the half-filled two-orbital Hubbard-Kanamori model, particle-hole symmetry can be utilized to perform sign-problem-free studies. Here, using DQMC we show that the bilayer Hubbard model possesses a Berezinskii–Kosterlitz–Thouless (BKT) transition to an inter-layer biexciton condensate at intermediate coupling and finite electron-hole doping. For the Hubbard-Kanamori model, we treat the full rotationally invariant interaction, including the Hubbard and Hund’s coupling terms, using a decoupling scheme which involves a 12-state auxiliary field per site, and present the magnetic correlation and phase transition results obtained from DQMC. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W62.00010: Novel states of charge-imbalanced polariton condensates Artem Strashko, Francesca Marchetti, Allan Macdonald, Jonathan Keeling Polariton condensation is a well-established phenomenon featuring all the signatures of an ordinary condensate. However, in the context of polariton condensation, almost exclusively balanced systems, with equal densities of electrons and holes, have been studied. This misses a whole class of potential exotic imbalanced condensed states like an FFLO or a breached-pair state. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W62.00011: π-ton contributions to optical conductivity in correlated electron systems Anna Kauch, Petra Pudleiner, Katharina Astleithner, Paul Worm, Clemens Watzenböck, Patrik Thunström, Tin Ribic, Karsten Held The interaction of light with solids gives rise to new bosonic quasiparticles, with the exciton being the most famous of these polaritons. While excitons are the generic polaritons of semiconductors, we show that for strongly correlated systems another polariton is prevalent [1] -- originating from the dominant antiferromagnetic or charge density wave fluctuations in these systems. As these are usually associated with a wave vector (π,π,...) or close to it, we call the derived polaritons π-tons. |
Friday, March 6, 2020 10:12AM - 10:24AM |
W62.00012: Quantum nematicity in EuB6 Gabrielle Beaudin, Andrea Bianchi, William Witczak-Krempa Our experimental discovery of a quantum nematic phase in the colossal magnetoresistive material EuB6 constitutes a breakthrough in quantum material research. Unlike the quantum nematics in the cuprate and arsenide superconductors, there is no nearby lattice instability complicating the interpretation, making EuB6 a clean model system for studying electronic nematicity. Interestingly, the nematic appears in the same region of the temperature-magnetic field phase diagram where experiments indicate magnetic polarons. This not only indicates a new understanding of nematicity but also paves the way for obtaining novel magnetoresistive transistor action in spintronic devices. |
Friday, March 6, 2020 10:24AM - 10:36AM |
W62.00013: Strong polaron–spin fluctuations drive the insulator–semimetal transition in low-density electron gases Andrea Bianchi, Gabrielle Beaudin, Alexandre Désilets-Benoit, Stavros Samothrakitis, Kilian Stenning, Michael Nicklas, Simon Gerber, Nikola Egetenmeyer, Jorge Gavilano, Michel Kenzelmann, Robert Cubitt, Charles Dewhurst, Mark Laver One of the outstanding questions in physics is the formation of magnetic moments in metallic systems of low carrier density. For the design of materials for spintronic applications, we need to understand the physics in a regime where mobile charge carriers couple strongly to the magnetic moments. One way to realise such a strong interaction is to work with materials with a low carrier density where two competing electronic ground states are close in energy. Switching between these different electronic ground states then leads to large effects in the electronic transport. What makes the physics in these colossal magnetoresistance (CMR) materials fascinating is that the CMR effects stem from an electronic phase separation at the nanometre scale, despite the fact that these materials are chemically homogeneous. ``Magnetic polarons'' are thought to be responsible for the CMR effects in some materials. Here we present results of a small angle neutron scattering (SANS) study showing the dramatic influence of magnetic polarons on ferromagnetic fluctuations in EuB6, giving clear evidence for their involvement in the insulator-to-metal transition as the result of large scale magnetic fluctuations. |
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