Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R48: Superconductivity: Theories and Models II |
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Sponsoring Units: DCMP Chair: Herbert Fotso, University at Albany Room: Mile High Ballroom 1A |
Thursday, March 5, 2020 8:00AM - 8:12AM |
R48.00001: Chirality polarization and spectral bulk-boundary correspondence Akito Daido, Youichi Yanase Surface physics dominated by bulk properties is attracting much attention. For example, bulk-boundary correspondence predicts topological boundary states. Another important context is the electric polarization: Surface charge is the bulk property modulo a quantum [1]. This relation is analogous to BBC, in the sense that physics apparently sensitive to the surface details is the bulk property. It is an interesing question whether this "BBC" can be generalized to the other physical quantities. |
Thursday, March 5, 2020 8:12AM - 8:24AM |
R48.00002: Exciton-induced Charge Fluctuations in One-dimensional Quasi-periodic “Metallic” Chains: A Possible Embodiment of Room Temperature Superconductivity Paul Grant It is well known that a purely periodic chain of odd-electron atoms, nominally expected to exhibit metallic behavior, is unstable to charge/spin spatial displacement which lowers its ground state energy by gapping its multi-degenerate Fermi surface, in this case consisting of nesting parallel sheets. It is largely for these reasons that superconductivity is not observed in highly one-dimensional metals -- it is simply energetically more favorable for CDW/SDW gaps to form, rather than a BCS state, at least one mediated by electron-phonon coupling. In this talk, we explore the hypothetical electronic properties of a nominally ``metallic'' quasi-periodic chain using both an analytical approach and computationally with density functional theory, searching for configurations which yield ``gap-lets'' sufficiently small to permit the formation of BCS pairs as the new energetically favored ground state. The particular embodiment we examine is a string of aluminum atoms with interatomic spacing determined by a Fibonacci sequence. We propose a path to attempt synthesis of such a structure for experimental examination, and perhaps leading to an entirely new class of higher temperature superconductors1. |
Thursday, March 5, 2020 8:24AM - 8:36AM |
R48.00003: A New Superfast Method for Combinatorial Search of High-Temperature Hydride Superconductors under Extreme Pressures Tianran Chen, Taner Yildirim Due to the low atomic mass and high electron-phonon coupling strength of hydrogen, hydride compounds under extreme pressures are most promising in the search for high-Tc superconductors. First-principles based computational search has become extremely important not only predicting new materials but also guiding high-pressure experimental |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R48.00004: Emergent unconventional superconductivity in doped valence bond solid insulators Zixiang Li, Dunghai Lee In this talk, I will discuss the unconventional superconductivity and non-Fermi liquid behavior in the doped valence bond solid insulators. Through sign-problem-free Quantum Monte Carlo simulation, we investigate the superconductor emergent from the doped valence bond solid insulators, which shares many properties with the superconductivity in doped resonance valence bond phases. Close to the valence bond solid quantum critical point, we observe the Non-Fermi liquid properties from the transport behavior. Our study sheds new light on Cooper pairing in the presence of strong Coulomb interaction. Moreover, this result might point to a new direction for searching high-Tc superconducting materials. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R48.00005: Possible counterintuitive enhancement of superconductivity in ladder-type cuprates by longitudinal compression Kazuhiko Kuroki, Hikaru Sakamoto We explore how the electron hoppings of the ladder type cuprates are affected when uniaxial compression or tension is applied to the lattice in the leg or rung directions, and investigate its consequences to superconductivity. For simplicity, we consider the two-leg ladder cuprate without the chains, that is, SrCu2O3, although this material is known to be difficult to dope carriers. By constructing a model based on first principles calculation, we surprisingly find that the ratio tr/tl, where tr(tl) is the nearest neighbor hopping in the rung (leg) direction, is enhanced when the lattice is compressed in the leg direction or stretched in the rung direction. This counterintuitive manner of the hopping variation can be attributed to the on-site hybridization between Cu dx2−y2 and Cu 4s orbitals, which arises due to the low symmetry of the lattice. We apply the fluctuation exchange approximation to the model and study how superconductivity is affected under uniaxial deformation. Due to the above mentioned variation of the hoppings, we find that the superconducting transition temperature is enhanced when the lattice is compressed in the leg direction, opposed to an intuitive expectation. The effect is expected to be strong especially in the electron-doped regime. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R48.00006: Ground state phase diagram of the doped Hubbard model on a two-leg ladder Cheng Peng, Yi-Fan Jiang, Yuval Gannot, Chao-Ming Jian, Steven Kivelson, Hong-Chen Jiang We have revisited the problem of the two-leg Hubbard ladder using DMRG methods but keeping larger number of states than in previous studies so as to obtain definitive results on the long-distance behavior of relevant ground-state correlation functions. In light of recent evidence that relatively small changes in the ratio of next nearest height for nearest neighbor hopping matrix, t'/t, can cause transitions between distinct ground-state phases, we have studied the model for a range of t'/t, electron density per site, n, and for several "intermediate" values of U/t. We find for a broad range of t'/t the ground-state exhibits quasi-long-range singlet superconducting order. For t'/t negative and large enough, two additional phases are found: One is characterized by commensurate charge-density-wave long-range order and a single gapless spin mode. Unexpectedly, for a relatively narrow range of negative t'/t, there exists an exotic intermediate phase with gapless charge and spin excitations, but one less gapless mode than for the non-interacting system. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R48.00007: Finding the breakdown of the linear Holstein model using determinant quantum Monte Carlo Steven Johnston, Philip Dee, Jennifer Coulter A large-scale effort is underway to determine when vertex corrections are required to the Migdal approximation for electron-phonon (e-ph) coupled systems. Most often, the underlying model for these studies is the simple Holstein Hamiltonian, which captures e-ph interactions as an on-site coupling of electrons to the linear displacement of the phonon field operators. When these displacements are large, however, introducing nonlinear corrections to the e-ph interaction can significantly alter measured correlations. Using determinant quantum Monte Carlo, we compared pairing and charge density wave correlations obtained with and without second-order nonlinear corrections. We find signs of disagreement between the linear and nonlinear models for relatively small values of the dimensionless e-ph coupling λ, and importantly, these discrepancies often occur in the same parameter regions where Migdal's theorem breaks down. Our results suggest that questions of the validity of Migdal's theorem go hand in hand with questions of the validity of the linear Holstein model. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R48.00008: The superconducting Tc and the metal-insulator transition in the Hubbard-Holstein model Tae-Ho Park, Han-Yong Choi We present that the maximum superconducting Tc is on the phase boundary between metallic and insulating states of the normal state in the Hubbard-Holstein model. It is a prototype model including the local Coulomb interaction U and the electron-phonon coupling g. We performed the dynamical mean field theory (DMFT) calculations employing the numerical renormalization group (NRG) technique and identified the normal state phase diagram for the metal-insulator transition. Interestingly, local maximum Tc occurs along the lower critical phase boundary gc1 of the 1st order metal-insulator transition in the normal state where the quantum fluctuation becomes maximum. Implications of this finding will be discussed in comparison with the quantum critical superconductivity. In addition, we obtained the electron-phonon coupling spectra α2F(ω) and examined whether the maximum Tc calculated from the Eliashberg theory lies on the gc1. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R48.00009: Pairing correlations in the cuprates: a numerical study of the three-band Hubbard model Peizhi Mai, Steven Johnston, Thomas Maier We study the three-band Hubbard model for the copper oxide plane of the high-temperature superconducting cuprates using exact diagonalization, determinant quantum Monte Carlo, and dynamical cluster approximation (DCA) with an emphasis on pairing correlations. Using these methods, we provide a comprehensive view of pairing in the model. Specifically, we compute the pair-field susceptibility for these methods and study its dependence on temperature, doping, interaction strength, and charge-transfer gap. Using DCA, we also solve the Bethe-Salpeter equation for two-particle Green’s function to identify the dominant pairing correlations and its orbital composition, and to determine the transition temperature to the superconducting state. |
Thursday, March 5, 2020 9:48AM - 10:00AM |
R48.00010: Explaining the Anomalous Isotope Shift in Cuprate High Temperature Superconductivity Dennis Newns, Glenn Martyna, Soumya Kanti Ganguly The pairing mechanism of cuprate High Temperature Superconductors is still controversial. A vibrational component to the pairing mechanism is suggested by the anomalous Isotope Shift (IS), which is of order the BCS value at dopings well below the maximum Tc, but is very low at dopings close to the maximum Tc. Here, based on the Fluctuating Bond Model [1] (FBM), we re-examine the IS issue focusing on the oxygen soft mode associated in the FBM with the pseudogap - knowing that a soft mode plus a van Hove singularity in the electronic density of states is one mechanism capable of leading to the cuprate-type anomalous IS [2]. A quantum mechanical treatment of the FBM model in Born-Oppenheimer approximation shows soft excitations of a few meV. Solving the Eliashberg equation we evaluate the IS, and will present the results. |
Thursday, March 5, 2020 10:00AM - 10:12AM |
R48.00011: Tendency of nematicity in the extremely underdoped t-J model YiHsuan Liu, Wei-Lin Tu, Ting-Kuo Lee Underdoped cuprates exhibiting electronic nematicity have been reported by various measurements such as scanning tunneling microscopy (STM) [1, 2] and resistivity [3]. These results indicate that nematicity already exists in very lightly doped thin film, which motivates us to study the extremely underdoped t-J model and Heisenberg model via variational Monte Carlo (VMC) method and renormalization mean-field theory(RMFT). In the previous VMC study of the Heisenberg model, various groups have confirmed that the uniform RVB+AFM trial wave function has the lowest energy[4], which is very close to the best numerical estimation of the exact ground state energy. However, we found that various C4 broken states are almost degenerate with the uniform solution. Also, we will study the lightly doped cases with one hole only. |
Thursday, March 5, 2020 10:12AM - 10:24AM |
R48.00012: Nematic superconductivity in twisted bilayer graphene Dmitry Chichinadze, Laura Classen, Andrey Chubukov Twisted bilayer graphene (TBG) shows insulating and superconducting phases in connection with an exceptional |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R48.00013: Magnetic and superconducting correlation in monolayer and twisted bilayer graphene Tianxing Ma Using exact quantum Monte Carlo method, we identify the phase diagram of the half filled, the lightly doped and heavily doped graphene. At half filling, the system is driven to a Mott insulator with antiferromagnetic long range order by increasing interaction, and a transition from a d+id pairing to a p+ip pairing is revealed, depends on the next-nearest hoping and the electronic fillings. We also examine the recent novel electronic states seen in magic-angle graphene superlattices. We reveal that an antiferromagnetically ordered Mott insulator emerges beyond a critical Uc at half filling, and with a small doping, the pairing with d+id symmetry dominates over other pairings at low temperature. The effective d+id pairing interaction strongly increase as the on-site Coulomb interaction increases, indicating that the superconductivity is driven by electron-electron correlation. Our non-biased numerical results demonstrate that the twisted bilayer graphene share the similar superconducting mechanism of high temperature superconductors, which is a new and idea platform for further investigating the strongly correlated phenomena. |
Thursday, March 5, 2020 10:36AM - 10:48AM |
R48.00014: Superconductivity in the doped quantum spin liquid on the triangular lattice Hong-Chen Jiang Broad interest in quantum spin liquid (QSL) phases was triggered by the notion that they can be viewed as insulating phases with preexisting electron-pairs, such that upon light doping they might automatically yield high temperature superconductivity. Yet despite intense efforts, definitive evidence showing that doping QSLs leads to superconductivity has been lacking. We address the problem of a lightly doped QSL through a large-scale density-matrix renormalization group study of the t-J model on the triangular lattice with a small but non-zero concentration of doped holes. We provide direct evidences that doping QSL can naturally give rise to superconductivity. The ground state is consistent with a Luther-Emery liquid with power-law superconducting and charge-density-wave correlations but short-range spin-spin correlations. Specifically, the superconducting correlations are the dominant correlations on both four-leg and six-leg cylinders, indicating that long-range superconductivity would emerge in doping QSL in two dimensions. |
Thursday, March 5, 2020 10:48AM - 11:00AM |
R48.00015: Supoerconductiviy in the doped chiral spin liquid on triangular lattice Yi-Fan Jiang, Hong-Chen Jiang Theory proposes that doping chiral spin liquid (CSL) can give rise to topological superconductivity. Yet it has been extensively studied in the past, definitive evidences showing this has been lacking. We address this problem by studying the t-J model supplemented by spin chiral interaction (with coupling Jχ) on the triangular lattice using density-matrix renormalization group. It has been established that the ground state of the system at half-filling is a CSL when 0.32≤Jχ/J≤0.56, or equivalently ν=1/2 bosonic fractional quantum Hall state. Interestingly, we find that the ground state of the system upon light doping the CSL is consistent with a Luther-Emery liquid, which is characterized by quasi-long-range superconducting and charge-density-wave correlations but with a finite spin gap and one gapless charge mode. Furthermore, we show that the superconducting correlations are the dominant correlations and its pair symmetry is consisent with d+id-wave. |
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