Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V30: Superconductivity: General Theory |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Brian Moritz, SLAC National Accelerator Laboratory Room: LACC 406B |
Thursday, March 8, 2018 2:30PM - 2:42PM |
V30.00001: Symmetry Analysis of Odd- and Even-Frequency Superconducting Gaps for Time-Reversal Symmetric Interactions Richard Geilhufe, Alexander Balatsky We present a general formalism for the classification of superconducting states of matter incorporating time-reversal symmetry. The analysis is based on a group theoretical classification of the Bethe-Salpeter equation. Our approach extends previous ground-laying work [2] by including both even- and odd-frequency superconducting states [1]. |
Thursday, March 8, 2018 2:42PM - 2:54PM |
V30.00002: Does Non-Hermitian superconductivity make sense? Tanmoy Das, Ananya Ghatak, Partha Rana For real world problems, we either consider Hermitian Hamiltonians, or include its Hermitian conjugate. Recently, a new class of non-Hermitian Hamiltonians is studied which are invariant under parity (P), and time-reversal (T) symmetries (or equivalent symmetries), and give real eigenvalues and unitary eigenstates. We study a non-Hermitian, PT-symmetric superconducting Hamiltonian that possesses real quasiparticle spectrum and unitary Bogoluybov states. We find that real quasiparticle energies are possible when the superconducting order parameter itself is either Hermitian or anti-Hermitian. Within the corresponding BCS theory, we find that several properties are characteristically distinct and novel in the non-Hermitian pairing case than its Hermitian counterpart. For example, anti-Hermitian pairing produces a robust first order phase transition. Finally, we discuss how such a PT -symmetric pairing can emerge from an anti-symmetric potential, such as the Dzyloshinskii-Moriya interaction, but with an external bath, or complex potential, among others. [1] A. Ghatak, T. Das, arXiv:1708.09108. |
Thursday, March 8, 2018 2:54PM - 3:06PM |
V30.00003: Broken time-reversal symmetry through non-degenerate superconducting instabilities Sudeep Ghosh, James Annett, Jorge Quintanilla Superconductivity is a manifestation of broken global gauge symmetry. In some materials other symmetries may also be broken at the superconducting transition. In the usual Ginzburg-Landau picture, time-reversal symmetry has a special status in that it can only be broken when the pairing instability has several degenerate channels. Higher-order terms in the free energy can then select a non-trivial phase factor between the different components of the order parameter, leading to a state with broken time-reversal symmetry. This is in contrast to other broken symmetries, such as C4 symmetry in a d-wave superconductor, which are broken intrinsically at the level of the diverging pairing susceptibility. Here we show that time-reversal symmetry can also be broken in this way. We use simple models of a symmorphic and a non-symmorphic crystal, and show that their symmetry allows for non-degenerate instabilities into a superconducting state with intrinsically broken time-reversal symmetry. We consider singlet, on-site pairing involving several bands emerging from orbitals related by internal symmetries of the crystal. We apply these ideas to the recently discovered Re6X (X=Zr, Hf) superconducting material. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V30.00004: Gap and Pseudogap States in YBa2Cu3O7 from First-Principles Guang-Lin Zhao For high Tc cuprate superconductors, there exists an energy gap well above Tc, over a wide region of compositions and temperatures, which is called pseudogap. The origin of this pseudogap and its relation to the superconducting gap are still remained as an outstanding problems in physics. In order to shed some light on the problem, we integrated the first-principles calculation of electronic structure of YBa2Cu3O7 (YBCO) into the theory of many-body physics for superconductivity, and further studied gap and pseudogap states from first-principles. We solved the four-dimensional Eliashberg gap equation in strong coupling theory utilizing first-principles electronic structure of YBCO. The results showed that there is a large anisotropy of superconducting gap on the Fermi surface of YBCO for T< Tc. Above Tc, the real part of gap function is not zero at finite frequency on some parts of the Fermi surface, although for frequency near 0, the real part of gap function is zero. The work was funded in part by NSF (Award # HRD 1736136 ) and ARO (Award # W911NF-15-1-0483). |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V30.00005: Superconducting order parameter in layered systems Balazs Ujfalussy, Gabor Csire In this talk we sketch the solution of the fully relativistic spin-polarized Kohn-Sham Bogoliubov de Gennes equations for multilayers within multiple scattering theory. In this formalism the Green function and consequently various physical quantities can be calculated easily. Particularly interesting is the so called anomalous charge, as it can be related to the order parameter. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V30.00006: Generality of high-Tc superconductivity originating from coexisting wide and incipient narrow bands Kazuhiko Kuroki, Karin Matsumoto, Daisuke Ogura An ideal situation for realizing high Tc superconductivity is to have light electron mass and strong pairing interaction at the same time, but usually the two are not compatible with each other. In ref.[1], one of the present authors proposed a way to overcome this problem ; in a system consisting of wide and narrow bands, light effective mass and strong pairing interaction is simultaneously realized when the Fermi level sits in the vicinity of, but does not intersect, the narrow band. The 2-leg Hubbard ladder with diagonal hoppings was studied as a system in which such a situation is realized, where a possible occurrence of extremely high Tc was suggested. Here, we extend this study, and show that this high Tc mechanism works in a variety of systems that consist of wide and narrow (of flat) bands, such as the diamond lattice [2] and the 3-leg ladder. We apply the FLEX approximation to the Hubbard model on these lattices, and show that superconductivity is strongly enhanced when the Fermi level sits close to the narrow band. The present study shows the generality of the high Tc superconductivity originating from wide and incipient narrow bands. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V30.00007: Temperature-driven BCS–BEC crossover in a coupled boson–fermion system Maciej Maska, Nandini Trivedi Motivated by strongly correlated and frustrated systems, we propose a coupled bose-fermi model |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V30.00008: A Modern Theory for the Orbital Magnetisation in a Superconductor Joshua Robbins, James Annett, Martin Gradhand The chiral p-wave superconducting state is comprised of spin triplet Cooper pairs carrying a finite orbital magnetic moment. Due to its net orbital magnetisation, such a state may sustain a variety of anomalous phenomena, such as the Kerr effect and edge currents. Calculating the orbital magnetisation in a periodic lattice presents a challenge, however, as the circulation operator r x p is not well defined in the Bloch representation. This difficulty has been overcome in the normal state, for which a modern theory is firmly established. Here, we show the extension of this normal state theory to a general superconducting state. We subsequently present model calculations of the magnetisation in the superconducting state of Sr2RuO4, which is considered a strong candidate for chiral p-wave pairing. The results suggest that the magnitude of the elusive edge current in Sr2RuO4 is below experimental resolution. This provides a possible resolution to the long-standing controversy concerning the theoretical predictions versus the experimental observation of the orbital magnetic moment in Sr2RuO4. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V30.00009: Quasiparticle interference and resonant states in normal and superconducting line nodal semimetals Chandan Setty, Philip Phillips, Awadhesh Narayan We study impurity scattering in the normal and d-wave superconducting states of line nodal semimetals and show that, due to additional scattering phase space available for impurities on the surface, the quasiparticle interference pattern acquires an extended character instead of a discrete collection of delta function peaks. Moreover, using the T-matrix formalism, we demonstrate that the conventional behavior of a scalar impurity in a d-wave superconductor breaks down on the surface of a line nodal semimetal in the quasi flat band limit. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V30.00010: Possible Unconventional High-Temperature Iron-Based Superconductors in Hexagonal Structure Jinfeng Zeng, Shengshan Qin, Congcong Le, Jiangping Hu Since the discovery of iron-based superconductors, many rich physics in these materials have been discovered and well understood, however the superconductivity mechanism remains a controversial subject. The essential difficulty lies on how to identify indispensable features that are directly tied to high temperature superconductivity among complex electronic structure and physical phenomena. A successful identification should also lead us to predict new families of high temperature superconductors. In this talk, we propose that hexagonal iron-based transition metal pnictides could be high temperature superconductors. We investigate the electronic and magnetic structures of the layered hexagonal transition metal pnictides with A=(Sr, Ca), M=(Cr, Mn, Fe, Co, Ni) and B=(As, P, Sb). We find that the family of materials shares critical similarities with those of tetragonal structures that include the famous iron-based superconductors. In both families, the next nearest neighbor effective antiferromagnetic exchange couplings reach the maximum value in the iron-based materials. We also find that the superconducting state is a time reversal symmetry broken d+id state upon doping in iron-base hexagonal materials. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V30.00011: Quantum Mechanism of Condensation and High Tc Superconductivity Shouhong Wang, Tian Ma First, we introduce a new interpretation of quantum mechanical wave functions, by postulating that the wave function $\psi=|\psi| e^{i \varphi}$ is the common wave function for all particles in the same class determined by the external potential $V(x)$, $|\psi(x)|^2$ represents the distribution density of the particles, and $\frac{\hbar}{m} \nabla \varphi$ is the velocity field of the particles. Second, we show that for superconductivity, the formation of superconductivity comes down to conditions for the formation of electron-pairs, and for the electron-pairs to share a common wave function. Thanks to the recently developed PID interaction potential of electrons and the average-energy level formula of temperature, these conditions for superconductivity are explicitly derived. Furthermore, we obtain both microscopic and macroscopic formulas for the critical temperature. Third, we derive the field and topological phase transition equations for condensates, and make connections to the quantum phase transition, as a topological phase transition. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V30.00012: Prediction of Universal Peak in Electronic Thermal Conductivity
of High Temperature Superconductors via Dipolon Theory Ram Sharma
|
Thursday, March 8, 2018 4:54PM - 5:06PM |
V30.00013: Dispersion dynamics in the Hall effect and pair bonds in HiTc Antony Bourdillon The key to understanding HiTc is the Chemical hole [1] that replaces the isotope effect and lattice distortions of LoTc. Hall coefficients are positive in the former superconductor and negative in the latter. Since the Lorentz force can influence neither voids nor immobile ions, how do HiTc materials show positive Hall coefficients? Moreover, in the superconducting state, where E=0, how is charge transported without resistance? Clear answers are given in dispersion dynamics [1] based on the stable wave packet. [1] High temperature superconductors – science and processing, Bourdillon A and Tan-Bourdillon NX, 1993, Academic. [2] Dispersion dynamics in the Hall effect and pair bonds in HiTc, Bourdillon AJ, 2017, Nova Science. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V30.00014: Competition between superconductivity and charge density wave order in the Holstein model Benjamin Nosarzewski, Edwin Huang, Ilya Esterlis, Phillip Dee, Brian Moritz, Steven Johnston, Steven Kivelson, Thomas Devereaux A common theme in the study of superconductivity in strongly correlated materials is the competition between superconductivity (SC) and a charge density wave (CDW). While superconductivity in these materials may involve more exotic pairing mechanisms such as spin fluctuations, the basic electron-phonon problem remains of broad interest in the study of superconductivity, exhibits both SC and CDW phases, and has no exact analytic solution. This motivates our study of SC/CDW competition in the 2D Holstein model using numerically exact determinant quantum Monte Carlo. We describe the optimal conditions for superconductivity, the transition from the weak-coupling Migdal-Eliashberg regime to the strong-coupling polaronic regime, and the behavior of the charge density wave ordering wave vector as a function of the dimensionless electron-phonon coupling strength and the adiabatic ratio of the phonon frequency to the Fermi energy. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V30.00015: Interplay between pseudogap, Van Hove singularity and entropy maximum within the CDMFT phase diagram of the 2D Hubbard model Alexis Reymbaut, Simon Bergeron, M. Thénault, Maxime Charlebois, Patrick Sémon, A.-M. Tremblay Recent experiments found that the pseudogap temperature T* of hole-doped cuprates drops suddently at a critical doping where charge-carrier density exhibits a sharp change[1,2,3]. The sudden drop in T* is accompanied by a nearly vertical line in the phase diagram where entropy is maximum [4]. Theoretical work studied the link between the change in electronic properties and the location of the Van Hove singularity[5,6]. We address this multi-faceted problem by exploring the 2D Hubbard model with cellular dynamical mean-field theory (CDMFT) solved by continuous-time quantum Monte Carlo (CTQMC) in the hybridization expansion. We characterize the dependence of the above quantities on interaction U, next nearest-neighbor hopping t’ and Zeeman-coupled magnetic field h. The short-range correlation-driven pseudogap is resilient to typical values of h. We recover the aforementioned experimental observations. |
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