Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H22: Theory and Simulations of Novel SuperconductorsFocus
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Sponsoring Units: DCOMP Chair: Barry Klein, University of California, Davis Room: 321 |
Tuesday, March 15, 2016 2:30PM - 3:06PM |
H22.00001: Computational search of novel superconductors Invited Speaker: Zhiping Yin The recently discovered 200 K high temperature superconductivity in the hydrogen sulfur material under high pressure was first successfully predicted by first-principles computation in a quantitative fashion, demonstrating the power of computation in the search of new superconductors. With the rapid advancement of theory, algorithm, and computer power, computation will play an increasingly important role. In this talk, I will first summarize the key features of different families of high temperature superconductors, including the iron pnictide and chalcogenide superconductors, the transition metal chloronitrides, and Bi-based superconductors. Then I will show how to use the key features as guidance to design novel candidate materials of high temperature superconductivity by utilizing a combination of different computational methods and tools, including evolutionary structural search method, density functional theory and dynamical mean field theory. A few candidate materials will be given towards the end of the talk for interested experimentalists and theorists to test and explore [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:42PM |
H22.00002: Superconductivity in compressed sulfur hydride: Dependences on pressure, composition, and crystal structure from first principles Invited Speaker: Ryosuke Akashi The recent discovery of high-temperature superconductivity in sulfur hydride under extreme pressure has broken the long-standing record of superconducting transition temperature (Tc) in the Hg-cuprate. According to the isotope effect measurement and theoretical calculations, the superconducting transition is mainly ascribed to the conventional phonon-mediated pairing interaction. It is, however, not enough for understanding the high-Tc superconductivity in the sulfur hydride. To elucidate various possible effects on Tc with accuracy, we have analyzed Tc with first-principles methods without any empirical parameters. First, for various pressures and theoretically proposed crystal structures, we calculated Tc with the density functional theory for superconductors (SCDFT) to examine which structure(s) can explain experimentally measured Tc data [Akashi et al., PRB 91, 224513(2015)]. We next solved the Eliashberg equations without introducing the renormalized Coulomb parameter mu*, which is the Green-function-based counterpart of the SCDFT, and evaluated the effects of rapidly varying electron density of states, atomic zero-point motion, and phonon anharmonic corrections on Tc [Sano et al, in preparation]. In the talk, we review these results and discuss the dominant factors for the Tc and their relation to the experimental results. We also report some crystal structures that we recently found with first-principles calculations, which could have a key role for the pressure-induced transformation to the high-Tc phase. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H22.00003: van Hove Singularities and Spectral Smearing in High Temperature Superconducting H$_3$S Yundi Quan, Warren E. Pickett The superconducting phase of hydrogen sulfide at T$_c$=200 K observed by Drozdov and collaborators at pressures around 200 GPa is simple bcc $Im{\bar 3}m$ H$_3$S reopens questions about what is achievable in high T$_c$. The various "extremes" that are involved -- pressure, implying extreme reduction of volume, extremely high H phonon energy scale around 1400K, extremely high temperature for a superconductor -- necessitate a close look at new issues raised by these characteristics in relation to high T$_c$. We have applied first principles methods to analyze the H$_3$S electronic structure, particularly the van Hove singularities (vHs) and the effect of sulfur. Focusing on the two closely spaced vHs near the Fermi level that give rise to the impressively sharp peak in the density of states, the implications of strong coupling Migdal-Eliashberg theory are assessed. The electron spectral density smearing due to virtual phonon emission and absorption, as done in earlier days for A15 superconductors, must be included explicitly to obtain accurate theoretical predictions and a correct understanding. Means for increasing T$_c$ in H$_3$S-like materials will be mentioned. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H22.00004: Structure, lattice dynamics, and high-Tc superconductivity in hydrogen sulfide under high pressure Linda Hung, Taner Yildirim The recent discovery of superconductivity near 200 K in hydrogen sulfide under pressures ~200 GPa has sparked interest in the search for hydrogen-rich superconducting materials. The observed large isotope effect and earlier first-principles calculations convincingly suggest that the high Tc is due to strong electron-phonon coupling, i.e., that hydrogen sulfide is a conventional superconductor. Hence, first-principles predictions of phonon properties can be used in the search for new phases that can superconduct at even higher temperatures and lower pressures. In this talk, we present structural and lattice dynamics calculations of various phases of H$_2$S/H$_3$S, examining the electron-phonon coupling and superconductivity in each phase using the finite-displacement and frozen-phonon approaches. The effect of anharmonicity on electron-phonon coupling, isotope effect, and superconducting temperature is discussed. Finally, we explore the properties of potential new hydrogen-sulfide-based materials. [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H22.00005: Hydrogen bond symmetrization by proton quantum motion and high Tc superconductivity in Sulfur hydrides at high pressure. Ion Errea, Matteo Calandra, Chris Pickard, Joseph Nelson, Richard Needs, Y Li, Hanyu Liu, Yunwey Zhang, Yamming Ma, Francesco Mauri Atoms in a crystal are quantum particles differing substantial from classical particles. The vibrational energy associated to the quantum oscillations can strongly modify the static energy landscape, even changing the ground state derived from the Born-Oppenheimer energy surface (BOES) minimum. Here, making use of density-functional theory and of the Stochastic Self-consistent Harmonic Approximation, we show that the ground state of the high Tc superconductor hydrogen sulfide at 155 GPa is completely determined by quantum fluctuations. Indeed, despite the minimum of the BOES is obtained for a rhombohedral structure with covalently bonded H3S units and hydrogen bonds between them, quantum fluctuations favor a fully symmetric cubic structure in which the covalent and hydrogen bonds equalize. The quantum hydrogen-bond symmetrization and the large anharmonic effects are crucial to understand the pressure dependence of the observed extraordinary T$_c$=205 K at 155 GPa. We finally show how, the dependence of Tc as a function of pressure can be completely explained in the framework of a phonon mediated pairing mechanism in the presence of large anharmonic effects. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H22.00006: Novel properties of Tungsten ditelluride Huimei Liu Tungsten ditelluride has attracted intense research interest due to the recent discovery of its large unsaturated magnetoresistance up to 60 Tesla. By using density functional theory calculations, we qualitatively reproduced the observed spin texture. Since the spin texture would forbid back scatterings that are directly involved in the resistivity, we suggest that the SOC and the related spin and orbital angular momentum textures may play an important role in the anomalously large magnetoresistance of WTe2. Motivated by the presence of a small, sensitive Fermi surface of 5d electronic orbitals, we also boost the electronic properties by applying a high pressure, and introduce superconductivity successfully. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H22.00007: Superconductivity in organic conductors: ab-initio results from parallel DMRG Adrian Kantian, Michele Dolfi, Matthias Troyer, Thierry Giamarchi The U-V model at quarter filling is considered the canonical minimal model to explain unconventional superconductivity in the organic Bechgaard and Fabre salts [1]. Yet it has so far resisted solution to show that it actually can support superconducting order. Here, we use a heavily parallelized version of the density-matrix renormalization group (DMRG) to provide the first ab-initio solutions for the U-V model in the regime suspected to support superconducting order. Our results support the existence of a phase marked by spin-singlet pairing, thus ruling out antiferromagnetic order (the usual close competitor to unconventional superconductivity). In this phase we will extend our analysis by studying the response to explicit bias fields. Our work is complemented by analysis of the two-leg U-V ladder through a combination of DMRG and analytical RG of a bosonized low-energy theory. [1] C. Bourbonnais, D. Jerome, “The Physics of Organic Superconductors and Conductors” (ed. Lebed, A.) pp. 358 (Springer, 2007). [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H22.00008: Exchange and correlation effects on the superconducting transition of two-dimensional multivalley semiconductors Betul Pamuk, Jacopo Baima, Roberto Dovesi, Matteo Calandra, Francesco Mauri It has recently been shown that the enhancement in the superconducting temperature of two-dimensional semiconductors at low doping is linked to the electron-electron interaction enhancing the response to the valley polarization that is due to the electron-phonon coupling [1]. In this work, we extend this study to analyze the exchange and correlation effects on the electronic, magnetic, and vibrational properties of Li-doped ZrNCl and HfNCl - typical examples of two-dimensional two-valley semiconductors. We show that these properties can be calculated by {\it ab initio} density functional theory only by using approximations beyond the generalized gradient approximation. Finally, we present the link between the enhancement of the spin susceptibility and superconducting temperature by demonstrating that the electron-phonon coupling is acting as a pseudo-magnetic field causing the valley polarization. [2] [1] M. Calandra, P. Zoccante and F. Mauri, Phys. Rev. Lett. {\bf 114}, 077001 (2015) [2] B. Pamuk {\it et al.}, in preparation [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H22.00009: Correlated properties of the doped Hubbard model on a honeycomb lattice Tianxing Ma, Lufeng Zhang, Hai-Qing Lin Low doped graphene has a finite density of state, while heavily doped graphene have a Van Hove sigularity in the density of states, in combination with pronounced antiferromagnetic spin fluctuations close to half filling, and strong ferromagnetic correlation as doping is bellow the location of Van Hove singularity, which may lead to different unconventional superconductivity. We performed a systematic quantum Monte Carlo study of the pairing correlation in the Hubbard model on a honeycomb lattice. Close to half filling, we find that pairing with d$+$id symmetry dominates over pairing with extended-s symmetry. When the next-nearest-neighbor t' is larger than t/6, the single-particle spectrum is featured by the continuously distributed Van Hove saddle points at the band bottom, where the density of states diverges in a power law. We investigate possible unconventional superconductivity in such systems with the Fermi level close to the band bottom by employing both random-phase-approximation and determinant quantum Monte Carlo approaches. Our study reveals a possible triplet p$+$ip superconductivity with appropriate interactions in low-filled graphene. We also explore the effect of the disorder and spin-orbit coupling on the magnetic correlation in doped graphene. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H22.00010: Helical Majorana fermions in d+id'-wave topological superconductivity of doped correlated quantum spin Hall insulators Chung-Hou Chung, Shih-Jye Sun, Yung-Yeh Chang, Wei-Feng Tsai, Fuchun Zhang Large Hubbard $U$ limit of the Kane-Mele model on a zigzag ribbon of honeycomb lattice near half-filling is studied via a renormalized mean-field theory. The ground state exhibits time-reversal symmetry (TRS) breaking $d_{x^2-y^2} + {\it i}d_{xy}$-wave superconductivity. At large spin-orbit coupling, the $Z_2$ topological phase with non-trivial spin Chern number in the pure Kane-Mele model is persistent into the TRS broken state (called ``spin-Chern phase''), and has two pairs of counter-propagating helical Majorana modes at the edges. As the spin-orbit coupling is reduced, the system undergoes a topological quantum phase transition from the spin-Chern to chiral superconducting states. Possible relevance of our results to adatom-doped graphene and irridate compounds is discussed.Ref.:Shih-Jye Sun, Chung-Hou Chung, Yung-Yeh Chang, Wei-Feng Tsai, and Fu-Chun Zhang, arXiv:1506.02584. [Preview Abstract] |
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