Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y47: Theory of Strongly Correlated Superconductivity |
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Sponsoring Units: DCMP Chair: Brian Moritz, SLAC National Accelerator Laboratory Room: Mile High Ballroom 4F |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y47.00001: d-wave superconducting phase diagram of the two dimensional Hubbard model Andre Marie Tremblay, Giovanni Sordi, Patrick Semon Superconductivity and Mott insulating state intertwine in materials such as cuprates and organic conductors. We study the d-wave superconducting phase at finite temperature in the two-dimensional Hubbard model on the square lattice within cellular dynamical mean-field theory and continuous-time quantum Monte Carlo. The whole phase diagram as a function of temperature, doping and interaction strength shows that a transition directly to the superconducting state from a Mott insulator is possible at the cellular dynamical mean-field level, whether the transition is bandwidth or doping driven. The dynamical mean-field superconducting transition temperature $T_c^d$ does not scale with the superconducting order parameter when there is a normal-state pseudogap. $T_c^d$ corresponds to the local pair formation temperature observed in tunneling experiments and is distinct from the pseudogap temperature, suggesting that pseudogap and superconductivity are distinct phenomena. Refs: G. Sordi et al., PRB 041101 (2013), G. Sordi et al. PRL 108 2164101 (2012) [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y47.00002: Resilience of d-wave superconductivity to nearest-neighbor repulsion A.G.R. Day, D. Senechal, V. Bouliane, A.-M.S. Tremblay Many theoretical approaches find $d$-wave superconductivity in the one-band Hubbard model for high-temperature superconductors. At strong-coupling ($U\geq W$, where $U$ is the on-site repulsion and $W=8t$ the bandwidth) pairing is controlled by the exchange energy $J=4t^2/U$. One may then surmise, ignoring retardation effects, that near-neighbor Coulomb repulsion $V$ will destroy superconductivity when it becomes larger than $J$, a condition that is easily satisfied in cuprates for example. Using Cellular Dynamical Mean-Field theory with an exact diagonalization solver for the extended Hubbard model, we show that pairing {\it at strong coupling} is preserved, even when $V\gg J$, as long as $V \la U/2$. While at weak coupling $V$ always reduces the spin fluctuations and hence $d$-wave pairing, at strong coupling, in the underdoped regime, the increase of $J=4t^2/(U-V)$ caused by $V$ increases binding at low frequency while the pair-breaking effect of $V$ is pushed to high frequency. These two effects compensate in the underdoped regime, in the presence of a pseudogap. While the pseudogap competes with superconductivity, the proximity to the Mott transition that leads to the pseudogap, and retardation effects, protect $d$-wave superconductivity from $V$. PRB 87, 075123 (2013) [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y47.00003: Competition between antiferromagnetism and superconductivity -- a quantum Monte-Carlo study Da Wang, Yi Li, Congjun Wu The competition between antiferromagnetism (AFM) and superconductivity (SC) remains a challenging problem in condensed matter physics because of the lack of non-perturbative method to handle strong correlations. Quantum Monte Carlo (QMC) simulations often suffers from the notorious fermion sign problem. It has been found that in a multi-band Hubbard model, or, equivalently, a large-spin Hubbard model, the sign problem can be removed at arbitrary fillings in a parameter regime in which SC competes with AFM. We have performed QMC simulations to investigate the phase diagram as doping and interaction strength. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y47.00004: A fully gapped superconducting state in small cuprate islands MIkael Fogelstr\"om, David Gustafsson, Dmitri Golubev, Annica Black-Schaffer, Tord Claeson, Sergey Kubatkin, Thilo Bauch, Floriana Lombardi We present a spectroscopic technique, based on an high-Tc superconducting nanoscale device that allows an unprecedented energy resolution thanks to Coulomb blockade effects, a regime practically inaccessible earlier in these materials. We found that the energy required to add an extra electron depends on the parity (odd/even) of excess electrons on the island and increases with magnetic field. This is inconsistent with a pure $d_{x^2-y^2}$ wave symmetry and demonstrates a complex order parameter component that needs to be encompassed in any theoretical model for high-Tc superconductivity. To address this inconsistency, we investigate subdominant order parameters stabilizing at low temperatures in nano-scale high-T$_c$ cuprate islands. Using complementary quasi-classical and tight-binding Bogoliubov-de Gennes methods, we show on distinctly different properties dependent on the symmetry being $d_{x^2-y^2}+i s$ or $d_{x^2-y^2}+i d_{xy}$. We find that a surface-induced $d_{x^2-y^2}+i s$ phase creates a global spectroscopic gap which increases with applied magnetic field, consistent with experimental observation. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y47.00005: Fermi-surface-free Superconductivity near a Topological Transition in Cuprates Peter Mistark, Hasnain Hafiz, Robert Markiewicz, Arun Bansil The phase diagram for cuprates has grown from a simple superconducting dome to one which includes a pseudogap phase and other potential orders which cut the superconducting dome into distinct parts. Investigating the doping dependence of the Fermi surface (FS) for hole doping of an antiferromagnetic (AFM) plus superconducting (SC) system, we find a transition from a FS with nodal hole pockets to one which also includes antinodal electron pockets. The key experimental signature of this transition is that the antinodal spectral weight increases dramatically after the appearance of the electron pocket. Just preceding this transition superconductivity can take advantage of the density of states associated with the antinodal band above the FS, creating a superconducting gap in the absence of a band crossing the FS: fermi-surface-free superconductivity. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y47.00006: The Puzzle of Anomalous Isotope Effect in Zr, Nb$_{3}$Sn, and YBa$_{2}$Cu$_{3}$O$_{7}$ Guang-Lin Zhao Anomalously small isotope effect in some high and low T$_{c}$ superconductors such as Zr, Nb$_{3}$Sn, YBa$_{2}$Cu$_{3}$O$_{7}$ created a great challenge for understanding. To shed light on a clue to solve this puzzle, a new methodology was implemented by integrating first-principles calculations of electronic structures of the materials into the theory of many-body physics for superconductivity. The aim is to seek a unified methodology to calculate the electronic and superconducting properties of these materials. It is shown that the electronic structures of Zr, Nb$_{3}$Sn, YBa$_{2}$Cu$_{3}$O$_{7}$ are very complex. The electron densities of states around the Fermi level in Zr, Nb$_{3}$Sn, YBa$_{2}$Cu$_{3}$O$_{7}$ possess sharp variations that could have a significant contribution to the anomalous isotope effect in these superconductors. However, there still exist some differences between the calculated and experimental results that require further research work. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y47.00007: Dynamics of competing orders in two-dimensional metals with antiferromagnetic exchange interactions Wenbo Fu, Ling-Yan Hung, Subir Sachdev We study the dynamics of bond order parameters after a quantum quench in a two-dimensional square lattice model with nearest-neighbor exchange and repulsion, using an unrestricted time-dependent Hartree-Fock computation. The mean-field model can be constructed by a set of operators, including $d$-wave Cooper pair and particle-hole pair, which form a SU(4) algebra, and thus their equations of motion are closed. After the quench, we find enhanced oscillation amplitude of the $d$-wave charge order below superconducting critical temperature ($T_c$) as observed in recent experiments in $YBa_2Cu_3O_{6+x}$. We also observe a phase shift when crossing $T_c$ and temperature-dependent frequencies. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y47.00008: A DFT study of rocksalt proxy copper monochalcogenide structures -- Implications for possible high-Tc superconductivity P.M. Grant, R.H. Hammond We report findings derived from a series of DFT calculations on the structural stability and paramagnetic ground states of four idealized copper monochalcogenide (CuO, CuS, CuSe, CuTe) rocksalt structures. Note that none of these target compounds occur naturally, but can possibly be fabricated using ``forced epitaxy'' MBE methods, as has been done to grow CuO tetragonal rocksalt films 5-6 monolayers thick.\footnote{W. Siemons, et al., PRB 79, 195122 (2009), DOI: 10.1103/PhysRevB.79.195122.}$^,$\footnote{P. M. Grant, J. of Physics: CS 129, 012042 (2008), DOI: 10.1088/1742-6596/129/1/012042} Therefore, we treat all examples we report herein as proxies intended to explore candidate implications for possible future high-T$_{\mathrm{C}}$ materials. In particular, we find, as might be expected from the long accepted Van Vleck-Anderson-Hubbard formalism describing antiferromagnetic insulators, the Neel temperature scales upward roughly as the width of the spin-carrying bands near or adjacent to the Fermi level or energy gap. We conclude such trend might result in higher superconducting transition temperatures should this be mediated by carrier-spin excitation/fluctuation driven pairing scaled by T$_{\mathrm{N}}$. Finally, we briefly discuss synthetic paths to realizing actual embodiments of our proxy exercises. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y47.00009: The Microscopic Spin fluctuation theory of superconductivity in Spin Density Wave metals Wenya Rowe, Ilya Eremin, Astrid R{\O}mer, Brian Andersen, Peter Hirschfeld We revisit the problem of electron pairing by spin waves in the commensurate spin density wave ordered state, and generalize the existing theory to include situations with electron pockets, hole pockets, or both. We derive simple analytic forms and the leading instabilities for the fluctuation exchange pairing vertex in these cases. In general pairing arises both from transverse spin waves and from gapped longitudinal charge and spin fluctuations, they acts primarily within one type of pocket. Only the d-wave state in the electron doped case is robust in the limit of weak magnetism and doping. By contrast, in the hole-doped case, we find that the spin-singlet odd parity $p$-wave state allowed in the SDW represents the leading instability. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y47.00010: General Rule of Negative Effective Ueff System \& Materials Design of High-Tc Superconductors by ab initio Calculations Hiroshi Katayama-Yoshida, Akitaka Nakanishi, Hiroki Uede, Yuki Takawashi, Tetsuya Fukushima, Kazunori Sato Based upon ab initio electronic structure calculation, I will discuss the general rule of negative effective U system by (1) exchange-correlation-induced negative effective U caused by the stability of the exchange-correlation energy in Hund's rule with high-spin ground states of d5 configuration, and (2) charge-excitation-induced negative effective U caused by the stability of chemical bond in the closed-shell of s2, p6, and d10 configurations. I will show the calculated results of negative effective U systems such as hole-doped CuAlO2 and CuFeS2. Based on the total energy calculations of antiferromagnetic and ferromagnetic states, I will discuss the magnetic phase diagram and superconductivity upon hole doping. I also discuss the computational materials design method of high-Tc superconductors by ab initio calculation to go beyond LDA and multi-scale simulations. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y47.00011: Superconductivity in anisotropic ferromagnets near a transverse saturation field Ilya Vekhter, Kazushi Aoyama, Hiroaki Ikeda In the uranium compounds such as URhGe, UCoGe, and UGe2, superconductivity emerges inside ferromagnetic phases and often exhibits a reentrant behavior in a magnetic field. Motivated by this experimental observation, we consider a model for superconductivity in an anisotropic ferromagnet under transverse field. We derive the spectrum of critical magnetic excitations near the saturation field, derive the pairing interaction due to exchange of these spin fluctuations, and compute the transition temperature into the superconducting state. We compare our results with experiments on U-based ferromagnetic superconductors and with recent theoretical analyses. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y47.00012: Topological Odd-Parity Superconductivity Close to Type-II 2D Van Hove Singularities Hong Yao, Fan Yang We study unconventional superconductivity induced by weak repulsive interactions in 2D electronic systems at Van Hove singularity (VHS) where electronic density of states is logarithmically divergent. We define two types of VH saddle points. For type-I VH systems, weak repulsive interactions generically induce unconventional singlet pairing. However and more interestingly, for type-II VH systems renormalization group treatment shows that weak repulsive interactions favor triplet pairing (e.g. p-wave) when the Fermi surface has no good nesting. When such type-II VH systems respecting tetragonal or hexagonal point group symmetry, topological superconductivity (chiral p$+$ip or time reversal invariant Z2 p$+$ip pairing) will generally occur. We shall also discuss implications of this study to recently discovered BiS2-based superconductors and other superconducting materials that host type-II VH singularities in their Fermi surfaces. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y47.00013: Single-polaron properties for double-well electron-phonon coupling Clemens Adolphs, Mona Berciu We introduce a new model to describe electron-phonon coupling in systems such as one-dimensional intercalated chains or two-dimensional $\mathrm{CuO}_2$ planes, where symmetry dictates that the linear coupling term vanishes. We show that, under certain conditions, an additional charge carrier dynamically changes the local lattice potential from a harmonic well into a double well. We use the Momentum Average approximation to study the properties of this model in the single-polaron limit. A detailed analysis reveals that despite some qualitative similarities to the linear Holstein model, a renormalized Holstein model cannot account for all of the physics of the double-well model. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y47.00014: DMFT analysis of the superconductivity in the Holstein-Hubbard model -- Interplay of strong Coulomb interaction and electron-phonon coupling Yuta Murakami, Philipp Werner, Naoto Tsuji, Hideo Aoki Phonon-mediated superconductivity when, as in the alkali-doped fullerides and aromatic compounds, the Coulomb interaction, electron-phonon coupling and phonon frequencies are all comparable to the electronic band width poses an intriguing question. In order to obtain insights into the superconductivity in this regime, we have analyzed the Holstein-Hubbard model with the dynamical mean-field theory with a continuous-time quantum Monte Carlo impurity solver. We focus on the s-wave superconducting state when the Hubbard repulsion $U$, the phonon mediated attractive interaction $\lambda$ and the phonon energy ($\omega_0$) are comparable to the bandwidth. A particular interest is the effects of the retardation and the strong Coulomb interaction on the behavior of the transition temperature $T_C$, the superconductivity order parameter and gap in spectrum ($\Delta$). We find that the Tc-dome against $U_{\rm eff} = U-\lambda$ significantly deviates from that in the anti-adiabatic limit, and that an effective model in the polaron representation reproduces the effect of the retardation and the Coulomb interaction well even for $\omega_0$ smaller than the bandwidth. We also show an unusual isotope effect for fast phonons and deviation of $2\Delta/k_B T_C$ from BCS value. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y47.00015: Unconventional Superconductivity by Fermi Surface Mismatch: A Diagrammatic Monte Carlo Study Jan Gukelberger, Evgeny Kozik, Lode Pollet, Kris Van Houcke, Nikolay Prokof'ev, Boris Svistunov, Matthias Troyer The conventional BCS pairing mechanism for s-wave superconductivity relies on spin rotation symmetry ensuring coinciding Fermi surfaces for both spin species. We study attractively interacting fermions on a square lattice where this symmetry is broken by imposing either a spin imbalance or a spin-dependent hopping anisotropy. The resulting Fermi surface mismatch disfavours conventional superconductivity making room for new kinds of order such as inhomogeneous or triplet superconductivity. We present unbiased numeric results for the low temperature phase diagrams of these models obtained with Diagrammatic Monte Carlo, a new technique for correlated fermionic systems based on sampling Feynman diagrammatic series directly in the thermodynamic limit. [Preview Abstract] |
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