Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M25: Theory of Superconductivity I |
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Sponsoring Units: DCMP Chair: Harold Jeschke, ITP, Frankfurt Room: 203B |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M25.00001: Ab initio molecular dynamics simulations of electron doped ZrNCl Antia S. Botana, Francois Gygi, Warren E. Pickett When electron doped, the layered transition metal nitride ZrNCl becomes superconducting with an impressive critical temperature T$_c$= 15K. Its isovalent sisters become superconducting at 17K (TiNCl) and 26K (HfNCl). This class has very strong 2D character, as cuprates, iron pnictides, and MgB$_2$, but the pairing interaction is not magnetic nor conventional electron-phonon in origin. To explore the dynamical behavior of electrons doped into a highly ionic insulator, ab initio molecular dynamics calculations have been carried out on doped ZrNCl. The simulations allow us to analyze the electronic response in real space, and to study the differences in behavior of the two experimental mechanisms of doping electronic carriers: alkali metal intercalation and Cl vacancies, both of which give nearly the same high critical temperature. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M25.00002: Density Functional Theory for Superconductors: new functionals and applications Antonio Sanna, E.K.U. Gross Density functional theory for superconductors (SCDFT) is a fully parameter-free approach that allows for accurate predictions of the critical temperature and other properties of superconductors. We report on the most recent extensions of this theoretical framework, in particular the development of new functionals to: \begin{itemize} \item incorporate in a correct fashion Migdal's theorem; \item compute the excitation spectrum; \item include spin-fluctuation mediated pairing \end{itemize} Applications and predictions are shown for a set of materials including conventional and unconventional superconductors. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M25.00003: Unconventional superconductivity and charge fluctuations in Li$_{0.9}$Mo$_6$O$_{17}$ Natalia Lera, Jose Alvarez We study superconductivity in Li$_{0.9}$Mo$_6$O$_{17}$ considering charge and spin fluctuations as the pairing mechanism. We model the low-energy electronic properties of the material with a multiorbital extended Hubbard model and compute the superconducting vertex within the RPA. We found that charge collective modes induced by Coulomb repulsion favor the superconductivity both the singlet and triplet channels. Superconductivity is favored as the charge ordering transition of the model is approached by tuning the interaction. We compare with another quasi-one-dimensional approach where the superconductivity is an instability of a generalized Luttinger Liquid (LL) to higher dimensions, known as Quantum Smectic Metal or Sliding LL. We also discuss these results on the context of the dimensional crossover issue in the unconventional metallic phase of the material. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M25.00004: Superconducting phase diagram of itinerant antiferromagnets Astrid Tranum Roemer, Ilya Eremin, Peter J. Hirschfeld, Brian M. Andersen We investigate the formation of Cooper pairs in systems with itinerant antiferromagnetic (AF) order. Our theory is a generalization of earlier studies of spin fluctuation mediated pairing on top of AF [1,2]. The AF order is manifested by gapless transverse spin waves as well as gapped longitudinal spin fluctuations. Both branches contribute to the Cooper pairing. We map out the superconducting gap as a function of electron doping and find a robust d-wave gap on the electron pockets in the anti-nodal regions with no nodes at the Fermi surface. Close to the critical doping of onset of AF order we observe a highly non-monotonic form of the superconducting gap due to enhancements at the ``hot spots.'' In this doping regime we explore the presence of p- and f-wave triplet gaps competing with the d-wave solution [3].\\[4pt] [1] J. R. Schrieffer, X. G. Wen, and S. C. Zhang,~ Phys. Rev. B 39, 11663 (1989). \newline [2] D. M. Frenkel and W. Hanke, Phys. Rev. B 42, 6711 (1990). \newline [3] A. T. Roemer et al, preprint. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M25.00005: Dynamical Jahn-Teller instability in metallic fullerides Naoya Iwahara, Liviu Chibotaru Dynamical Jahn-Teller effect has escaped so far direct observation in metallic systems. It is particularly believed to be quenched also in correlated conductors with orbitally degenerate sites such as cubic fullerides. Here the Gutzwiller approach is extended to treat electron correlation over metals with Jahn-Teller active sites and applied to the investigation of the ground state of K$_3$C$_{60}$ [1]. It is shown that dynamical Jahn-Teller instability fully develops in this material when the interelectron repulsion $U$ on C$_{60}$ sites exceeds some critical value. The latter is found to be lower than the current estimates of $U$, meaning that dynamical Jahn-Teller effect takes place in all cubic fullerides. This leads to strong splitting of LUMO orbitals on C$_{60}$ sites and calls for reconsideration of the role of orbital degeneracy in the Mott-Hubbard transition in fullerides. \\[4pt] [1] N. Iwahara and L. F. Chibotaru, submitted to Phys. Rev. B. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M25.00006: Microscopic Theory of the Knight Shift Bianca Hall The Knight shift has been used for detecting possible parallel spin states, such as Sr$_{2}$RuO$_{4}$. However an improved theory of the Knight shift is needed to explain anomalies that have been found in a variety of superconductors below the superconducting transition temperature. The standard model includes the Zeeman and hyperfine interactions of the magnetic field and the electron-electron pairing interaction necessary for superconductivity. The new model now includes a term based on the Anderson model of local moments in metals. The local electron states in the original Anderson model now correspond to local atomic electron orbitals. When these orbital energies are singly occupied and close to the Fermi energy, the spins of the electrons can interact with the nuclear spins and the spins of the electrons in the conducting bands. Additionally, the paths of the electrons in the conduction bands depend on the direction and magnitude of the magnetic induction. With these new terms included, this model has the potential to describe the anomalous behavior seen in Knight shift measurements on unconventional superconductors. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M25.00007: On the existence of superfluidity between two critical temperatures Ilya Grigorenko, Roman Kezerashvili It is predicted the existence of the superfluid state between two critical temperatures. Superfluidity on a finite temperature interval can be observed in a system when the pairing takes place between two types of carriers, with the asymmetry caused by the difference in the carriers masses and their chemical potentials. It is found that the superfluid state is possible in a wide range of the asymmetry parameters, when they satisfy a simple linear relation. We also predict that at zero temperature with the change of the asymmetry parameters the system can undergo a quantum phase transition of the first order. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M25.00008: Amplitude mode in diluted O(N) models Yury Kiselev, Daniel Arovas The relativistic O(N) model has recently been shown to exhibit a robust amplitude mode peak in its scalar susceptibility which exhibits scaling in the vicinity of the quantum critical point [PRL 110, 140401 (2013)]. We report on the O(N) model with random site dilution, which we study using the worm algorithm [PRL 87, 160601 (2001)]. We present results for the scalar susceptibility and O(N) conductivity in the critical region and follow the system's behavior as the dilution tends to the percolation threshold. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M25.00009: Phonon Fano effect in RIXS across a CDW instability Thomas Devereaux I this talk I will present a theory for resonant photoexcitation of phonons using x-ray inelastic spectroscopy. I will show how the strength of electron-phonon can be characterized in a momentum dependent way, and demonstrate how the phonon-continuum lineshape displays a characteristic Fano effect as an incipient charge density wave state forms. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M25.00010: Thermodynamics of superconducting quantum metamaterials Pierre-Luc Dallaire-Demers, Frank Wilhelm-Mauch Left-handed matematerials are capacitively coupled layers of inductive pieces of conductors. These systems are well studied in the context of microwave metamaterials but their full quantum description or their embedding in highly correlated materials like superconductors are still an open problem. Notably, they are known to have a Van Hove singularity in the density of states at low energy and high pseudo-momentum that could effectively couple and condense Cooper pairs. The goal of this research is to analyze the thermodynamical properties of the order parameter of stacked layers of superconductors with a small repulsive Coulomb interaction. A 3D toy model of such a material is mapped to a Fermi-Hubbard lattice. The temperature dependent anomalous correlation functions are computed variationally from a self-energy functional of a small cluster where inter-cluster tunneling is treated perturbatively. The effect of the repulsive interaction on the Cooper pairs binding can then be seen from the momentum distribution of the condensation amplitude. Such a material could potentially be realized with optical lattices or nanoscaled superconductors. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M25.00011: Transport in thin insulating films close to the Boson-Fermion Crossover J.C. Joy, X. Zhang, S.M. Hollen, C. Zhao, G. Fernandes, J.M. Xu, J.M. Valles, Jr. In two-dimensional systems, sufficient levels of disorder are known to localize Cooper Pairs into a phase incoherent insulating state. While many theoretical and experimental works have shown this state's existence, its ubiquity close to the disorder tuned Superconductor to Insulator transition is still an open problem. Recent experiments on nanopatterned Pb$_{0.9}$Bi$_{0.1}$ films have suggested a crossover from Bosonic to Fermionic transport deep in the insulating phase, indicating that the Cooper Pair Insulator (CPI) only persists to a finite level of microscopic disorder. The normal state resistance at which this crossover occurs is governed by the extent coupling constant inhomogeneities on the scale of the coherence length, which allow the formation of locally phase coherent superconducting islands in the insulating state. By tuning the scale of these inhomogeneities and examining the extent of the CPI state, we argue that the disorder tuned Superconductor to Insulator transition proceeds via pair breaking and Anderson localization of fermions when the level spacing in the islands approaches the size of the mean field gap. This work was supported by the NSF through grants No. DMR-1307290 and DMR-0907357 and by the AFRL, the ONR, and the AFOSR. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M25.00012: Tuning non-equilibrium superconductors with lasers Michael Sentef, Alexander F. Kemper, Antoine Georges, Corinna Kollath The study of the real-time dynamics dynamics of solids perturbed by short laser pulses is an intriguing opportunity of ultrafast materials science. Previous theoretical work on pump-probe photoemission spectroscopy revealed spectroscopic signatures of electron-boson coupling [1, 2], which are reminiscent of features observed in recent pump-probe photoemission experiments on cuprate superconductors [3, 4]. Here we investigate the ordered state of electron-boson mediated superconductors subject to laser driving using Migdal-Eliashberg theory on the Kadanoff-Baym-Keldysh contour. We extract the characteristic time scales on which the non-equilibrium superconductor reacts to the perturbation, and their relation to the coupling boson and the underlying order. [1] M. Sentef et al., Phys. Rev. X 3, 041033 (2013). [2] A. F. Kemper et al., Phys. Rev. B 90, 075126 (2014). [3] J. Graf et al., Nat. Phys. 7, 805 (2011); W. Zhang et al., Nat. Comm. 5, 4959 (2014). [4] J. D. Rameau et al., Phys. Rev. B 89, 115115 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M25.00013: Probing the charge-vortex duality near the superfluid-to-insulator transition Snir Gazit, Daniel Podolsky, Assa Auerbach We study the charge vortex duality near the superfluid-to-insulator quantum phase transition in d=2+1 dimensions. We use a generalized reciprocity relation between charge and vortex conductivities at complex frequencies to identify the capacitance in the insulating phase as a measure of vortex condensate stiffness. We then compute the ratio of boson superfluid stiffness to vortex condensate stiffness at mirror points to be 0.21(1). This corroborates and provides a quantitative measure to the non self-dual nature of the charge-vortex duality. We further study deviations from self-duality at finite frequency by computing the product of Matsubara frequency conductivities at mirror points across the phase transition. Finally, we propose experimental realizations that test our predictions in THz spectroscopy of disordered superconductors and cold atomic systems trapped in an optical lattice. [1] S. Gazit, D. Podolsky, A. Auerbach, arXiv:1407.1055 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M25.00014: Theory of phase-slip-center effects on superconducting kinetic inductance Masahiko Machida, Yukihiro Ota Fluctuations significantly alter the conventional picture on transport phenomena in 1D superconductors. We study the current-dependence of the kinetic inductance in superconducting wires using the Ginzuburg-Landau approach with fluctuations. We obtain a non-monotonic current-dependence of the kinetic inductance when the thermal fluctuations predominate the behaviors of the superconducting phase. This result is ascribed to the occurrence of phase-slip centers from thermally-activated processes. We also find that our approach qualitatively reproduces the experiment by Annunziata et al. [Nanotechnology 21, 445202 (2010)]. Moreover, we discuss the effects of thermal fluctuations on the characteristics of superconducting detectors. We will also discuss the effects of quantum phase slips on superconducting detectors. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M25.00015: Pairing in doped Hubbard model on a honeycomb lattice: A quantum Monte Carlo study Tianxing Ma Inspired by the recent discovered graphene, we performed a systematic QMC study of the magnetic and pairing correlation in the t-U-V 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. As the doping increases, the next-nearest-neighbor t' tends to be important and when {\$}t'\textless -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 power-law. We investigate possible unconventional superconductivity in such system with Fermi level close to the band bottom, and our studies reveal a possible triplet p $+$ ip superconductivity with appropriate interactions. By including the spin-orbit coupling, it is shown that the d$+$id pairing is enhanced while the p$+$ip pairing is decreased by increasing spin-obit coupling. Our results might provide a possible route to look for triplet superconductivity with relatively-high transition temperature in a doped graphene and other similar systems. [Preview Abstract] |
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