Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W35: Superconducting Theory II |
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Sponsoring Units: DCMP Chair: Michael Norman, Argonne National Laboratory Room: 405 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W35.00001: Gap anisotropy and universal pairing scale in a spin fluctuation model for cuprates Artem Abanov, Andrey Chubukov, Michael Norman We consider the evolution of $d_{x^2-y^2}$ pairing, mediated by nearly critical spin fluctuations, with the coupling strength. We show that the onset temperature for pairing, $T^*$, smoothly evolves between weak and strong coupling, passing through a broad maximum at intermediate coupling. At strong coupling, $T^*$ is of order the magnetic exchange energy $J$. We argue that for all couplings, pairing is confined to the vicinity of the Fermi surface. We also find that thermal spin fluctuations only modestly reduce $T^*$, even at criticality, but they substantially smooth the gap anisotropy. The latter evolves with coupling, being the largest at weak coupling. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W35.00002: Magneto-oscillations in Underdoped Cuprates Chandra Varma The conventional interpretation of the recent magneto-oscillation experiments in underdoped Cuprates, requires that there be a state of altered translational symmetry in the pseudogap state which is not supported by structural and Angle Resolved Photoemission Spectroscopy (ARPES) experiments. I show that the observed oscillations may be reconciled with the conclusion arrived in ARPES experiments that the fermi-surface, suitably defined, has the shape of four arcs which shrink to four points as the temperature T approaches 0. Experiments, including infrared absorption in a magnetic field, are suggested to distinguish between such a state from that obtained by the conventional interpretation of the magneto-oscillations. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W35.00003: Phase Diagrams for Stripe Phases with a Spin gap Akbar Jaefari, Siddhartha Lal, Eduardo Fradkin We consider the problem of competing orders in a stripe phase with a large spin gap. In developing the phase diagram, we discuss the phases arising from the stabilization of the Superconducting (SC) and Charge Density Wave (CDW) orders by inter-stripe couplings. This is particularly relevant for stripe phases in High Temperature Superconductivity arising from the Josephson tunneling between neighboring stripes, and is essentially a problem of dimensional crossover. Using inter-chain Mean Field Theory, we present results for the gaps, critical temperatures, and critical exponents in terms of the inter-chain couplings and interaction parameters of the model. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W35.00004: Phase separation of electrons strongly coupled with phonons in cuprates and manganites Sasha Alexandrov Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron-electron correlations. I argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron-phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott-Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons. They form clusters localized by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Froehlich EPI as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR and high and low-resistance phase coexistence near the ferromagnetic transition of doped manganites. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W35.00005: Coherent Lattice Vibrations, Kohn Anomalies, and Pseudogaps in Superconductors Alan M. Kadin A recent analysis has proposed [1] that the superconducting state is associated with charge density standing waves at k=2k$_{F}$, coupled to coherent lattice vibrations at 2k$_{F}$-G, where G is a reciprocal lattice vector. Independently, Aynajian et al. [2] have recently observed phonon spectral anomalies in Nb and Pb that correspond to Kohn anomalies in the Fermi surface, at energies matching the low T energy gap 2$\Delta $(0). Since Kohn anomalies are also defined by k=2k$_{F}$-G, these observations appear consistent with [1]. This also suggests that Kohn anomalies and an associated pseudogap provide a universal precursor of the superconducting state. Further experiments are proposed that should provide direct evidence of the coherent lattice vibrations in the superconducting state of conventional electron-phonon superconductors, and of alternative coherent oscillations (spin waves, etc.) in unconventional materials. \\[0pt] [1] A.M. Kadin, ``Coherent Lattice Vibrations in Superconductors'', Physica C 468, 255 (2008); http://arxiv.org/abs/0706.0338. \\[0pt] [2] P. Aynajian, et al., ``Energy gaps and Kohn anomalies in elemental superconductors'', Science 319, 1509 (2008); http://arxiv.org/abs/0808.1028. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W35.00006: Phase-fluctuations model for the pseudogap of high temperature superconductors Wonkee Kim, Yan Chen, C. S. Ting Within the phase fluctuation picture for the pseudogap state of a high-$T_{c}$ superconductor, we incorporate the phase fluctuations generated by the classical XY model with the Bogoliubov-de Gennes formalism utilizing a field-theoretical method. This picture delineates the essential characteristics of spatially varying local order parameters observed in high-$T_{c}$ superconductors above $T_{c}$. We also compute the local density of states near a non-magnetic impurity with a strong scattering potential. The resonance peak smoothly evolves as temperature increases through $T_{c}$ without showing any sudden broadening, which is consistent with recent experimental findings. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W35.00007: Antiferromagnetic to singlet transition in the quarter-filled band R.T. Clay, S. Mazumdar One of the greatest challenges in constructing a theory of superconductivity in the presence of strong electron-electron (e-e) interactions is to describe how a transition can occur from antiferromagnetic to singlet order. Transitions between antiferromagnetism (AFM) and singlet order are well known in several specific cases such as the spin-Peierls (SP) transition, dimerization in the presence of antiferromagnetic nearest neighbor and second neighbor couplings, and the rung-based singlet in the rectangular spin ladder. In all three examples, the transition is a consequence of confinement within a quasi-one-dimensional lattice. Similar AFM/singlet transitions have not been found in the two dimensional (2D) 1/2-filled band. We present evidence that an AFM/singlet transition can occur in a 2D 1/4-filled anisotropic triangular lattice. A key difference is that at 1/4 filling, inhomogeneity in the form of coexisting charge, bond, and spin orders occur due to e-e and electron-phonon interactions. We show that with increasing lattice frustration the ground state of the 1/4-filled band anisotropic triangular lattice changes from AFM to a charge-ordered state with local singlets. Our results have direct implications for the 1/4-filled organic superconductors as well as related inorganic materials such as Na$_x$CoO$_2$, LiTi$_2$O$_4$, CuRh$_2$S$_4$. Supported by DOE grant DE-FG02-06ER46315. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W35.00008: Collective Fluctuations of the Loop-Current Phase in Cuprates Arkady Shekhter, Vivek Aji, Chandra Varma We have calculated the collective modes of the loop-current ordered phase observed in underdoped Cuprates. Besides the modes given by the fluctuations of the Ashkin- Teller model, we find that the current fluctuations introduce a mode whose properties are similar to that of electro-magnetic vector potentials. We calculate the coupling of such a mode to the Ashkin-Teller modes and to the Fermions. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W35.00009: Superconductivity in Spin-Chain Ladder Cuprate Shigeru Koikegami, Takashi Yanagisawa We study the superconductivity in the three-dimensional d-p model with the quasi-one-dimensional structure in which CuO$_2$- chain and Cu$_2$O$_3$-ladder are alternately stacked with each other. When we control the hole density on each Cu site in our model, we have two or three Fermi surfaces, on which the fully- gapped superconductivity develops. Both the inter-band nesting and the large density of states around Van Hove singulality points play essential roles to achieve the superconductivity, and these two factors can coexist easily owing to the electron transfer between chain and ladder. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W35.00010: Superconducting Fluctuations in Strongly Correlated Electronic Systems William Putikka Superfluid behavior is relatively common in strongly correlated fermion systems. This suggests there is a common reason for this behavior rooted in the strong correlations. I propose such a mechanism, developed in the context of the 2D $t$-$J$ model, where $d_{x^2-y^2}$ superconducting fluctuations have recently been observed$^1$. The $d_{x^2-y^2}$ fluctuations are {\it not} due to antiferromagnetic fluctuations; the AF fluctuations compete with superconducting fluctuations. Pair fluctuations have their own, separate origin based in the strong correlations. If the on site repulsion is strong enough it can affect the electronic degrees of freedom while the entropy still dominates the free energy. This requires the entropy to be maximized under the constraint of no double occupancy, thereby rearranging the electronic degrees of freedom into separate spin and charge excitations. These excitations have different statistics and very different energy scales, allowing the charges to develop pair correlations before the spin degrees of freedom become coherent. Below the spin coherence temperature, the spins determine the symmetry of the pair wave function for the {\it electronic} pair fluctuations. The symmetry which best {\it avoids} the AF fluctuations on a square lattice is $d_{x^2-y^2}$.\\ 1. WO Putikka and MU Luchini, PRL{\bf 96}, 247001 (2006). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W35.00011: The quantum effective potential and the condensation of topological excitations in Josephson junction arrays. Said Sakhi I analyze the radiative corrections to the effective potential for an Abelian gauge theory relevant to Josephson junction arrays (JJA). This model consists of two disorder fields related to electric and magnetic charges coupled to topologically gauge fields described by Maxwell terms and a mixed Chern-Simons term. The symmetry of the ground state is studied through the effective potential which takes into account radiative corrections in the theory. Here zero condensates for the topological charge excitations describe [1] insulating phases of JJA, and nonzero condensates describe superconducting phases. The gauge fields contribution to the one-loop effective potential is evaluated and its effect on the spontaneous symmetry breaking is examined. Effects of dissipation driven coupling in JJA systems connected to a reservoir of gapless single-particle excitations are also studied. Coupling to gapless fermions is shown to induce radiative corrections in the effective potential which favor transitions between an insulating state and a superconducting state. [1] S. Sakhi, Europhys. Lett. 73 (2), 267 (2006). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W35.00012: Topological Confinement and Superconductivity Khaled Al-Hassanieh, Cristian Batista, Pinaki Sengupta, Adrian Feiguin We derive a Kondo Lattice model with a correlated conduction band from a two-band Hubbard Hamiltonian. This mapping allows us to describe the emergence of a robust pairing mechanism in a model that only contains repulsive interactions. The mechanism is due to topological con?nement and results from the interplay between antiferromagnetism and delocalization. By using Density-Matrix-Renormalization Group (DMRG) we demonstrate that this mechanism leads to dominant superconducting correlations in a 1D-system. [1] K. A. Al-Hassanieh, C. D. Batista, P. Sengupta, and A. E. Feiguin, preprint arXiv:0808.3735. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W35.00013: Vortex lattice structures of spin triplet superconductors Daniel Agterberg, Suk Bum Chung, Eun-Ah Kim Motivated by recent interest in spin triplet superconductors, we investigate the vortex lattice structures for this class of unconventional superconductors. We discuss how the order parameter symmetry can give rise to U(1)$\times$U(1) symmetry in same sense as in spinor condensates, making the half- quantum vortex (1/2-qv) topologically stable. We then calculate the vortex lattice structure of 1/2-qv's, with particular attention on the roles of the crystalline lattice, the Zeeman coupling, and Meissner screening, all absent in spinor condensates. Finally, we consider how spin-orbit coupling leads to a breakdown of the U(1) $\times$ U(1) symmetry and the fate of the 1/2-qv lattice. As examples, we consider models for spin-triplet superconductivity in Sr$_2 $RuO$_4$ and more speculative spin-triplet models for Na$_x$CoO$_2 \cdot y$H$_2$O and Bechgaard salts. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W35.00014: Strong coupling limit of superconductivity in anti-ferromagnetic phase: Extended hardcore boson picture of d-wave order and phase fluctuation Yucel Yildirim, Wei Ku Strong coupling limit (local-pairing $>>$ kinetic energy) of the superconductivity in High-$T_c$ superconductors is investigated within the anti-ferromagnetic phase. An extended hardcore boson picture consisting of paired holes results from general considerations of paired fermions, in which directional near-neighbor occupations of bosons are forbidden. By use of Wannier function of the low-energy sector of the bosonic Hilbert space, our simple picture provides a natural separation of the phase of the superconducting order parameter into local and external ones. Within a realistic parameter range, the local structure is found to be of d-wave symmetry, driven by the kinetic energy. On the other hand, the genuine behavior of the superconductivity is controlled by the phase coherence of the external phase, which leads to experimentally observed linear reduction of super fluid density. Interestingly, due to the competition with p-wave symmetry, the effective mass of the boson is enormously enhanced from that of the Fermion, explaining the very small stiffness. Connections to recent observation of 4-period d-wave CDW in the ``stripe'' phase, and C2-symmetry bond-centered charge profile will also be addressed. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W35.00015: A new method for solving the inhomogeneous Bogoliubov - de Gennes equations Lucian Covaci, Mona Berciu Inhomogeneities (surface, interfaces, impurities, etc.) in superconductors give rise to interesting phenomena, like broken time-reversal states, bound states near surfaces, etc. Numerical solutions of the self-consistent Bogoliubov-de Gennes mean field equations become computationally intensive for systems whose translational symmetry is broken. We propose a new method of solving the mean-field equations based on the Kernel Polynomial Method. We expand the Green's functions in terms of Chebyshev polynomials and calculate the order parameters self-consistently. Because the most expensive operation is only the sparse matrix-vector multiplication, the benefits of this method are multiple: usage of large systems, easy implementation of symmetries, multiple bands. Although we apply this method to a specific example (formation of Andreev states in 2D superconductors), it is applicable to any mean-field calculation. [Preview Abstract] |
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