Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W11: Theory of Superconductivity in Cuprates II |
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Sponsoring Units: DCMP Chair: Mohit Randeria, Ohio State University Room: Morial Convention Center RO9 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W11.00001: Theory of Strongly Correlated Superconductivity 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 spin fluctuations; the AF fluctuations compete with superconducting fluctuations. Pairing fluctuations have their own origin based on strong correlations. The key to understanding how this comes about is the high temperature entropy. Strong correlations not only create order at low temperatures, they also reduce the entropy at high temperatures. The response of the system is to rearrange the degrees of freedom into separate spin and charge excitations. These excitations have very different energy scales, allowing the charges to develop pairing correlations before the spin degrees of freedom become coherent. At lower temperatures where the spins become coherent they determine the symmetry of the electronic pair wavefunction in a manner to best {\it avoid} the magnetic fluctuations in the system.\\ 1. WO Putikka and MU Luchini, PRL{\bf 96}, 247001 (2006). [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W11.00002: Magnetism and superconductivity in the t-t'-J model Federico Becca, Leonardo Spanu, Massimo Lugas, Sandro Sorella We present a systematic study of the phase diagram of the $t{-}t^\prime{-}J$ model by using the Green's function Monte Carlo (GFMC) technique, implemented within the fixed-node (FN) approximation and a wave function that contains both antiferromagnetism and d-wave pairing. This enables us to study the interplay between these two kinds of order and compare the GFMC results with the ones obtained by the simple variational approach. By using a generalization of the forward-walking technique, we are able to calculate true FN ground-state expectation values of the pair-pair correlation functions. In the case of $t^\prime=0$, there is a large region with a coexistence of superconductivity and antiferromagnetism, that survives up to $\delta_c \sim 0.10$ for $J/t=0.2$ and $\delta_c \sim 0.13$ for $J/t=0.4$. The presence of a finite $t^\prime/t<0$ induces a strong suppression of both magnetic (with $\delta_c \alt 0.03$, for $J/t=0.2$ and $t^\prime/t=-0.2$) and pairing correlations. In particular, the latter ones are depressed both in the low-doping regime and around $\delta \sim 0.25$, where strong size effects are present. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W11.00003: Phase Fluctuations in high-Tc Superconductors Yucel Yildirim, Weiguo Yin, Wei Ku Thermal fluctuation induced destruction of phase coherence of superconductivity in High-Tc superconductors is investigated via numerical quantum Monte Carlo solution of a new theoretical model at finite-temperature. Our simple effective boson Hamiltonian, derived from the pairing sector of the t'-t''-J model, can be considered as a natural extension of the hard-core boson with additional information about the internal structure of the local fermion pairs. The local solution is found to consist of d-wave pairing, hybridizing with neighboring p-wave pairs. The possible connection with the pseudo-gap phase will also be addressed. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W11.00004: Suppression of Superfluid Density in Underdoped Cuprates Wei-Cheng Lee, Jairo Sinova, Anton A. Burkov, Yogesh Joglekar, Allan H. MacDonald A key challenge for theories of high-T$_{c}$ cuprates is to explain why the superfluid density vanishes as the antiferromagnetic insulator state is approached. Viewing a cuprate as a doped Mott insulator gives a natural explanation of this property, but one that is not obviously compatible with the fact that superconductivity always vanishes at finite doping. We propose another possible explanation, starting from weak-coupling conventional d-wave BCS theory and calculating the correlation contribution to the superfluid density. We show that triplet fluctuations of the d-wave superconducting order parameter are canonically conjugate to antiferromagnetic fluctuations, and that this causes the correlation energy to increase in magnitude when superconductivity is weakened by a phase gradient. In our theory the inelastic neutron scattering resonance has the character of a magnetic plasmon rather than the character of an exciton. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W11.00005: Stability of inhomogeneous superconducting states in the doped t-J model Manuela Capello, Didier Poilblanc By using Variational quantum Monte Carlo techniques, we have investigated the stability of inhomogeneous RVB superconducting states in the t-J model at doping 1/8. We show that introducing half-filled charge domain walls involves very small energy costs, especially in the presence of tetragonal-lattice distortions [1]. This suggests that spontaneous unidirectional modulations of the hole density and the superconducting order parameter could easily be stabilized in real materials, in the presence of small perturbations. We argue that such a scenario is at play in the recently observed patterns of unidirectional domains in high-T$_c$ superconductors [1]. \newline [1] M. Capello et al. (in preparation). \newline [2] Y. Kohsaka et al. Science 315, 1380 (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W11.00006: ``Glue" approximation for the pairing interaction in the Hubbard model with next nearest neighbor hopping Ehsan Khatami, Alexandru Macridin, Mark Jarrell Recently, several authors have employed the ``glue" approximation for the Cuprates in which the full pairing vertex is approximated by the spin susceptibility. We study this approximation using Quantum Monte Carlo Dynamical Cluster Approximation methods on a 2D Hubbard model. By considering a reasonable finite value for the next nearest neighbor hopping, we find that this ``glue" approximation, in the current form, does not capture the correct pairing symmetry. Here, d-wave is not the leading pairing symmetry while it is the dominant symmetry using the ``exact" QMC results. We argue that the sensitivity of this approximation to the band structure changes leads to this inconsistency and that this form of interaction may not be the appropriate description of the pairing mechanism in Cuprates. We suggest improvements to this approximation which help to capture the the essential features of the QMC data. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W11.00007: Pairing glue in the Hubbard and t-J models: Insights from a dynamic cluster approximation study Thomas Maier, Douglas Scalapino, Mark Jarrell We will discuss dynamic cluster non-crossing approximation studies of the superconducting gap function $\Phi(k,\omega)$ in the 2D Hubbard and t-J models. The momentum and frequency dependence of the gap reflect the spatial structure and dynamics of the pairing interaction responsible for d-wave pairing. For both models we find that two mechanisms are simultaneously active. An instantaneous contribution to the gap arises from states whose energies reflect the Mott scale and which give rise to the exchange coupling $J$. The dominant contribution, however, is retarded and comes from energies that correspond to the spin fluctuation spectrum. It is therefore reasonable to speak of a dynamic ``pairing glue'' which gives rise to d-wave superconductivity in the Hubbard and t-J models. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W11.00008: Finite-Temperature Phase Diagram of the $d=3$ $tJ$ Model with Quenched Disorder A. Nihat Berker, Michael Hinczewski We study a quenched disordered $d=3$ $tJ$ Hamiltonian with static vacancies as a model of nonmagnetic impurities in high-$T_c$ materials.[1,2] Using a position-space renormalization-group approach, we calculate the evolution of the finite-temperature phase diagram with impurity concentration $p$, and find several features with close experimental parallels: away from half-filling we see the rapid destruction of a spin-singlet liquid phase (analogous to the superconducting phase in cuprates) which is eliminated for $p \ge 0.05$; in the same region for these dilute impurity concentrations we observe an enhancement of antiferromagnetism. The antiferromagnetic phase near half-filling is robust against impurity addition, and disappears only for $p \ge 0.40$. \newline \noindent [1] M. Hinczewski and A.N. Berker, Eur. Phys. J. B {\bf 51}, 461 (2006). \newline \noindent [2] M. Hinczewski and A.N. Berker, arXiv:cond-mat/0607171v1 [cond-mat.str-el]. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W11.00009: Dynamical Layer Decoupling in a Stripe-Ordered High-$T_c$ Superconductor Eduardo Fradkin, Erez Berg, Eun-Ah Kim, Steve Kivelson, Vadim Oganesyan, John Tranquada, Shoucheng Zhang In the stripe-ordered state of a strongly correlated two-dimensional electronic system, under a set of special circumstances, the superconducting condensate, like the magnetic order, can occur at a nonzero wave vector corresponding to a spatial period double that of the charge order. In this case, the Josephson coupling between near neighbor planes, especially in a crystal with the special structure of La$_{2-x}$Ba$_x$CuO$_4$, vanishes identically. We propose that this is the underlying cause of the dynamical decoupling of the layers recently observed in transport measurements at $x =1/8$. \newline [1] E. Berg et al, PRL {\bf 99}, 127003 (2007) [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W11.00010: Exact diagonalization study of electronic nematic and $d$-density wave states Hyeonjin Doh, Thomas Grzesiak, Hae-Young Kee It was proposed that the $d$-density wave (ddw) and the electronic nematic states are relevant phases in the phase diagram of high $T_C$ cuprates. The two phases break different symmetries, and their order parameters have been used to describe the characteristic broken symmetries. Here we show that the two order parameters transform into each other under a local gauge transformation, which implies that the order parameters cannot represent different broken symmetric states, if a Hamiltonian is invariant under such a transformation. The two order parameters describe distinctly different states, when the nearest neighbor hopping integral is finite, but the states are nearly degenerate at a strong coupling limit. We also present a phase diagram of a Hamiltonian with correlated hopping, and nearest neighbor repulsive interactions using Lanczos exact diagonalization where we find an interesting interplay between the ddw and nematic states. We compare our results with the previous mean field calculation, and discuss a possible relevance to a $d$-wave superconducting state. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W11.00011: Competition between superconductivity and antiferromagnetism as a boson-fermion mixture problem Gerardo Ortiz, Leonid Isaev, Cristian D. Batista It is widely believed that the key to understanding the mechanism of superconductivity in cuprate materials and heavy fermion compounds lays in the competition between two quantum orders: superconductivity and antiferromagnetism, which complicates the choice of relevant effective degrees of freedom. Here we present a new isomorphic mapping, preserving the dimension of the on-site Hilbert space, where holons are represented by fermionic and spinons -- by bosonic degrees of freedom, which, we claim, are the relevant ones in a certain regime (low density). Starting from t-J -- like models we show that the original Hamiltonian can be exactly mapped onto a mixture of interacting fermion and boson gases. It is then demonstrated that this system may be considered as dilute under certain experimentally reasonable conditions. In this limit we show that the fermion gas exhibits a pairing instability, caused by a two boson exchange, which implies superconductivity of holons in the original repulsive model. Although the long-range Coulomb interaction is, in principle, capable of suppressing this state, we argue about a physical mechanism for its compensation, which is relevant for real materials. Our results also lead to a mechanism for superfluidity in ultracold boson -- fermion gas mixtures. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W11.00012: Stability of nodal quasi-particles in superconductors with coexisting orders Erez Berg, Cheng-Chien Chen, Steven A. Kivelson The possible existence of nodal quasi-particles is one of the most distict properties of unconventional superconductors. Nodal quasi-particles have many unique experimental fingerprints, such as a linear temperature dependence of the superfluid density. It is therefore interesting to ask under what conditions can they exist generically. Here, we establish a condition for the perturbative stability of zero energy nodal points in the quasi-particle spectrum of superconductors in the presence of a general coexisting \textit{commensurate} order. The nodes are found to be stable if the Hamiltonian is invariant under time reversal followed by a lattice translation. The principle is demonstrated with a few examples. Some experimental implications of various types of assumed order are discussed in the context of the cuprate superconductors. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W11.00013: S-wave superconductivity from predominantly d-wave pairing Paul Oreto, Boris Spivak, Steven Kivelson We have studied the zero temperature properties of a system consisting of dilute superconducting droplets embedded in a weakly disordered Fermi liquid metal. Even under the assumption that the pairing interaction occurs exclusively in the d-wave channel, irregularities in the shape of the droplets, or impurities within the droplet generically induce local mixing of s-wave and d-wave components of the superconducting order. Moreover, we find that the Josephson coupling between the s-wave components of the order parameter on two droplets separated by a distance r falls off as a slower power law than does the d-wave component. Thus, for dilute enough droplets, even if the local gap structure within each droplet is dominantly d-wave-like, the globally phase coherent portion of the superconducting order must have s-wave symmetry. We speculate that this effect could lead to a region of globally s-wave superconductivity in highly overdoped cuprates. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W11.00014: Absence of superconductivity in the $\frac{1}{2}$-filled band Hubbard model on a triangular lattice Hongtao Li, R. Torsten Clay, S. Mazumdar The superconducting $\kappa$-(BEDT-TTF)$_2$X salts are strongly dimerized, with one hole per dimer unit cell. The occurrence of antiferromagnetism under ambient pressure, and the appearance of superconductivity under pressure has led several investigators to suggest that the superconductivity can be explained within an anisotropic triangular lattice $\frac{1}{2}$-filled band Hubbard Hamiltonian. Within this picture, pressure takes the system closer to the isotropic limit, when antiferromagnetism gives way to d-wave superconductivity. We have performed exact diagonalizations on a 16-site periodic anisotropic triangular lattice as a function of the Coulomb interaction (Hubbard $U$) and the anisotropy to investigate this claim. We calculate bond orders, double occupancies, spin-spin correlations, spin structure factors and $d_{x^2-y^2}$ superconducting pair-pair correlations. We are able to confirm the Mott metal-insulator transition and antiferromagnetism, but we do not find any hint of long range superconducting correlations. Neither is there any region of the Hubbard $U$ where these correlations are enhanced by the interaction strength. \footnote{Supported by DE-FG02-06ER46315}. [Preview Abstract] |
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