Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D33: Superconducting Theory I |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Dennis Newnes, IBM Room: 403 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D33.00001: Giant nonlinear electron-lattice interaction in cuprate superconductors, and origin of the pseudogap Dennis Newns The pseudogap is a key property of the cuprate superconductors, whose understanding should illuminate the pairing mechanism. Recent data support a close connection between the pseudogap and an oxygen-driven C4 symmetry breaking within the CuO$_{2}$ plane unit cell. Using \textit{ab initio} Molecular Dynamics, we show the existence of a strong nonlinear electron-oxygen vibrator coupling in two cuprates. In a mean field approach applied to this coupling, we derive a C4 splitting/pseudogap phase diagram in agreement with experiment - providing an explanation for the pseudogap phenomenon from first principles. The implications for superconductivity, Fermi surface arcing, and other properties are discussed. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D33.00002: An explanation of the dichotomy between Fermi arcs and Fermi pockets in underdoped high-$T_{c}$ superconductors Xun Jia, Sudip Chakravarty We have numerically computed the spectral function $A(\vec{k},\omega)$ of an underdoped cuprate superconductor for the $d$-density wave state subject to a long range correlated disorder. The intensity of the spectral function is significantly reduced for the electron pockets for an intermediate range of correlation length, but the Fermi arcs remain quite intact. This result provides one possible explanation as to why the electron pockets are not observed in angle resolved photoemission experiments. A calculation of Shubnikov-de Haas (SdH) oscillations using a real space transfer matrix method shows that two main frequencies are still present in the presence of a moderate amount of white noise disorder. The SdH oscillations in other relevant broken symmetry states are also computed. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D33.00003: Quantum oscillations in a highly renormalized Fermi liquid model of Fermi pockets in underdoped cuprates Tudor Stanescu, Victor Galitski, Dennis Drew Motivated by the recent experimental observation of quantum oscillations in the underdoped cuprates, which suggest the existence of small electron pockets characterized by a relatively large cyclotron mass, we address two basic questions: 1) How can one explain the relatively large cyclotron effective mass observed experimentally and its relation with the effective Hall mass? 2) Why the electron pockets are not seen in ARPES experiments? We propose an explanation based on a model of a highly renormalized Fermi liquid characterized by a reconstructed Fermi surface and strongly momentum-dependent quasiparticle properties. We find that the cyclotron mass is enhanced by a factor $< 1/Z >$, while the effective Hall mass is proportional to $< Z >/< Z^2 >$, where $<...>$ implies an averaging over the Fermi surface. If the Z-factor becomes small in some part of the Fermi surface, the cyclotron mass is enhanced sharply while the infrared Hall mass may remain small. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D33.00004: Superconductivity near structural phase transition: the case of NbN Simon Blackburn, Michel C\^ot\'e, Steven G. Louie, Marvin L. Cohen Using density functional theory within the local density approximation we report the study of the electron-phonon coupling in NbC$_{1- x}$N$_{x}$ crystals in the rocksalt structure. The Fermi surface of the system allows important nesting. The associated Kohn's anomaly greatly increases the electron-phonon coupling and induces a structural instability when the electronic density of states reaches a critical value. Our results reproduce the observed rise in T$_{c}$ from 11.2 K to 17.3 K as the nitrogen doping is increased. To further understand the important effect of the structural instability to the superconducting temperature, we model the Eliashberg spectral function with two contributions, one for the unstable phonons and the other for the unaffected phonons. Using the McMillan formula, we can predict the evolution of T$_{c}$ within this simple model that reproduces well our \textit{ab initio} results and the experimental data. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D33.00005: Competition between antiferromagnetism and superconductivity, electron-hole doping asymmetry and ``Fermi Surface" topology in cuprates Sandeep Pathak, Vijay Shenoy, Nandini Trivedi, Mohit Randeria We study the asymmetry between electron and hole doping in a 2D Mott insulator, and the resulting competition between antiferromagnetism (AF) and d-wave superconductivity (SC), using variational Monte Carlo for projected wave functions. We find that key features of the $T=0$ phase diagram, such as critical doping for SC-AF coexistence and the maximum value of the SC order parameter, are determined by a single parameter $\eta$ which characterizes the topology of the ``Fermi surface" at half filling defined by the bare tight-binding parameters. Our results give insight into why AF wins for electron doping, while SC is dominant on the hole doped side. We also suggest using band structure engineering to control the $\eta$ parameter for enhancing SC. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D33.00006: Density Matrix Renormalization Group Study of a Dynamic Hubbard Model, a Comparative Study Fatih Dogan, Frank Marisglio A one-dimensional model of holes locally coupled to pseudo spin degrees of freedom is studied using density matrix renormalization group. The model used in this talk is one in the family of dynamic Hubbard models. We look at density-density correlations, and frequency dependent functions to see existence and nature of the attraction of the holes in a electron-hole asymmetric system. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D33.00007: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 3:54PM - 4:06PM |
D33.00008: Valence bond glassy order and the pseudogap phase in underdoped high Tc cuprates Liang Ren, Ziqiang Wang Different origins of the pseudogap phenomena in underdoped high Tc cuprate have been proposed over the years, but a consistent theory has been challenging. We argue that the low-energy fluctuations of the valence bond, originating from the superexchange interaction, are pinned by the doping induced electronic disorder to give rise to a valence bond glass (VBG) pseudogap phase Using an extended t-J model within the Gutzwiller approximation, we show that the normal state VBG phase exhibits a genuine Fermi arc and a V-shaped average density of state at low energies. In the superconducting phase below Tc, the VBG can coexist and compete with an inhomogeneous d-wave superconductor, leading to the two-gap phenomena. We discuss the evolution of the local and momentum-space spectroscopy with doping and temperature, which capture the salient properties of the pseudogap phenomena and electronic disorder observed by recent ARPES and STM experiments. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D33.00009: On stability of odd-frequency superconducting state Dmitry Solenov, Ivar Martin, Dmitry Mozyrsky Odd-frequency pairing mechanism has been investigated for several decades. Nevertheless the properties of such superconducting phase as well as its thermodynamic stability have remained unclear. In particular it has been argued by numerous authors that the odd-frequency state is thermodynamically unstable, has an unphysical Meissner effect (at least within the mean-field approximation), and therefore can not exist as a homogeneous phase in equilibrium physical systems. We argue that such a conclusion is incorrect because it relies on an inappropriate assumption that the odd-frequency superconductor can be described by an effective Hamiltonian that breaks the U(1) symmetry. We show that the odd-frequency state can be appropriately formulated within the functional integral representation by using the effective action to describe such a superconducting state within the mean field approximation. We find that the odd-frequency superconductor is thermodynamically stable and exhibits ordinary Meissner effect, and therefore, in principle, it can be realized in equilibrium solid state systems. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D33.00010: Wave function for composite odd-frequency superconductors. Hari Dahal, Elihu Abrahams, Dmitry Mozyrsky, Yukio Tanaka, Alexander Balatsky Berezinskii proposed a new class of superconducting state that has an anomalous gap function that is odd function of frequency.$^{1}$ Following initial work of Berezinskii there has been growing interest in properties of such superconductors.$^{2-3 }$We propose a BCS-like wave function for an s-wave triplet odd-frequency superconductor. The wave function describes a condensate of spin-0 Cooper pair and spin-1 magnon; a composite order. By minimizing a Hamiltonian, suggested earlier in Ref. [3] to study the odd-frequency superconductor, we derive the quasiparticle dispersions, the self-consistent gap equation, and the density of states. We show that the quasiparticle excitations are gapless and the superconducting transition requires a critical coupling. References: \begin{enumerate} \item V. L. Berezinskii, JETP Lett. \textbf{20, }287 (1974). \item A. Balatsky, and E. Abrahams, PRB \textbf{45}, 13125 (1992). \item E. Abrahams, A. Balatsky, D. J. Scalapino and J. R. Schrieffer, PRB \textbf{52}, 1271(1995). \end{enumerate} [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D33.00011: Ginzburg Landau Theory for Cuprate Superconductivity Tiruppattur Ramakrishnan, Sumilan Banerjee, Chandan Dasgupta We propose and develop the consequences of a theory in which the free energy $F$ of a cuprate is expressed as a functional of the complex nearest neighbour spin singlet bond pair order parameter $\Delta_{ij} \exp{(i\phi_{ij})}$. $F$ is a sum of two terms $\sum_m(a \Delta_m^2+b\Delta_m^4)$, and $F_1= c\sum_{mn}\Delta_{mn}\cos{(\phi_m-\phi_n)}$; here $m$ is the site corresponding to $ij$ on the dual lattice (also square) and $m, n$ are nearest neighbours. The doping $x$ and temperature $T$ dependences of $a, b$ and $c$ are rationalized ( eg, $c\propto x$ for small $x$). The pseudogap ( due to incoherent bond pairs) and the parabolic $x$ dependence of $T_c$ ( AF ordering of the 2d-XY spin $\Delta_m\exp{(i\phi_m)}$ leading to d wave superconductivity) are described. The observed $C_v(T)$ behaviour is shown to be due to order parameter fluctuations. Detailed calculations of the vortex structure show a crossover from a Josephson like to a BCS like form with increasing doping, mirroring a similar change in superconductivity. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D33.00012: Levitation and lateral forces between a small magnet and superconducting sphere and the stability of the magnet H. Al-Khateeb, M. Alqadi, F. Alzoubi, B. Albiss, M. Hasan, N. Ayoub Using the dipole-dipole interaction model, we obtained analytical expressions for the levitation and lateral forces act on a small magnet for anti-symmetric magnet/spherical superconductor system. Breaking the symmetry of the system, allow as to study the lateral force which is important in the stability of the magnet above superconducting sphere in the Meissner state. Our formulas are written in terms of the radius of the superconductor as well as the height, the lateral displacement and the orientation of the magnetic moment of the magnet. We found that the levitation force is linearly dependent on the lateral displacement whereas the lateral force is independent of the lateral displacement. Moreover, the levitation and lateral forces are varying solinoudally with the polar and azimuthal angle of the orientation of the moment of the magnet. The stability of the magnet has been discussed for special orientations of the moment of the magnet. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D33.00013: HTSC Measurements Explained by Defended Superconductivity Theory John James Resonance between the superconducting gap and the low energy optical phonon mode creates a burst of coherent phonons which prevents the unstable superconducting state from collapsing. Surrounding lattice defects with charge states prevents interactions which would stop the coherent phonons from forming. This simple model of why Tc is high can explain many other phenomena, such as; insufficient density of states and instability in BCS model, variation of the gap energy in a single sample, the irreversibility line, sensitivity of Tc to impurities, high slope resistivity transition with impurities, isolation of magnetic impurities accept praeseodymium, long lived quasiparticles at low T, pseudogap, gapless pairs above Tc, proximity of phases, phase diagrams, low and high values of the tunneling gap, rise and fall of Tc with number of Cu layers, sensitivity to radiation damage at low temperature, normal state metal at the surface of samples cleaved at low temperature, eventual lowering of Tc with overdoping, remaining states in the photoemission gap, peak in the conductivity below Tc and many more. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D33.00014: A Dynamical Mean Field Study of the Three-Band Copper-Oxide Model Xin Wang, Luca de' Medici, Capone Massimo, Andrew J. Millis We apply the dynamical mean field theory to study the three-band Copper-Oxide Model related to the High-Tc Cuprates. Both continuous-time quantum Monte Carlo and Exact Diagonalization impurity solvers are used. The spectral function, mass enhancement and optical conductivity in both paramagnetic and antiferromagnetic case are computed. We determine the contribution of antiferromagnetic order to the gap observed in the undoped material. We show that in the paramagnetic Mott insulating regime the quasiparticle mass enhancement is larger for hole than for electron-doped compounds, but an opposite trend in matrix elements means that a the optical conductivity in these two cases is comparable. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D33.00015: Retarded vs instantaneous interactions in high-temperature superconductors. What is the glue? Bumsoo Kyung, David Senechal, A.-M.S. Tremblay In BCS theory, the phononic origin of the attraction that leads to Cooper pairs was confirmed by theoretical and experimental developments that clearly showed that the interaction was retarded and that the corresponding energy scales were associated with phonons. Using Cellular Dynamical Mean-Field Theory with exact diagonalization at $T=0$, we identify retardation effects in pairing and associate the corresponding energy scales with the spectral function of short-range spin fluctuations. These fluctuations are clearly seen in neutron and optical spectroscopy probes. Since the pairs have vanishing wave function at zero distance, the energy scale $U$ is absent from the pair dynamics. That dynamics can be monitored by the anomalous spectral weight. The Heisenberg exchange $J$ is a characteristic energy scale of that spectral weight and it appears in a manner analogous to what is found in mean-field theories. However, the anomalous spectral weight has additional structure caused by retardation. [Preview Abstract] |
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