Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A40: Focus Session: Phonons and Electron Correlations in High Temperature Superconductors I |
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Sponsoring Units: DCMP Chair: Alexander Balatsky, Los Alamos National Laboratory Room: F151 |
Monday, March 15, 2010 8:00AM - 8:36AM |
A40.00001: Strong-coupling theory of high-temperature superconductivity beyond BCS Invited Speaker: Alexandre Alexandrov We have extended the BCS theory to the strong-coupling regime, where carriers are small lattice polarons and bipolarons. Attractive electron correlations are caused by an almost unretarded electron-phonon interaction (EPI) sufficient to overcome the direct intersite Coulomb repulsion in this regime. Here I present our recent theoretical results, which in conjunction with a number of experimental observations provide a definite answer to the fundamental question on a key pairing interaction in high-temperature superconductors. Theoretical studies using advanced numerical (QMC) techniques have shown that purely repulsive models do not account for high- temperature superconductivity [1]. On the other hand our recent QMC studies have found that even a relatively weak finite-range EPI induces substantial d-wave superconducting order in strongly-correlated Mott-Hubbard insulators [2], while the strong EPI provides superlight small bipolarons, which bose-condense at high temperatures [3]. I propose that the true origin of high-temperature superconductivity is found in a proper combination of strong electron-electron correlations with the significant finite-range EPI, so that charge carriers are small mobile polarons and bipolarons in cuprate superconductors. We have shown that the conventional EPI explains the unconventional symmetry of the superconducting order parameter both in the weak-coupling (BCS) and in the strong-coupling (bipolaronic) regimes [4]. Bipolarons account for the normal state diamagnetism [5], unusual ARPES [6], superconducting and normal state (pseu)dogaps in the tunnelling spectra of cuprates [7]. \\[4pt] [1] T. Aimi and M. Imada, J. Phys. Soc. Jpn. 76, 113708 (2007).\\[0pt] [2] T. M. Hardy, J. P. Hague, J. H. Samson, and A.S. Alexandrov, Phys. Rev. B79, 212501 (2009).\\[0pt] [3] J. P. Hague, P. E. Kornilovitch, J. H. Samson, and A. S. Alexandrov, Phys. Rev. Lett. 98 (2007) 037002.\\[0pt] [4] A. S. Alexandrov, Phys. Rev. B77 (2008) 094502.\\[0pt] [5] A. S. Alexandrov, Phys. Rev. Lett. 96 (2006) 147003 .\\[0pt] [6] A. S. Alexandrov and K. Reynolds, Phys. Rev. B76 (2007) 132506.\\[0pt] [7] A. S. Alexandrov and J. Beanland, arXiv: 0910.4295. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A40.00002: Testing polaron coherence and the pairing symmetry in cuprate superconductors by local probe methods Annette Bussmann-Holder, Hugo Keller, Jose Mustre de Leon, Arndt Simon, Alan Bishop A variety of local structural probes have demonstrated that local lattice distortions take place in cuprates, which correlate with the onset of the pseudogap phase (PG) and superconductivity (SC). We show here that these lattice responses can be a consequence of polaron formation, local coherence in the pseudogap phase, and global coherence in the superconducting phase. In addition, we demonstrate that the results are consistent with a complex s+d wave order parameter in the SC phase. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A40.00003: La2CuO4 isolator gap, AF structure and pseudogaps from spin-space entangled orbitals in the Hartree-Fock scheme Alejandro Cabo Montes de Oca, Alejandro Cabo Bizet It is argued that a Hartree-Fock (HF) solution of a simple model of the Cu-O planes in La2CuO4, is able to predict its insulator character and antiferromagnetic (AF) order. Pseudogap HF states are also naturally emerging from the discussion. These results follow from the deletion of symmetry restrictions usually imposed on the variational orbitals. One of them is the simplification of the spinor projection to be +1/2 or -1/2, which strongly reduces the searching space of orbitals. We also remove the demand on the orbitals to have a Bloch structure in the starting lattice. It turns out that the most stable HF solution of the problem is an AF and insulating state associated to ``spin-space'' entangled orbitals. The evaluated magnetic moment per cell is 0.67 $\mu $B, a result that satisfactorily reproduces the measured value of 0.68 $\mu $B. Another HF state having higher energy arises which is paramagnetic and shows a pseudogap. It follows after only requiring the Bloch structure in the original lattice. A third paramagnetic but metallic solution is received by including both of the mentioned constraints. [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A40.00004: Bipolaron in the t-J Model Coupled to Longitudinal and Transverse Quantum Lattice Vibrations Janez Bonca, Lev Vidmar, Sadamichi Maekawa, Takami Tohyama We explore the influence of two different polarizations of quantum oxygen vibrations on the spacial symmetry of the bound magnetic bipolaron in the context of the $t-J$ model by using exact diagonalization within a limited functional space. Linear as well as quadratic electron phonon coupling to transverse polarization stabilize $d-$wave symmetry. The existence of a magnetic background is essential for the formation of a $d-$wave bipolaron state. With increasing linear electron phonon coupling to longitudinal polarization the symmetry of a $d$-wave bipolaron state changes to a $p$-wave. Bipolaron develops a large anisotropic effective mass. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A40.00005: Fulleride Superconductors are Phonon-Driven and Strongly Correlated Erio Tosatti, Massimo Capone, Claudio Castellani, Michele Fabrizio Superconductivity in trivalent alkali fullerides is believed to be phonon-driven and s-wave, similar in that to ordinary BCS systems. There is nonetheless in these materials a metal-Mott insulator transition upon lattice expansion, indicating exceedingly strong electron-electron correlations. Using Dynamical Mean Field Theory we solved a 3-band Hubbard model, including both electron-electron and (simplified) electron-phonon interactions, which yields a phase diagram [1] in striking agreement with the experimental one for the recently discovered expanded fulleride Cs3C60 as a function of pressure.[2] A dome-shaped superconducting order parameter, a pseudogap phase, and the subsequent Mott transition upon expansion thus assimilate the phonon driven fulleride superconductors to cuprates and to 2D organics, despite their obvious differences. Some experimental predictions are made, including a kinetic energy gain and a Drude weight increase in the superconducting state relative to the normal state, contrary to BCS, but similar to cuprates. [1] M. Capone, et al., Rev. Mod. Phys. 81,943 (2009); [2] Y. Takabayashi et al., Science 323, 1585 (2009). [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A40.00006: Theory of time-resolved spectral function in high-temperature superconductors with bosonic modes Jianmin Tao, Jian-Xin Zhu Quasiparticle properties are of fundamental importance toward the understanding of high-temperature superconductivity. The nature of bosonic modes, to which electrons are strongly coupled in these systems, is a topic of current extensive discussion. Here we propose to use the time-resolved spectral function to uncover the mystery. First we develop a three-temperature model to simulate the time dependence of electronic and phononic temperatures. The advantage of this model is that it not only takes the tight-binding electronic structure into account, but also is valid in superconducting state. Based on this model, we then calculate the time-resolved spectral function via the double-time Green's functions. We find that the dip-hump structure evolves with the time delay. More interestingly, new phonon structures are obtained when the phonons are excited by a laser field. This signature may serve as a direct evidence for electron-vibration mode coupling. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A40.00007: Spontaneous phase separation instabilities in nanoclusters: bottom up approach Armen Kocharian, Gayanath Fernado, Kalum Palandage, James Davenport Pairing instabilities and inhomogeneities found from exact diagonalization of small (4 - 8 atoms) clusters in bipartite and non-bipartite topologies provide novel insights into several mysterious many body problems in condensed matter physics and ultracold fermionic atoms. Rigorous Nagaoka type criteria are formulated for spontaneous phase separation, electron pairing and magnetism driven by interaction strength, geometrical frustration, inter-site couplings (connectivity) and transverse magnetic field. The spin-charge separation of the bare electron degrees are manifested in spin and charge density response functions under variation of electron concentration, magnetic flux and temperature. The calculated phase diagrams in low and high spin regions display level crossings, pairing and magnetic instabilities seen in assembled clusters and nanoparticles. The phase diagrams in ensemble of octahedrons and pyramids also are remarkably similar to the number of inhomogeneous paired phases, spin pseudogap, coherent and incoherent pairings found in concentrated perovskites such as high Tc superconductors, manganites, magnetoelectric multiferroic nanomaterials probed by scanning tunneling spectroscopy. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A40.00008: Strong coupling picture of superconductivity in underdoped cuprates I: weak phase stiffness and mass divergence of d-wave superfluid Yucel Yildirim, Wei Ku Despite more than two decades of intensive investigations, the true nature of high temperature superconductivity observed in the cuprates remains elusive to the researchers. In particular, in the so-called ``underdoped'' region, the overall behavior of superconductivity deviates qualitatively from the standard BCS description. Recently, the importance of phase fluctuation of the superconducting order parameter, has gained significant support from various experiments. However, the microscopic mechanism responsible for the surprisingly soft phase remains one of the most important unsolved puzzles. Here, opposite to the standard BCS starting point, we propose a simple, solvable low-energy model in the strong coupling limit, which maps the superconductivity literally into a well-understood physics of superfluid in a special dilute bosonic system of local pairs. In the prototypical material $(La_{1-\delta}Sr_{\delta})_{2}CuO_4$, without the use of any free parameter, a d-wave superconductivity is obtained for doping above 5.2$\%$, below which unexpected incoherent p-wave pairs dominate. Throughout the whole underdoped region, very soft phases are found to originate from enormous mass enhancement of the pairs. Furthermore, a striking mass divergence is predicted that dictates the occurrence of the observed quantum critical point. Finally, good theoretical agreement with experiments will be presented. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A40.00009: Strong coupling picture of superconductivity in underdoped cuprates II: quasi-particle gap and its symmetry Wei Ku, Yucel Yildirim Recent observations of quasi-particle superconducting gap via ARPES and STM revealed important clues to the nature of superconductivity in the underdoped cuprates. Here, we study the quasi-particle gap theoretically in the presence of a larger pseudo-gap, within the strong coupling limit. Without any free parameter, the resulting quasi-particle gap is found to agree very well with the experimental observations. Most surprisingly, at very low doping, the symmetry is found to deviate significantly from the simple $d$-wave shape of the order parameter. In contrast to the Bogoliubov excitation, a new interpretation of the nature of the quasi-particles at the edge of the gap will be presented that is more suitable for the strong coupling region. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A40.00010: Superconductivity mediated by anharmonic phonons: application to $\beta$-pyrochlore oxides Kazumasa Hattori, Hirokazu Tsunetsugu We investigate three dimensional anharmonic phonons under tetrahedral symmetry and superconductivity mediated by these phonons. Three dimensional anharmonic phonon spectra are calculated directly by solving Schr\"odinger equation and the superconducting transition temperature is determined by using the theory of strong coupling superconductivity assuming an isotropic gap function. With increasing the third order anharmonicity $b$ of the tetrahedral potential, we find a crossover in the energy spectrum to a quantum tunneling regime. We obtain strongly enhanced transition temperatures around the crossover point. The first order transition observed in KOs$_2$O$_6$ is discussed in terms of the first excited state energy $\Delta$, and the coupling constant $\lambda$ in the strong coupling theory of superconductivity. Our results suggest that the decrease of $\lambda$ and increase of $\Delta$ below the first order transition temperature. We point out that the change in the oscillation amplitude $\langle x^2\rangle$ and $\langle xyz \rangle$ characterizes this isomorphic transition. The chemical trends of the superconducting transition temperature, $\lambda$, and $\Delta$ in the $\beta$-pyrochlore compounds are also discussed. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A40.00011: Electron phonon interaction in cuprates: manifestations in spectroscopy Andrey Mishchenko The ARPES and optical conductivity (OC) of cuprates is studied in the framework of the t-J-Holstein model. Both magnetic excitations of the t-J model and strong coupling to phonons are important to explain the ARPES and OC in the underdoped compounds [1]. The infrared OC of the underdoped compounds reveal the two band structure with low energy band arising due to phonon scattering and high energy peak emerging because of simultaneous emission of magnon and several phonons [2,3]. Both t-J and Holstein model fail to describe anomalous temperature dependence of ARPES whereas the t-J-Holstein model is in semi quantitative agreement with experiment [4]. Further generalization of the model to nonlocal electron-phonon couplings is considered [5]. Analysis of ARPES and OC data for larger dopings suggests fast decrease of the electron-phonon coupling strength with doping having universal dependence of the coupling strength on the in-plain hole doping concentration [2].\\[4pt] [1] A. S. Mishchenko, N. Nagaosa, Phys. Rev. Lett, 93 036402 (2004);\\[0pt] [2] A. S. Mishchenko et al, Phys. Rev. Lett, 100 166401 (2008);\\[0pt] [3] G. De Filippis et al, Acceepted to Phys. Rev. B (2009);\\[0pt] [4] V. Cataudella et al, Phys. Rev. Lett, 99 226402 (2007);\\[0pt] [5] G. De Filippis et al, Phys. Rev. Lett, 99 146405 (2007). [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A40.00012: Strong correlations and inhomogeneous environments in holographic superconductors S. Papanikolaou, R. Flauger, E. Pajer Holographic superconductors are strongly coupled superconductors that enjoy an explicit weakly coupled gravity dual description via the AdS/CFT correspondence. Even though the dual description includes a condensing U(1) scalar field, the superconducting state is certainly unconventional, but not explicitly known. We study the role played by inhomogeneous environments in holographic superconductors and compare it with the expectations from standard BCS-like theories. We study the variations of the critical temperature and the coherence length as a function of the amplitude and modulation wavelength of the imposed perturbations. Through the exact solutions of this model, we shed light and provide intuition on the role of inhomogeneities in unconventional superconductors. [Preview Abstract] |
Monday, March 15, 2010 10:48AM - 11:00AM |
A40.00013: Self-Trapping and Binding of Particles from Singular Pockets in the AFM Mott Insulator State of Cuprates Alvaro Rojo Bravo, Serguei Brazovski We study theoretically the formation of quasi-particules due to interactions with impurities and gap distortions in the insulating phase in a antiferromagnetic Mott insulator. Wave functions for single particles are distorted by interactions with collective modes as local distortions of the insulating gap or impurities, as shown by the experimental data. This effects give place to the apparition of bound sates which lower the total particle energy below the nominal level of the insulating gap. This features take a special importance in the lightly electron-doped materials, where the van Hove singularity (vHS) at the anti-nodal points $\left( \pi, 0\right)$ of the Brillouin zone, plays a major role in enhancing the effective mass of these bound states and in the self-localization through local distortion of the gap. [Preview Abstract] |
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