Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J1: Cuprate Superconductors: Underdoping and Pseudogaps |
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Sponsoring Units: DCMP Chair: Catherine Kallin, Brockhouse Institute for Materials Research Room: Colorado Convention Center Four Seasons 2-3 |
Tuesday, March 6, 2007 11:15AM - 11:51AM |
J1.00001: Nernst effect, fluctuation diamagnetism and vortices above Tc in cuprates Invited Speaker: Nernst-effect and torque magnetometry experiments have provided evidence that, in the hole-doped cuprates, long-range phase coherence vanishes at the critical temperature $T_c$, while the pair condensate survives to a much higher ``onset" temperature $T_{onset}$. In the Nernst experiment, the vortex current produced by a gradient generates a Josephson $E$-field perpendicular to the applied field $\bf H$. In cuprates, this large Nernst signal $e_N$ persists to $T_{onset}\sim$ 130 K. Extensive Nernst experiments in the cuprates LSCO, Bi 2201, and 2212 yield a 3D phase diagram $(x,T,H)$ in fields up to 45 T. This picture has been confirmed by high-resolution torque magnetometry. In a tilted $\bf H$, local planar supercurrents associated with vortices above $T_c$ produce a torque that deflects a cantilever. At each $T$, the diamagnetic magnetization inferred matches the field profile of the Nernst $e_N$. The high-resolution measurement of the diamagnetic susceptibility $\chi$ over 5 field decades uncovers an unusual, fragile ``London rigidity'' that exists in the pseudogap state of Bi 2212 and 2201. The magnetization curves below $T_c$ also provide a reliable determination of the upper critical field $H_{c2}$ which is found to scale linearly with $T_{onset}$. I will also preview evidence for pairing without phase coherence at 0.35 K in LSCO for $x < x_c$ in fields to 30-45 T. \newline \newline In collaboration with Yayu Wang, Lu Li, Joseph G. Checkelsky, Michael Naughton, Seiki Komiya, Shimpei Ono, Yoichi Ando, Shin-ichi Uchida and Genda Gu. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:27PM |
J1.00002: Controversial Issues in High-T$_c$ Superconductivity - a Specific Heat Perspective Invited Speaker: We briefly review specific heat data on the evolution with hole doping of HTS cuprates and discuss the results in terms of current models. We see a universal progression from insulator to overdoped metal via a states-non-conserving approximately V-shaped pseudogap in the qp DOS. The gap shrinks with p due to the accumulation of new spectral weight ($\sim$ 1 state per doped hole) on the shoulders of the pseudogap (the antinodal regions) and closes abruptly close to optimal doping accompanied by a rapid increase in superconducting (SC) condensation energy. Thermodynamic measurements show no features (even broadened) at the temperature T* at which the pseudogap is generally presumed to close, and that the spectral weight loss persists to temperatures well above T*. This suggests that the pseudogap is not due to a Fermi surface instability or precursor SC fluctuations and that the pristine Fermi surface is not restored at T*. Specific heat and NMR measurements also reveal a rather high degree of SC homogeneity, casting doubt on the popular inference of gross SC gap inhomogeneity revealed by some tunnelling studies. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 1:03PM |
J1.00003: From Fermi Arcs to the Nodal Metal Invited Speaker: The pseudogap phase in the copper oxide superconductors is a most unusual state of matter, and understanding its nature will likely resolve the issue of what interactions give rise to the superconductivity itself. Angle resolved photoemission has revealed that the pseudogap phase is characterized by a partially truncated Fermi surface, denoted as a Fermi arc. We have found that the arc length is proportional to T/T*, where T* is the pseuodgap temperature. Therefore, in the zero temperature limit, the pseudogap phase has the same nodal structure as the d-wave superconducting phase. Attempts to explain this novel behavior by a variety of theoretical models will be discussed, as well as the fate of these Fermi arcs once superconductivity sets in. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:39PM |
J1.00004: Effect of strong correlations on transport properties of disordered cuprates Invited Speaker: The theory of thermal transport in a $d$-wave superconductor predicts a universal $T$-linear term $\kappa_0$ at low temperatures. Measurements on several cuprate families down to the 50 milliKelvin range indicate that the linear term decreases with underdoping, from which a substantial increase of the slope of the order parameter near the nodes is usually deduced by comparison with the standard theory. We discuss ways in which low-$T$ universal transport can break down, and in particular focus on the importance of strong correlations, which can induce local magnetism in the presence of disorder or other spatial perturbations. Static magnetism coexisting with superconductivity has been detected in some but not all cuprate families, particularly at low temperatures and for strongly underdoped samples, We present an interpretation of this superconducting ``spin glass'' state as local antiferromagnetic order driven by dopant atoms, particularly in the LSCO and BSCCO systems. Within this framework, recent NMR experiments on Zn-doped YBCO can also be quantitatively explained, down to detailed description of the lineshapes. Both the strong correlations and the quantum interference of impurity states appear to be vital to understand these results. In either more disordered or more underdoped systems, the tendency towards static magnetism is enhanced. Numerical solutions of the Bogoliubov-de Gennes equations of a disordered $d$-wave superconductor with Hubbard-like correlations show that in this case $\kappa_0$ is in fact strongly suppressed, universality of quasiparticle transport is violated and $\kappa_0$ may no longer be used to extract the size of the gap near the node directly. \footnote{B.M. Andersen and P.J. Hirschfeld, cond-mat/0607682, J.W. Harter et al., cond-mat/0609721 } [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 2:15PM |
J1.00005: Inelastic Tunneling, Electronic Nanoscale Inhomogeneities and Local Pairing in Superconductor with Inhomogeneous Bosonic Modes Invited Speaker: There is a growing experimental evidence that nanoscale electronic inhomogeneity plays defining role in a growing classes of materials. Recently scanning tunneling spectroscopy has reached the stage where electronic properties of materials can be imaged on a nanometer scale. Local tunneling data that indicate strong nanoscale inhomogeneity of superconducting gap in high temperature superconductors[1,2]. Strong local nanoscale inhomogeneity in the bosonic scattering mode has also been observed in the same samples. We argue that these two inhomogeneities are directly related to each other. To address local boson scattering effects, we would need to develop a local strong coupling model of pairing in a coarse grained superconducting state. I will present a simple strong coupling model that yields features that are broadly consistent with the doping and isotope substitution trends observed experimentally. Oxygen isotope substitution O16 -$>$O18 reveals nontrivial changes in boson mode energy. These changes and changes in electron-boson coupling will also be discussed. [1] A. V. Balatsky and J.-X. Zhu, Local Strong Coupling Pairing in d-wave superconductors, Phys. Rev. B \textbf{74}, 094517 (2006). A.V. Balatsky, Ar. Abanov and J.X. Zhu, Inelastic Tunneling Spectroscopy in d-wave Superconductor, Phys. Rev. B 68, 214506 (2003). J. X. Zhu et al, Effects of Collective Spin Resonance Mode on STM spectra in d-wave Superconductor, Phys. Rev. Lett. v 92, 017002, (2004). [2] J. Lee et al, Interplay of electron--lattice interactions and superconductivity in Bi2Sr2CaCu2O8 , Nature, v \textbf{442}, p 546, (2006). [Preview Abstract] |
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