Quasi-1D dynamics and nematic phases in the 2D Emery model

We consider the Emery model of a Cu-O plane of the high temperature superconductors. We show that in a strong-coupling limit, with strong Coulomb repulsions between electrons on nearest-neighbor O sites, the electron-dynamics is strictly one dimensional, and consequently a number of asymptotically exact results can be obtained concerning the electronic structure. In particular, we show that a nematic phase, which spontaneously breaks the point- group symmetry of the square lattice, is stable at low enough temperatures and strong enough coupling.Comment: 8 pages, 5 eps figures; revised manuscript with more detailed discussions; two new figures and three edited figuresedited figures; 14 references; new appendix with a detailed proof of the one-dimensional dynamics of the system in the strong coupling limi

Avoided Critical Behavior in a Uniformly Frustrated System

We study the effects of weak long-ranged antiferromagnetic interactions of strength $Q$ on a spin model with predominant short-ranged ferromagnetic interactions. In three dimensions, this model exhibits an avoided critical point in the sense that the critical temperature $T_c(Q=0)$ is strictly greater than $\lim_{Q\to 0} T_c(Q)$. The behavior of this system at temperatures less than $T_c(Q=0)$ is controlled by the proximity to the avoided critical point. We also quantize the model in a novel way to study the interplay between charge-density wave and superconducting order.Comment: 32 page Latex file, figures available from authors by reques

Properties of an orbital Kondo array

We consider a solvable model of a one dimensional electron gas interacting with an array of dynamical scattering centers, whose state is specified by a pseudospin variable. In the dilute limit, for frequencies w and temperatures T below the single-center Kondo scale but above a coherence scale {delta}, the physics is governed by the fixed point of the single-impurity two-channel Kondo problem. In that region, the physical properties are governed by composite (or odd-frequency) pairing fluctuations and they are reminiscent of the normal state of high temperature superconductors. As {omega} and T {yields} 0, three susceptibilities are equally divergent: (1) conventional, spin-singlet even-parity pairing (2) composite spin-singlet odd-parity 77-pairing and (3) odd parity pseudospin

Quasi-Fermi Distribution and Resonant Tunneling of Quasiparticles with Fractional Charges

We study the resonant tunneling of quasiparticles through an impurity between the edges of a Fractional Quantum Hall sample. We show that the one-particle momentum distribution of fractionally charged edge quasiparticles has a quasi-Fermi character. The density of states near the quasi-Fermi energy at zero temperature is singular due to the statistical interaction of quasiparticles. Another effect of this interaction is a new selection rule for the resonant tunneling of fractionally charged quasiparticles: the resonance is suppressed unless an integer number of {\em electrons} occupies the impurity. It allows a new explanation of the scaling behavior observed in the mesoscopic fluctuations of the conductivity in the FQHE.Comment: 7 pages, REVTeX 3.0, Preprint SU-ITP-93-1

Distribution of spectral weight in a system with disordered stripes

The ``band-structure'' of a disordered stripe array is computed and compared, at a qualitative level, to angle resolved photoemission experiments on the cuprate high temperature superconductors. The low-energy states are found to be strongly localized transverse to the stripe direction, so the electron dynamics is strictly one-dimensional (along the stripe). Despite this, aspects of the two dimensional band-structure Fermi surface are still vividly apparent.Comment: 10 pages, 11 figure

Pairing and Phase Coherence in High Temperature Superconductors

Mobile holes in an antiferromagnetic insulator form a slowly fluctuating array of quasi one-dimensional metallic stripes, which induce a spin gap or pseudogap in the intervening Mott-insulating regions. The mobile holes on an individual stripe acquire a spin gap via pair hopping between the stripe and its environment; i.e. via a magnetic analog of the usual superconducting proximity effect. This process is the analog of pairing in conventional superconductors. At non-vanishing stripe densities, Josephson coupling between stripes produces a dimensional crossover to a state with long-range superconducting phase coherence. In contrast to conventional superconductors, the superconducting state is characterised by a high density of (spin) pairs, but the phase stiffness, which is determined by the density and mobility of holes on the stripes, is very low.Comment: 4 pages. Proceedings of MMS-High Temperature Superconductivity, V (1997) To be published in Physica

Making High Tc_c Higher: A Theoretical Proposal

There is considerable evidence that the highest $T_c$ obtainable in a copper-oxide plane is limitted by the competition between two effects: On the one hand, as the concentration of doped-holes, $x$, is increased, the pairing scale, which is related to the properties of a doped Mott insulator, decreases. On the otherhand, the superfluid density, which controls the stiffness of the system to phase fluctuations, vanishes as $x \to 0$, and increases with increasing $x$. Optimal $T_c$ is obtained at a crossover from a phase ordering dominated regime at small $x$ to a pairing dominated regime at large $x$. If this description is valid, then higher $T_c$'s can be obtained in an array of coupled planes with different doped hole concentrations, such that a high pairing scale is derived from the underdoped planes and a large phase stiffness from the optimally or overdoped ones.Comment: 6 page

Metallic stripes in high-temperature superconductors

A phenomenological approach is applied to explore signatures of disordered charge stripes and antiphase spin domains in single-particle properties of the high-temperature superconductors. Stripe phases are shown to explain many experimentally observed unusual features measured in angle-resolved photoemission and optical spectroscopy. It is argued that disordered and fluctuating stripe phases are a common feature of high-temperature superconductors, supported by the additional evidence from neutron scattering and NMR

Can mesoscopic fluctuations reverse the supercurrent through a disordered Josephson junction?

We calculate the Josephson coupling energy $U_J(\phi)$ (related to the supercurrent $I=(2e/\hbar) dU_J/d\phi$) for a disordered normal metal between two superconductors with a phase difference $\phi$. We demonstrate that the symmetry of the scattering matrix of non-interacting quasiparticles in zero magnetic field implies that $U_J(\phi)$ has a minimum at $\phi=0$. A maximum (that would lead to a $\pi$-junction or negative superfluid density) is excluded for any realization of the disorder.Comment: 2 page

Crossovers and Phase Coherence in Cuprate Superconductors

High temperature superconductivity is a property of doped antiferromagnetic insulators. The electronic structure is inhomogeneous on short length and time scales, and, as the temperature decreases, it evolves via two crossovers, before long range superconducting order is achieved. Except for overdoped materials, pairing and phase coherence occur at different temperatures, and phase fluctuations determine both T$_c$ and the temperature dependence of the superfluid density for a wide range of doping. A mechanism for obtaining a high pairing scale in a short coherence length material with a strong poorly-screened Coulomb interaction is described.Comment: 5 pages, Latex, Revte