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D. J. Scalapino

Publications and source records attributed to D. J. Scalapino.

At least 91 records · Page 5Linked to original sources
arXiv2002

Differences Between Hole and Electron Doping of a Two-Leg CuO Ladder

Here we report results of a density-matrix-renormalization-group (DMRG) calculation of the charge, spin, and pairing properties of a two-leg CuO Hubbard ladder. The outer oxygen atoms as well as the rung and leg oxygen atoms are included along with near-neighbor and oxygen-hopping matrix elements. This system allows us to study the effects of hole and electron doping on a system which is a charge transfer insulator at a filling of one hole per Cu and exhibits power law, d-wave-like pairing correlations when doped. In particular, we focus on the differences between doping with holes or electrons.

cond-mat.str-el↗
arXiv2002

Calculation of Delta (k,omega) for a 2D t-J cluster

Using numerical techniques, the diagonal and off-diagonal superconducting one-electron Green's functions are calculated for a two-dimensional (2D) t-J model on a periodic 32-site cluster at low doping. From these Green's functions, the momentum and frequency dependence of the pairing gap $Δ({\bf k}, ω)$ are extracted. It has $d_{x^2-y^2}$ symmetry and exhibits $ω$-dependent structure which depend upon J/t. We find that the pairing gap persists down to small J/t values. The frequency- and momentum-dependent renormalized energy and renormalization factor are also calculated.

cond-mat.str-el↗
arXiv2001

Electronic Correlations Near a Peierls-CDW Transition

Results of a phenomenological Monte carlo calculation for a 2D electron-phonon Holstein model near a Peierls-CDW transition are presented. Here the zero Matsubara frequency part of the phonon action is dominant and we approximated it by a phenomenological form that as an Ising-like Peierls-CDW transition. The resulting model is studied on a 32 by 32 lattice. The single particle spectral weight A(k,ω), the density of states N(ω), and the real part of the conductivity σ_1(ω) all show evidence of a pseudogap which develops in the low-energy electronic degrees of freedom as the Peierls-CDW transition is approached

cond-mat.str-el↗
arXiv2001

The 2-Leg Hubbard Ladder: Computational Studies of New Materials

Computational studies of basic models of strongly-correlated electron systems can provide guidance in the search for new materials as well as insight into the physical mechanisms responsible for their properties. Here, we illustrate this by examining what numerical computations have shown us about the 2-leg Hubbard ladder.

cond-mat.str-el↗
arXiv2001

Fractionalized Phase in an XY-Z_2 Gauge Model

We study a model with fractional quantum numbers using Monte Carlo techniques. The model is composed of bosons interacting though a $Z_2$ gauge field. We find that the system has three phases: a phase in which the bosons are confined, a fractionalized phase in which the bosons are deconfined, and a phase in which the bosons are condensed. The deconfined phase has a ``topological'' order due to the degeneracy in the ground state of the gauge field. We discuss an experimental test proposed by Senthil and Fisher that uses the topological order to determine the existence of a deconfined, fractionalized phase.

cond-mat.str-el↗
arXiv2001

Rung-Rung Current Correlations on a 2-Leg t-J Ladder

We report the results of numerical calculations of rung-rung current correlations on a 2-leg t-J ladder with J/t=0.35 for dopings x=0.125 and x=0.19. We find that the amplitude of these correlations decays exponentially. We argue that this can be understood within a bosonization framework in terms of the pinned phase variables associated with a C1SO phase with d_{x^2-y^2}-like power law pairing correlations.

cond-mat.str-el↗
arXiv2000

A New Approach to Doped Mott Insulators

We describe a new microscopic approach for analyzing interacting electron systems with local moments or, in principle, any local order parameter. We specialize attention to the doped Mott insulator phase of the Hubbard model, where standard weak-coupling perturbation methods fail. A rotationally invariant Stratonovich-Hubbard field is introduced to decouple the static spin components of the interaction at each site. Charge degrees of freedom are then treated by ``slave'' Hartree-Fock in the presence of a spatially varying random spin field. The effective action reduces to a classical Heisenberg model at half filling, insuring that the system has (i) finite-range order at T>0 with an exponentially diverging correlation length and (ii) a one-electron Mott-Hubbard gap in the presence of disorder. Away from half-filling properties are determined by strongly non-Gaussian fluctuations in the amplitude and orientation of the local spin fields. The saddle point equations of the theory at zero temperature reduce to inhomogeneous Hartree-Fock, so that disordering of domain walls at finite temperature is in principle included. We present preliminary small system results for the intermediate and large interaction regimes obtained by Monte Carlo simulation of random spin field configurations.

cond-mat.str-el↗
arXiv2000

Some Recent Issues in Quantum Magnetism

Summary of Quantum Magnetism Conference, Institute for Theoretical Physics, University of California, Santa Barbara, California, August 16-20, 1999.

cond-mat.str-el↗
arXiv2000

Evolution of the Spin Gap Upon Doping a 2-Leg Ladder

The evolution of the spin gap of a 2-leg ladder upon doping depends upon the nature of the lowest triplet excitations in a ladder with two holes. Here we study this evolution using various numerical techniques for a t-t'-J ladder as the next-near-neighbor hopping t' is varied. We find that depending on the value of t', the spin gap can evolve continuously or discontinuously and the lowest triplet state can correspond to a magnon, a bound magnon-hole-pair, or two separate quasi-particles. Previous experimental results on the superconducting two-leg ladder Sr12Ca2Cu24O41 are discussed.

cond-mat↗
arXiv1999

Phase separation in t-J ladders

The phase separation boundary of isotropic t-J ladders is analyzed using density matrix renormalization group techniques. The complete boundary to phase separation as a function of J/t and doping is determined for a chain and for ladders with two, three and four legs. Six-chain ladders have been analyzed at low hole doping. We use a direct approach in which the phase separation boundary is determined by measuring the hole density in the part of the system which contains both electrons and holes. In addition we examine the binding energy of multi-hole clusters. An extrapolation in the number of legs suggests that the lowest J/t for phase separation to occur in the two dimensional t-J model is J/t~1.

cond-mat.str-el↗
arXiv1999

Frustrated Hubbard ladders and superconductivity in $κ$-BEDT-TTF organic compounds

Half-filled two-leg Hubbard ladders have spin-gapped short-range antiferromagnetic correlations while three-leg ladders have power law antoferromagnetic correlations, and both systems have d_{x^2-y^2}-power law pairing correlations when they are doped. Thus these ladders exhibit some of the phenomenology seen in the layered cuprates. Here we report results for half-filled frustrated Hubbard ladders, based upon ladder segments taken from a tight-binding model of kappa-BEDT-TTF. Although these ladders are half-filled, varying the degree of frustration can drive them across an insulator-metal transition. We suggest that the spin, charge and pairing correlations of these frustrated ladders near the insulator-metal transition provide support for the notion that kappa-BEDT-TTF is a strongly correlated superconductor.

cond-mat.supr-con↗
arXiv1999

Effect of the W-term for a t-U-W Hubbard ladder

Antiferromagnetic and d_{x2-y2}-pairing correlations appear delicately balanced in the 2D Hubbard model. Whether doping can tip the balance to pairing is unclear and models with additional interaction terms have been studied. In one of these, the square of a local hopping kinetic energy H_W was found to favor pairing. However, such a term can be separated into a number of simpler processes and one would like to know which of these terms are responsible for enhancing the pairing. Here we analyze these processes for a 2-leg Hubbard ladder.

cond-mat.str-el↗
arXiv1999

Pairing Correlations on t-U-J Ladders

Pairing correlations on generalized t-U-J two-leg ladders are reported. We find that the pairing correlations on the usual t-U Hubbard ladder are significantly enhanced by the addition of a nearest-neighbor exchange interaction J. Likewise, these correlations are also enhanced for the t-J model when the onsite Coulomb interaction is reduced from infinity. Moreover, the pairing correlations are larger on a t-U-J ladder than on a t-Jeff ladder in which Jeff has been adjusted so that the two models have the same spin gap at half-filling. This enhancement of the pairing correlations is associated with an increase in the pair-binding energy and the pair mobility in the t-U-J model and point to the importance of the charge transfer nature of the cuprate systems.

cond-mat.str-el↗
arXiv1999

Phase Transitions Between Topologically Distinct Gapped Phases in Isotropic Spin Ladders

We consider various two-leg ladder models exhibiting gapped phases. All of these phases have short-ranged valence bond ground states, and they all exhibit string order. However, we show that short-ranged valence bond ground states divide into two topologically distinct classes, and as a consequence, there exist two topologically distinct types of string order. Therefore, not all gapped phases belong to the same universality class. We show that phase transitions occur when we interpolate between models belonging to different topological classes, and we study the nature of these transitions.

cond-mat.str-el↗
arXiv1999

Phase separation and stripe formation in the 2D t-J model: a comparison of numerical results

We make a critical analysis of numerical results for and against phase separation and stripe formation in the t-J model. We argue that the frustrated phase separation mechanism for stripe formation requires phase separation at too high a doping for it to be consistent with existing numerical studies of the t-J model. We compare variational energies for various methods, and conclude that the most accurate calculations for large systems appear to be from the density matrix renormalization group. These calculations imply that the ground state of the doped t-J model is striped, not phase separated.

cond-mat.str-el↗
arXiv1999

Superconductivity and Spin Fluctuations

The organizers of the Memorial Session for Herman Rietschel asked that I review some of the history of the interplay of superconductivity and spin fluctuations. Initially, Berk and Schrieffer showed how paramagnon spin fluctuations could suppress superconductivity in nearly-ferromagnetic materials. Following this, Rietschel and various co-workers wrote a number of papers in which they investigated the role of spin fluctuations in reducing the T_c of various electron-phonon superconductors. Paramagnon spin fluctuations are also believed to provide the p-wave pairing mechanism responsible for the superfluid phases of $^3He$. More recently, antiferromagnetic spin fluctuations have been proposed as the mechanism for d-wave pairing in the heavy-fermion superconductors and in some organic materials as well as possibly the high T_c cuprates. Here I will review some of this early history and discuss some of the things we have learned more recently from numerical simulations.

cond-mat.str-el↗
arXiv1999

Nonlinear Current Response of a d-wave Superfluid

Despite several efforts the nonlinear Meissner effect in d-wave superconductors, as has been discussed by Yip and Sauls in 1992, has not been verified experimentally in high-Tc superconductors at present. Here, we reinvestigate the nonlinear response expected in a d-wave superconductor. While the linear H field dependence of the penetration depth, predicted by Yip and Sauls, is restricted by the lower critical field and can be masked by nonlocal effects, we argue that the upturn of the nonlinear coefficient of the quadratic field dependence is more stable and remains observable over a broader range of parameters. We investigate this by studying the influence of nonmagnetic impurities on the nonlinear response. We discuss the difficulties of observing this intrinsic d-wave signature in present day high-Tc films and single crystals.

cond-mat.supr-con↗