11 research outputs found
Quasiparticle Density of States, Localization, and Distributed Disorder in the Cuprate Superconductors
We explore the effects of various kinds of random disorder on the
quasiparticle density of states of two-dimensional d-wave superconductors using
an exact real-space method, incorporating realistic details known about the
cuprates. Random on-site energy and pointlike unitary impurity models are found
to give rise to a vanishing DOS at the Fermi energy for narrow distributions
and low concentrations, respectively, and lead to a finite, but suppressed, DOS
at unrealistically large levels of disorder. Smooth disorder arising from
impurities located away from the copper-oxide planes meanwhile gives rise to a
finite DOS at realistic impurity concentrations. For the case of smooth
disorder whose average potential is zero, a resonance is found at zero energy
for the quasiparticle DOS at large impurity concentrations. We discuss the
implications of these results on the computed low-temperature specific heat,
the behavior of which we find is strongly affected by the amount of disorder
present in the system. We also compute the localization length as a function of
disorder strength for various types of disorder and find that intermediate- and
high-energy states are quasi-extended for low disorder, and that states near
the Fermi energy are strongly localized and have a localization length that
exhibits an unusual dependence on the amount of disorder. We comment on the
origin of disorder in the cuprates and provide constraints on these based on
known results from scanning tunneling spectroscopy and specific heat
experiments.Comment: 29 pages, 19 figures, published version, includes minor change
Revisiting Quasiparticle Scattering Interference in High-Temperature Superconductors: The Problem of Narrow Peaks
We revisit the interpretation of quasiparticle scattering interference in
cuprate high- superconductors. This phenomenon has been very successful in
reconstructing the dispersions of d-wave Bogoliubov excitations, but the
successful identification and interpretation of QPI in scanning tunneling
spectroscopy (STS) experiments rely on theoretical results obtained for the
case of isolated impurities. We introduce a highly flexible technique to
simulate STS measurements by computing the local density of states using
real-space Green's functions defined on two-dimensional lattices with as many
as 100,000 sites. We focus on the following question: to what extent can the
experimental results be reproduced when various forms of distributed disorder
are present? We consider randomly distributed point-like impurities, smooth
"Coulombic" disorder, and disorder arising from random on-site energies and
superconducting gaps. We find an apparent paradox: the QPI peaks in the
Fourier-transformed local density of states appear to be sharper and better
defined in experiment than those seen in our simulations. We arrive at a no-go
result for smooth-potential disorder since this does not reproduce the QPI
peaks associated with large-momentum scattering. An ensemble of point-like
impurities gets closest to experiment, but this goes hand in hand with impurity
cores that are not seen in experiment. We also study the effects of possible
measurement artifacts (the "fork mechanism"), which turn out to be of
relatively minor consequence. It appears that there is an unknown mechanism at
work which renders the QPI peaks much sharper than they are based on present
theoretical understanding.Comment: 23 pages, 25 figures, published version, includes minor change
Revisiting quasiparticle scattering interference in high-temperature superconductors: The problem of narrow peaks
Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination
Complete theoretical understanding of the most complex superconductors
requires a detailed knowledge of the symmetry of the superconducting energy-gap
, for all momenta on the Fermi surface
of every band . While there are a variety of techniques for determining
, no general method existed to measure the signed
values of . Recently, however, a new technique based
on phase-resolved visualization of superconducting quasiparticle interference
(QPI) patterns centered on a single non-magnetic impurity atom, was introduced.
In principle, energy-resolved and phase-resolved Fourier analysis of these
images identifies wavevectors connecting all k-space regions where
has the same or opposite sign. But use of a single
isolated impurity atom, from whose precise location the spatial phase of the
scattering interference pattern must be measured is technically difficult. Here
we introduce a generalization of this approach for use with multiple impurity
atoms, and demonstrate its validity by comparing the
it generates to the determined from single-atom
scattering in FeSe where energy-gap symmetry is established. Finally,
to exemplify utility, we use the multi-atom technique on LiFeAs and find
scattering interference between the hole-like and electron-like pockets as
predicted for of opposite sign
Phase-sensitive determination of nodal d-wave order parameter in single-band and multiband superconductors
Determining the exact pairing symmetry of the superconducting order parameter in candidate unconventional superconductors remains an important challenge. Recently, a new method, based on phase sensitive quasiparticle interference measurements, was developed to identify gap sign changes in isotropic multiband systems. Here we extend this approach to the single-band and multiband nodal d-wave superconducting cases relevant, respectively, for the cuprates and likely for the infinite-layer nickelate superconductors. Combining analytical and numerical calculations, we show that the antisymmetrized correction to the tunneling density of states due to nonmagnetic impurities in the Born limit and at intermediate-scattering strength shows characteristic features for sign-changing and sign-preserving scattering wave vectors, as well as for the momentum-integrated quantity. Furthermore, using a realistic approach accounting for the Wannier orbitals, we model scanning tunneling microscopy data of Bi2Sr2CaCu2O8+delta, which should allow the comparison of our theory with experimental data
Puddle formation, persistent gaps, and non-mean-field breakdown of superconductivity in overdoped (Pb,Bi)2Sr2CuO6+{\delta}
The cuprate high-temperature superconductors exhibit many unexplained
electronic phases, but it was often thought that the superconductivity at
sufficiently high doping is governed by conventional mean-field
Bardeen-Cooper-Schrieffer (BCS) theory[1]. However, recent measurements show
that the number of paired electrons (the superfluid density) vanishes when the
transition temperature Tc goes to zero[2], in contradiction to expectation from
BCS theory. The origin of this anomalous vanishing is unknown. Our scanning
tunneling spectroscopy measurements in the overdoped regime of the
(Pb,Bi)2Sr2CuO6+{\delta} high-temperature superconductor show that it is due to
the emergence of puddled superconductivity, featuring nanoscale superconducting
islands in a metallic matrix[3,4]. Our measurements further reveal that this
puddling is driven by gap filling, while the gap itself persists beyond the
breakdown of superconductivity. The important implication is that it is not a
diminishing pairing interaction that causes the breakdown of superconductivity.
Unexpectedly, the measured gap-to-filling correlation also reveals that
pair-breaking by disorder does not play a dominant role and that the mechanism
of superconductivity in overdoped cuprate superconductors is qualitatively
different from conventional mean-field theory