452 research outputs found
Non-perturbative approach to nearly antiferromagnetic Fermi liquids
We present a non-perturbative approach to the problem of quasiparticles
coupled to spin-fluctuations. If the fully dressed spin-fluctuation propagator
is used in the Feynman graph expansion of the single-particle Green's function,
the problem of summing all spin-fluctuation exchange graphs (i.e without
virtual fermion loops) can be cast as a functional integral over gaussian
distributed random vector fields. A Monte Carlo sampling of this functional
integral does not suffer from the 'fermion sign problem' and offers an
attractive alternative to perturbative calculations. We compare the results of
our computer simulations with perturbation theory and self-consistent one-loop
calculations.Comment: 11 pages, 4 figure
The Influence of Magnetic Imperfections on the Low Temperature Properties of D-wave Superconductors
We consider the influence of planar ``magnetic" imperfections which destroy
the local magnetic order, such as Zn impurities or vacancies, on the
low temperature properties of the cuprate superconductors. In the unitary
limit, at low temperatures, for a pairing state such
imperfections produce low energy quasiparticles with an anistropic spectrum in
the vicinity of the nodes. We find that for the system, one
is in the {\em quasi-one-dimensional} regime of quasiparticle scattering,
discussed recently by Altshuler, Balatsky, and Rosengren, for impurity
concentrations in excess of whereas YBCO appears likely to be
in the true 2D scattering regime for Zn concentrations less than . We
show the neutron scattering results of Mason et al. \cite{Aeppli} on
provide strong evidence for ``dirty d-wave"
superconductivity in their samples. We obtain simple expressions for the
dynamic spin susceptibility and spin-lattice relaxation time,
, in the superconducting state.Comment: 10 pages; revtex; Los Alamos preprint LA-UR-94-53
Magnetically mediated superconductivity: Crossover from cubic to tetragonal lattice
We compare predictions of the mean-field theory of superconductivity for
nearly antiferromagnetic and nearly ferromagnetic metals for cubic and
tetragonal lattices. The calculations are based on the parametrization of an
effective interaction arising from the exchange of magnetic fluctuations and
assume that a single band is relevant for superconductivity. The results show
that for comparable model parameters, the robustness of magnetic pairing
increases gradually as one goes from a cubic structure to a more and more
anisotropic tetragonal structure either on the border of antiferromagnetism or
ferromagnetism.Comment: 16 pages 14 figure
Density Fluctuation Mediated Superconductivity
We conpare predictions of the mean-field theory of supercnductivity for
metallic systems on the border of a density instability for cubic and
tetragonal lattices. The calculations are based on a parametrisation of an
effective interaction arising from the exchange of density fluctuations and
assume that a single band is relevant for superconductivity. The results show
that for comparable model parameters, desnity fluctuation mediated pairing is
more robust in quasi-two dimensions than in three dimensions, and that the
robustness of pairing increases gradually as one goes from a cubic to a more
and more anisotropic tetragonal structure. We also find that the robustness of
density fluctuation mediated pairing can depend sensitively on the incipient
ordering wavevector. We discuss the similarities and differences between the
mean-field theories of superconductivity for density and magnetically mediated
pairing
Weak Pseudogap Behavior in the Underdoped Cuprate Superconductors
We report on an exact solution of the nearly antiferromagnetic Fermi liquid
spin fermion model in the limit \pi T << \omega_{sf}, which demonstrates that
the broad high energy features found in ARPES measurements of the spectral
density of the underdoped cuprate superconductors are determined by strong
antiferromagnetic (AF) correlations and precursor effects of an SDW state. We
show that the onset temperature, T^{cr}, of weak pseudo-gap (pseudoscaling)
behavior is determined by the strength, \xi, of the AF correlations, and obtain
the generic changes in low frequency magnetic behavior seen in NMR experiments
with \xi(T^{cr}) \approx 2, confirming the Barzykin and Pines crossover
criterion.Comment: REVTEX, 4 pages, 3 EPS figure
On the vertex corrections in antiferromagnetic spin fluctuation theories
We argue that recent calculations by Amin and Stamp (PRL 77, 301 (1996);
cond-mat/9601086) overestimate the strength of the vertex corrections in the
spin-fermion model for cuprates. We clarify the physical origin of the apparent
discrepancy between their results and earlier calculations. We also comment on
the relative sign of the vertex correction.Comment: 3 pages, Revtex, 1 figure, ps-file also available at
http://lifshitz.physics.wisc.edu/www/morr/morr_homepage.htm
Sensitivity of the Superconducting Transition Temperature to Changes in the Spin-Fluctuation Spectral Weight
In the simplest model of magnetic pairing, the transition temperature to the
superconducting state depends on the dynamical susceptibility . We discuss how is affected by different momentum and
frequency parts of for nearly antiferromagnetic and
nearly ferromagnetic metals in two dimensions. While in the case of
phonon-mediated superconductivity any addition of spectral weight to
at leads to an increase in , we find that
adding magnetic spectral weight at any momentum and low frequencies
( and for nearly antiferromagnetic and ferromagnetic
metals respectively) leads to a suppression of . The most effective
frequency and momentum range consists of large momenta
and frequencies around for nearly antiferromagnetic metals and small
momenta and frequencies of approximately for
nearly ferromagnetic metals.Comment: 18 pages, 39 figure
Diagrammatic perturbation theory and the pseudogap
We study a model of quasiparticles on a two-dimensional square lattice
coupled to Gaussian distributed dynamical fields. The model describes
quasiparticles coupled to spin or charge fluctuations and is solved by a Monte
Carlo sampling of the molecular field distributions. The non-perturbative
solution is compared to various approximations based on diagrammatic
perturbation theory. When the molecular field correlations are sufficiently
weak, the diagrammatic calculations capture the qualitative aspects of the
quasiparticle spectrum. For a range of model parameters near the magnetic
boundary, we find that the quasiparticle spectrum is qualitatively different
from that of a Fermi liquid in that it shows a double peak structure, and that
the diagrammatic approximations we consider fail to reproduce, even
qualitatively, the results of the Monte Carlo calculations. This suggests that
the pseudogap induced by a coupling to antiferromagnetic fluctuations and the
spin-splitting of the quasiparticle peak induced by a coupling to ferromagnetic
spin-fluctuations lie beyond diagrammatic perturbation theory
Electronic Structure of Sodium Cobalt Oxide: Comparing Mono- and Bilayer-hydrate
To shed new light on the mechanism of superconductivity in sodium cobalt
oxide bilayer-hydrate (BLH), we perform a density functional calculation with
full structure optimization for BLH and its related nonsuperconducting phase,
monolayer hydrate (MLH). We find that these hydrates have similar band
structures, but a notable difference can be seen in the band around
the Fermi level. While its dispersion in the direction is negligibly small
for BLH, it is of the order of 0.1 eV for MLH. This result implies that the
three dimensional feature of the band may be the origin for the
absence of superconductivity in MLH.Comment: 5 pages, 7 figures, to be published in Phys. Rev.
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