695 research outputs found
Gap States in Dilute Magnetic Alloy Superconductors
We study states in the superconducting gap induced by magnetic impurities
using self-consistent quantum Monte Carlo with maximum entropy and formally
exact analytic continuation methods. The magnetic impurity susceptibility has
different characteristics for T_{0} \alt T_{c0} and T_{0} \agt T_{c0}
(: Kondo temperature, : superconducting transition temperature)
due to the crossover between a doublet and a singlet ground state. We
systematically study the location and the weight of the gap states and the gap
parameter as a function of and the concentration of the
impurities.Comment: 4 pages in ReVTeX including 4 encapsulated Postscript figure
Competition between electron-phonon attraction and weak Coulomb repulsion
The Holstein-Hubbard model is examined in the limit of infinite dimensions.
Conventional folklore states that charge-density-wave (CDW) order is more
strongly affected by Coulomb repulsion than superconducting order because of
the pseudopotential effect. We find that both incommensurate CDW and
superconducting phases are stabilized by the Coulomb repulsion, but,
surprisingly, the commensurate CDW transition temperature is more robust than
the superconducting transition temperature. This puzzling feature is resolved
by a detailed analysis of perturbation theory.Comment: 13 pages in ReVTex including 3 encapsulated postscript files
(embedded in the text). The encapsulated postscript files are compressed and
uuencoded after the TeX file
The anharmonic electron-phonon problem
The anharmonic electron-phonon problem is solved in the infinite-dimensional
limit using quantum Monte Carlo simulation. Charge-density-wave order is seen
to remain at half filling even though the anharmonicity removes the
particle-hole symmetry (and hence the nesting instability) of the model.
Superconductivity is strongly favored away from half filling (relative to the
charge-density-wave order) but the anharmonicity does not enhance transition
temperatures over the maximal values found in the harmonic limit.Comment: 5 pages typeset in ReVTeX. Four encapsulated postscript files
include
Role of the van Hove Singularity in the Quantum Criticality of the Hubbard Model
A quantum critical point (QCP), separating the non-Fermi liquid region from
the Fermi liquid, exists in the phase diagram of the 2D Hubbard model
[Vidhyadhiraja et. al, Phys. Rev. Lett. 102, 206407 (2009)]. Due to the
vanishing of the critical temperature associated with a phase separation
transition, the QCP is characterized by a vanishing quasiparticle weight. Near
the QCP, the pairing is enhanced since the real part of the bare d-wave p-p
susceptibility exhibits algebraic divergence with decreasing temperature,
replacing the logarithmic divergence found in a Fermi liquid [Yang et. al,
Phys. Rev. Lett. 106, 047004 (2011)]. In this paper we explore the
single-particle and transport properties near the QCP. We focus mainly on a van
Hove singularity (vHS) coming from the relatively flat dispersion that crosses
the Fermi level near the quantum critical filling. The flat part of the
dispersion orthogonal to the antinodal direction remains pinned near the Fermi
level for a range of doping that increases when we include a negative
next-near-neighbor hopping t' in the model. For comparison, we calculate the
bare d-wave pairing susceptibility for non-interacting models with the usual
two-dimensional tight binding dispersion and a hypothetical quartic dispersion.
We find that neither model yields a vHS that completely describes the critical
algebraic behavior of the bare d-wave pairing susceptibility. The resistivity,
thermal conductivity, thermopower, and the Wiedemann-Franz Law are examined in
the Fermi liquid, marginal Fermi liquid, and pseudo-gap doping regions. A
negative next-near-neighbor hopping t' increases the doping region with
marginal Fermi liquid character. Both T and negative t' are relevant variables
for the QCP, and both the transport and the motion of the vHS with filling
suggest that they are qualitatively similar in their effect.Comment: 15 pages, 17 figure
Correlation and surface effects in Vanadium Oxides
Recent photoemission experiments have shown strong surface modifications in
the spectra from vanadium oxides as (V,Cr)_2O_3 or
(Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent
part of the surface spectrum is narrowed as compared to the bulk. The
quasiparticle weight is more sensitive at the surface than in the bulk against
bandwidth variations. We investigate these effects theoretically considering
the single-band Hubbard model for a film geometry. A simplified dynamical
mean-field scheme is used to calculate the main features of the interacting
layer-dependent spectral function. It turns out that the experimentally
confirmed effects are inherent properties of a system of strongly correlated
electrons. The reduction of the weight and the variance of the coherent part of
the surface spectrum can be traced back to the reduced surface coordination
number. Surface correlation effects can be strongly amplified by changes of the
hopping integrals at the surface.Comment: to appear in PRB; 8 pages, 6 figure
Weak-coupling expansions for the attractive Holstein and Hubbard models
Weak-coupling expansions (conserving approximations) are carried out for the
attractive Holstein and Hubbard models (on an infinite-dimensional hypercubic
lattice) that include all bandstructure and vertex correction effects. Quantum
fluctuations are found to renormalize transition temperatures by factors of
order unity, but may be incorporated into the superconducting channel of
Migdal-Eliashberg theory by renormalizing the phonon frequency and the
interaction strength.Comment: 10 pages, (five figures available from the author by request) typeset
with ReVTeX, preprint NSF-ITP-93-10
Iterated perturbation theory for the attractive Holstein and Hubbard models
A strictly truncated (weak-coupling) perturbation theory is applied to the
attractive Holstein and Hubbard models in infinite dimensions. These results
are qualified by comparison with essentially exact Monte Carlo results. The
second order iterated perturbation theory is shown to be quite accurate in
calculating transition temperatures for retarded interactions, but is not as
accurate for the self energy or the irreducible vertex functions themselves.
Iterated perturbation theory is carried out thru fourth order for the Hubbard
model. The self energy is quite accurately reproduced by the theory, but the
vertex functions are not. Anomalous behavior occurs near half filling because
the iterated perturbation theory is not a conserving approximation. (REPLACED
WITH UUENCODED FIGURES AT THE END. THE TEXT IS UNCHANGED)Comment: 27 pages, RevTex (figures appended at end
Superconductivity in the Two-Band Hubbard Model in Infinite Dimensions
We study a two-band Hubbard model in the limit of infinite dimensions, using
a combination of analytical methods and Monte-Carlo techniques. The normal
state is found to display various metal to insulators transitions as a function
of doping and interaction strength. We derive self-consistent equations for the
local Green's functions in the presence of superconducting long-range order,
and extend previous algorithms to this case. We present direct numerical
evidence that in a specific range of parameter space, the normal state is
unstable against a superconducting state characterized by a strongly frequency
dependent order-parameter.Comment: 12 pages (14 figures not included, available upon request), Latex,
LPTENS Preprint 93/1
Electronic Structure of Paramagnetic V_2O_3: Strongly Correlated Metallic and Mott Insulating Phase
LDA+DMFT, the computation scheme merging the local density approximation and
the dynamical mean-field theory, is employed to calculate spectra both below
and above the Fermi energy and spin and orbital occupations in the correlated
paramagnetic metallic and Mott insulating phase of V_2O_3. The self-consistent
DMFT equations are solved by quantum Monte Carlo simulations. Room temperature
calculations provide direct comparison with experiment. They show a significant
increase of the quasiparticle height in comparison with the results at 1160 K.
We also obtain new insights into the nature of the Mott-Hubbard transition in
V_2O_3. Namely, it is found to be strikingly different from that in the
one-band Hubbard model due to the orbital degrees of freedom. Furthermore we
resolve the puzzle of the unexpectedly small Mott gap in Cr-doped V_2O_3.Comment: 14 pages, 22 figure
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