93 research outputs found
Spin Liquid Behavior in Electronic Griffiths Phases
We examine the interplay of the Kondo effect and the RKKY interactions in
electronic Griffiths phases using extended dynamical mean-field theory methods.
We find that sub-Ohmic dissipation is generated for sufficiently strong
disorder, leading to suppression of Kondo screening on a finite fraction of
spins, and giving rise to universal spin-liquid behavior.Comment: 4 pages, minor changes included, typos correcte
Disorder Screening in Strongly Correlated Systems
Electron-electron interactions generally reduce the low temperature
resistivity due to the screening of the impurity potential by the electron gas.
In the weak-coupling limit, the magnitude of this screening effect is
determined by the thermodynamic compressibility which is proportional to the
inverse screening length. We show that when strong correlations are present,
although the compressibility is reduced, the screening effect is nevertheless
strongly enhanced. This phenomenon is traced to the same non-perturbative
Kondo-like processes that lead to strong mass enhancements, but which are
absent in weak coupling approaches. We predict metallicity to be strongly
stabilized in an intermediate regime where the interactions and the disorder
are of comparable magnitude.Comment: 4+epsilon pages, 3 figure
Transport properties of clean and disordered superconductors in matrix field theory
A comprehensive field theory is developed for superconductors with quenched
disorder. We first show that the matrix field theory, used previously to
describe a disordered Fermi liquid and a disordered itinerant ferromagnet, also
has a saddle-point solution that describes a disordered superconductor. A
general gap equation is obtained. We then expand about the saddle point to
Gaussian order to explicitly obtain the physical correlation functions. The
ultrasonic attenuation, number density susceptibility, spin density
susceptibility and the electrical conductivity are used as examples. Results in
the clean limit and in the disordered case are discussed respectively. This
formalism is expected to be a powerful tool to study the quantum phase
transitions between the normal metal state and the superconductor state.Comment: 9 page
Magnetic Correlations in the Two Dimensional Anderson-Hubbard Model
The two dimensional Hubbard model in the presence of diagonal and
off-diagonal disorder is studied at half filling with a finite temperature
quantum Monte Carlo method. Magnetic correlations as well as the electronic
compressibility are calculated to determine the behavior of local magnetic
moments, the stability of antiferromagnetic long range order (AFLRO), and
properties of the disordered phase. The existence of random potentials
(diagonal or ``site'' disorder) leads to a suppression of local magnetic
moments which eventually destroys AFLRO. Randomness in the hopping elements
(off-diagonal disorder), on the other hand, does not significantly reduce the
density of local magnetic moments. For this type of disorder, at half-filling,
there is no ``sign-problem'' in the simulations as long as the hopping is
restricted between neighbor sites on a bipartite lattice. This allows the study
of sufficiently large lattices and low temperatures to perform a finite-size
scaling analysis. For off-diagonal disorder AFLRO is eventually destroyed when
the fluctuations of antiferromagnetic exchange couplings exceed a critical
value. The disordered phase close to the transition appears to be
incompressible and shows an increase of the uniform susceptibility at low
temperatures.Comment: 10 pages, REVTeX, 14 figures included using psfig.st
Phase diagram of the -spin-interacting spin glass with ferromagnetic bias and a transverse field in the infinite- limit
The phase diagram of the -spin-interacting spin glass model in a
transverse field is investigated in the limit under the presence
of ferromagnetic bias. Using the replica method and the static approximation,
we show that the phase diagram consists of four phases: Quantum paramagnetic,
classical paramagnetic, ferromagnetic, and spin-glass phases. We also show that
the static approximation is valid in the ferromagnetic phase in the limit by using the large- expansion. Since the same approximation is
already known to be valid in other phases, we conclude that the obtained phase
diagram is exact.Comment: 16 pages, 4 figures. another additional author, some amendment
Universality in an integer Quantum Hall transition
An integer Quantum Hall effect transition is studied in a modulation doped
p-SiGe sample. In contrast to most examples of such transitions the
longitudinal and Hall conductivities at the critical point are close to 0.5 and
1.5 (e^2/h), the theoretically expected values. This allows the extraction of a
scattering parameter, describing both conductivity components, which depends
exponentially on filling factor. The strong similarity of this functional form
to those observed for transitions into the Hall insulating state and for the
B=0 metal- insulator transition implies a universal quantum critical behaviour
for the transitions. The observation of this behaviour in the integer Quantum
Hall effect, for this particular sample, is attributed to the short-ranged
character of the potential associated with the dominant scatterers
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