4,594 research outputs found
Random Magnetic Interactions and Spin Glass Order Competing with Superconductivity: Interference of the Quantum Parisi Phase
We analyse the competition between spin glass (SG) order and local pairing
superconductivity (SC) in the fermionic Ising spin glass with frustrated
fermionic spin interaction and nonrandom attractive interaction. The phase
diagram is presented for all temperatures T and chemical potentials \mu. SC-SG
transitions are derived for the relevant ratios between attractive and
frustrated-magnetic interaction. Characteristic features of pairbreaking caused
by random magnetic interaction and/or by spin glass proximity are found. The
existence of low-energy excitations, arising from replica permutation symmetry
breaking (RPSB) in the Quantum Parisi Phase, is shown to be relevant for the
SC-SG phase boundary. Complete 1-step RPSB-calculations for the SG-phase are
presented together with a few results for infinity-step breaking. Suppression
of reentrant SG - SC - SG transitions due to RPSB is found and discussed in
context of ferromagnet - SG boundaries. The relative positioning of the SC and
SG phases presents a theoretical landmark for comparison with experiments in
heavy fermion systems and high T_c superconductors. We find a crossover line
traversing the SG-phase with (\mu=0,T=0) as its quantum critical (end)point in
complete RPSB, and scaling is proposed for its vicinity. We argue that this
line indicates a random field instability and suggest Dotsenko-Mezard vector
replica symmetry breaking to occur at low temperatures beyond.Comment: 24 pages, 14 figures replaced by published versio
Pseudogaps and Charge Band in the Parisi Solution of Insulating and Superconducting Electronic Spin Glasses at Arbitrary Fillings
We report progress in understanding the fermionic Ising spin glass with
arbitrary filling. A crossover from a magnetically disordered single band phase
via two intermediate bands just below the freezing temperature to a 3-band
structure at still lower temperatures - beyond an almost random field
instability - is shown to emerge in the magnetic phase. An attempt is made to
explain the exact solution in terms of a quantum Parisi phase. A central
nonmagnetic band is found and seen to become sharply separated at T=0 by gaps
from upper and lower magnetic bands. The gap sizes tend towards zero as the
number of replica symmetry breaking steps increases towards infinity. In an
extended model, the competition between local pairing superconductivity and
spin glass order is discussed.Comment: 3 pages, contribution to "ECRYS-99
See-Saw Masses for Quarks and Leptons in SU(5)
We build on a recent paper by Grinstein, Redi and Villadoro, where a see-saw
like mechanism for quark masses was derived in the context of spontaneously
broken gauged flavour symmetries. The see-saw mechanism is induced by heavy
Dirac fermions which are added to the Standard Model spectrum in order to
render the flavour symmetries anomaly-free. In this letter we report on the
embedding of these fermions into multiplets of an SU(5) grand unified theory
and discuss a number of interesting consequences.Comment: 15 pages, 4 figures (v3: outline restructured, modified mechanism to
cancel anomalies
How to evaluate ground-state landscapes of disordered systems thermodynamical correctly
Ground states of three-dimensional EA Ising spin glasses are calculated for
sizes up to 14^3 using a combination of a genetic algorithm and cluster-exact
approximation. For each realization several independent ground states are
obtained. Then, by applying ballistic search and T=0 Monte-Carlo simulations,
it is ensured that each ground state appears with the same probability.
Consequently, the results represent the true T=0 thermodynamic behavior. The
distribution P(|q|) of overlaps is evaluated. For increasing size the width of
P(|q|) and the fraction of the distribution below q_0=0.5 converge to zero.
This indicates that for the infinite system P(|q|) is a delta function, in
contrast to previous results. Thus, the ground-state behavior is dominated by
few large clusters of similar ground states.Comment: 7 pages revtex, 6 figures, 27 reference
Semi-fermionic representation of SU(N) Hamiltonians
We represent the generators of the SU(N) algebra as bilinear combinations of
Fermi operators with imaginary chemical potential. The distribution function,
consisting of a minimal set of discrete imaginary chemical potentials, is found
for arbitrary N. This representation leads to the conventional temperature
diagram technique with standard Feynman codex, except that the Matsubara
frequencies are determined by neither integer nor half-integer numbers. The
real-time Schwinger-Keldysh formalism is formulated in the framework of complex
distribution functions. We discuss the continuous large N and SU(2) large spin
limits. We illustrate the application of this technique for magnetic and
spin-liquid states of the Heisenberg model.Comment: 11 pages, 7 EPS figures included, extended versio
Quantum Lubrication: Suppression of Friction in a First Principle Four Stroke Heat Engine
A quantum model of a heat engine resembling the Otto cycle is employed to
explore strategies to suppress frictional losses. These losses are caused by
the inability of the engine's working medium to follow adiabatically the change
in the Hamiltonian during the expansion and compression stages. By adding
external noise to the engine, frictional losses can be suppressed.Comment: references added some minor change
Suppression of vortex channeling in meandered YBa2Cu3O7-d grain boundaries
We report on the in-plane magnetic field (H) dependence of the critical
current density (Jc) in meandered and planar single grain boundaries (GBs)
isolated in YBa2Cu3O7-d (YBCO) coated conductors. The Jc(H)properties of the
planar GB are consistent with those previously seen in single GBs of YBCO films
grown on SrTiO3 bi-crystals. In the straight boundary a characteristic flux
channeling regime when H is oriented near the GB plane, associated with a
reduced Jc, is seen. The meandered GB does not show vortex channeling since it
is not possible for a sufficient length of vortex line to lie within it.Comment: Submitted to AP
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