226 research outputs found
Numerical Study of Competing Spin-Glass and Ferromagnetic Order
Two and three dimensional random Ising models with a Gaussian distribution of
couplings with variance and non-vanishing mean value are studied
using the zero-temperature domain-wall renormalization group (DWRG). The DWRG
trajectories in the () plane after rescaling can be collapsed on two
curves: one for and other for . In the first case
the DWRG flows are toward the ferromagnetic fixed point both in two and three
dimensions while in the second case flows are towards a paramagnetic fixed
point and spin-glass fixed point in two and three dimensions respectively. No
evidence for an extra phase is found.Comment: a bit more data is taken, 5 pages, 4 eps figures included, to appear
in PR
Quasiballistic correction to the density of states in three-dimensional metal
We study the exchange correction to the density of states in the
three-dimensional metal near the Fermi energy. In the ballistic limit, when the
distance to the Fermi level exceeds the inverse transport relaxation time
, we find the correction linear in the distance from the Fermi level.
By a large parameter this ballistic correction exceeds
the diffusive correction obtained earlier.Comment: 2 pages, 1 figur
Size-effects in the Density of States in NS and SNS junctions
The quasiparticle local density of states (LDOS) is studied in clean NS and
SNS junctions with increasing transverse size, from quasi-one-dimensional to
three-dimensional. It is shown that finite transverse dimensions are related to
pronounced effects in the LDOS, such as fast oscillations superimposed on the
quasiparticle interference oscillations (for NS) and additional peaks in the
bound state spectrum in the subgap region (for SNS). Also, the validity of the
Andreev approximation is discussed. It turns out to be an acceptable
approximation in all situations tested.Comment: 9 pages, RevTex, 5 figures, accepted in Phys. Rev.
High Pressure Study on MgB2
The hydrostatic pressure effect on the newly discovered superconductor MgB2
has been determined. The transition temperature Tc was found to decrease
linearly at a large rate of -1.6 K/GPa, in good quantitative agreement with the
ensuing calculated value of -1.4 K/GPa within the BCS framework by Loa and
Syassen, using the full-potential linearlized augmented plane-wave method. The
relative pressure coefficient, dlnTc/dp, for MgB2 also falls between the known
values for conventional sp- and d-superconductors. The observation, therefore,
suggests that electron-phonon interaction plays a significant role in the
superconductivity of the compound.Comment: 8 pages, 3 figures; submitted to Physical Review B (February 14,
2001; revised March 21, 2001); minor modifications, including a discussion of
the preprint by Vogt et a
Low Energy Excitations in Spin Glasses from Exact Ground States
We investigate the nature of the low-energy, large-scale excitations in the
three-dimensional Edwards-Anderson Ising spin glass with Gaussian couplings and
free boundary conditions, by studying the response of the ground state to a
coupling-dependent perturbation introduced previously. The ground states are
determined exactly for system sizes up to 12^3 spins using a branch and cut
algorithm. The data are consistent with a picture where the surface of the
excitations is not space-filling, such as the droplet or the ``TNT'' picture,
with only minimal corrections to scaling. When allowing for very large
corrections to scaling, the data are also consistent with a picture with
space-filling surfaces, such as replica symmetry breaking. The energy of the
excitations scales with their size with a small exponent \theta', which is
compatible with zero if we allow moderate corrections to scaling. We compare
the results with data for periodic boundary conditions obtained with a genetic
algorithm, and discuss the effects of different boundary conditions on
corrections to scaling. Finally, we analyze the performance of our branch and
cut algorithm, finding that it is correlated with the existence of
large-scale,low-energy excitations.Comment: 18 Revtex pages, 16 eps figures. Text significantly expanded with
more discussion of the numerical data. Fig.11 adde
Lockin to Weak Ferromagnetism in TbNi2B2C and ErNi2B2C
This article describes a model in which ferromagnetism necessarily
accompanies a spin-density-wave lockin transition in the borocarbide structure
provided the commensurate phase wave vector satisfies Q = (m/n)a* with m even
and n odd. The results account for the magnetic properties of TbNi2B2C, and are
also possibly relevant also for those of ErNi2B2C.Comment: 4 page
Minigap in a long disordered SNS junction: analytical results
We review and refine analytical results on the density of states in a long
disordered superconductor - normal-metal - superconductor junction with
transparent interfaces. Our analysis includes the behavior of the minigap near
phase differences zero and PI across the junction, as well as the density of
states at energies much larger than the minigap but much smaller than the
superconducting gap.Comment: 4 page
Calculation of ground states of four-dimensional +or- J Ising spin glasses
Ground states of four-dimensional (d=4) EA Ising spin glasses are calculated
for sizes up to 7x7x7x7 using a combination of a genetic algorithm and
cluster-exact approximation. The ground-state energy of the infinite system is
extrapolated as e_0=-2.095(1). The ground-state stiffness (or domain wall)
energy D is calculated. A D~L^{\Theta} behavior with \Theta=0.65(4) is found
which confirms that the d=4 model has an equilibrium spin-glass-paramagnet
transition for non-zero T_c.Comment: 5 pages, 3 figures, 31 references, revtex; update of reference
Generating droplets in two-dimensional Ising spin glasses by using matching algorithms
We study the behavior of droplets for two dimensional Ising spin glasses with
Gaussian interactions. We use an exact matching algorithm which enables study
of systems with linear dimension L up to 240, which is larger than is possible
with other approaches. But the method only allows certain classes of droplets
to be generated. We study single-bond, cross and a category of fixed volume
droplets as well as first excitations. By comparison with similar or equivalent
droplets generated in previous works, the advantages but also the limitations
of this approach are revealed. In particular we have studied the scaling
behavior of the droplet energies and droplet sizes. In most cases, a crossover
of the data can be observed such that for large sizes the behavior is
compatible with the one-exponent scenario of the droplet theory. Only for the
case of first excitations, no clear conclusion can be reached, probably because
even with the matching approach the accessible system sizes are still too
small.Comment: 11 pages, 16 figures, revte
Gap to Transition Temperature Ratio in Density Wave Ordering: a Dynamical Mean Field Study
We use the dynamical mean-field method to determine the origin of the large
ratio of the zero temperature gap to the transition temperature observed in
most charge density wave materials. The method is useful because it allows an
exact treatment of thermal fluctuations. We establish the relation of the
dynamical mean-field results to conventional diagrammatics and thereby
determine that in the physically relevant regime the origin of the large ratio
is a strong inelastic scattering.Comment: 4 figure
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