536 research outputs found
Spin-polarized tunneling through a thin-film
The effect of spin-disorder scattering on perpendicular transport in a
magnetic monolayer is considered within the single-site Coherent Potential
Approximation (CPA). The exchange interaction between a conduction electron and
localized moment of the magnetic ion is treated with the use of the
model. Electron-spin polarization is evaluated in the tunnel current which
comes from the different densities of spin-up, spin-down conduction electrons
at the Fermi level in a ferromagnetic semiconductor (EuS). Calculated results
are compared with some tunneling experiments.Comment: 5 pages, LaTex, 2 EPS figure
The temperature dependence of the spin polarization of field emitted electrons from a W-EuS-vacuum junction
Baum G, Kisker E, Mahan AH, Schröder K. The temperature dependence of the spin polarization of field emitted electrons from a W-EuS-vacuum junction. Journal of Magnetism and Magnetic Materials. 1976;3(1-2):4-6.We have measured the temperature dependence P(T) of the electron spin polarization of field emitted electrons from a W-EuS-vacuum junction. The shapes of the P(T) curves depend strongly on the annealing temperature of the EuS layer. Annealed at some temperature between 300°C and 600°C the polarization drops to zero at about 16 ± 2 K
Superconductor-to-Normal Phase Transition in a Vortex Glass Model: Numerical Evidence for a New Percolation Universality Class
The three-dimensional strongly screened vortex-glass model is studied
numerically using methods from combinatorial optimization. We focus on the
effect of disorder strength on the ground state and found the existence of a
disorder-driven normal-to-superconducting phase transition. The transition
turns out to be a geometrical phase transition with percolating vortex loops in
the ground state configuration. We determine the critical exponents and provide
evidence for a new universality class of correlated percolation.Comment: 11 pages LaTeX using IOPART.cls, 11 eps-figures include
Directed geometrical worm algorithm applied to the quantum rotor model
We discuss the implementation of a directed geometrical worm algorithm for
the study of quantum link-current models. In this algorithm Monte Carlo updates
are made through the biased reptation of a worm through the lattice. A directed
algorithm is an algorithm where, during the construction of the worm, the
probability for erasing the immediately preceding part of the worm, when adding
a new part,is minimal. We introduce a simple numerical procedure for minimizing
this probability. The procedure only depends on appropriately defined local
probabilities and should be generally applicable. Furthermore we show how
correlation functions, C(r,tau) can be straightforwardly obtained from the
probability of a worm to reach a site (r,tau) away from its starting point
independent of whether or not a directed version of the algorithm is used.
Detailed analytical proofs of the validity of the Monte Carlo algorithms are
presented for both the directed and un-directed geometrical worm algorithms.
Results for auto-correlation times and Green functions are presented for the
quantum rotor model.Comment: 11 pages, 9 figures, v2 : Additional results and data calculated at
an incorrect chemical potential replaced. Conclusions unchange
Cluster Monte Carlo Algorithm for the Quantum Rotor Model
We propose a highly efficient "worm" like cluster Monte Carlo algorithm for
the quantum rotor model in the link-current representation. We explicitly prove
detailed balance for the new algorithm even in the presence of disorder. For
the pure quantum rotor model with the new algorithm yields high
precision estimates for the critical point and the correlation
length exponent . For the disordered case, , we
find .Comment: 5 pages, 3 figure
Dynamical coherent-potential approximation approach to excitation spectra in 3d transition metals
First-principles dynamical CPA (Coherent-Potential Approximation) for
electron correlations has been developed further by taking into account
higher-order dynamical corrections with use of the asymptotic approximation.
The theory is applied to the investigations of a systematic change of
excitation spectra in transition metals from Sc to Cu at finite
temperatures. It is shown that the dynamical effects damp main peaks in the
densities of states (DOS) obtained by the local density approximation to the
density functional theory, reduce the band broadening due to thermal spin
fluctuations, create the Mott-Hubbard type bands in the case of fcc Mn and fcc
Fe, and create a small hump corresponding to the `6 eV' satellite in the case
of Co, Ni, and Cu. Calculated DOS explain the X-ray photoelectron spectroscopy
data as well as the bremsstrahlung isochromat spectroscopy data. Moreover, it
is found that screening effects on the exchange energy parameters are
significant for understanding the spectra in magnetic transition metals.Comment: To be published in Phys. Rev.
Simulation Studies on the Stability of the Vortex-Glass Order
The stability of the three-dimensional vortex-glass order in random type-II
superconductors with point disorder is investigated by equilibrium Monte Carlo
simulations based on a lattice XY model with a uniform field threading the
system. It is found that the vortex-glass order, which stably exists in the
absence of screening, is destroyed by the screenng effect, corroborating the
previous finding based on the spatially isotropic gauge-glass model. Estimated
critical exponents, however, deviate considerably from the values reported for
the gauge-glass model.Comment: Minor modifications made, a few referenced added; to appear in J.
Phys. Soc. Jpn. Vol.69 No.1 (2000
Phase diagram of a Disordered Boson Hubbard Model in Two Dimensions
We study the zero-temperature phase transition of a two-dimensional
disordered boson Hubbard model. The phase diagram of this model is constructed
in terms of the disorder strength and the chemical potential. Via quantum Monte
Carlo simulations, we find a multicritical line separating the weak-disorder
regime, where a random potential is irrelevant, from the strong-disorder
regime. In the weak-disorder regime, the Mott-insulator-to-superfluid
transition occurs, while, in the strong-disorder regime, the
Bose-glass-to-superfluid transition occurs. On the multicritical line, the
insulator-to-superfluid transition has the dynamical critical exponent and the correlation length critical exponent ,
that are different from the values for the transitions off the line. We suggest
that the proliferation of the particle-hole pairs screens out the weak disorder
effects.Comment: 4 pages, 4 figures, to be published in PR
Direct Mott Insulator-to-Superfluid Transition in the Presence of Disorder
We introduce a new renormalization group theory to examine the quantum phase
transitions upon exiting the insulating phase of a disordered, strongly
interacting boson system. For weak disorder we find a direct transition from
this Mott insulator to the Superfluid phase. In d > 4 a finite region around
the particle-hole symmetric point supports this direct transition, whereas for
2=< d <4 perturbative arguments suggest that the direct transition survives
only precisely at commensurate filling. For strong disorder the renormalization
trajectories pass next to two fixed points, describing a pair of distinct
transitions; first from the Mott insulator to the Bose glass, and then from the
Bose glass to the Superfluid. The latter fixed point possesses statistical
particle-hole symmetry and a dynamical exponent z, equal to the dimension d.Comment: 4 pages, Latex, submitted to Physical Review Letter
Crystallization of a supercooled liquid and of a glass - Ising model approach
Using Monte Carlo simulations we study crystallization in the
three-dimensional Ising model with four-spin interaction. We monitor the
morphology of crystals which grow after placing crystallization seeds in a
supercooled liquid. Defects in such crystals constitute an intricate and very
stable network which separate various domains by tensionless domain walls. We
also show that the crystallization which occurs during the continuous heating
of the glassy phase takes place at a heating-rate dependent temperature.Comment: 7 page
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