26,606 research outputs found
Quantum Phase Transitions in Josephson Junction Chains
We investigate the quantum phase transition in a one-dimensional chain of
ultra-small superconducting grains, considering both the self- and junction
capacitances. At zero temperature, the system is transformed into a
two-dimensional system of classical vortices, where the junction capacitance
introduces anisotropy in the interaction between vortices. This leads to the
superconductor-insulator transition of the Berezinskii-Kosterlitz-Thouless
type, as the ratios of the Josephson coupling energy to the charging energies
are varied. It is found that the junction capacitance plays a role similar to
that of dissipation and tends to suppress quantum fluctuations; nevertheless
the insulator region survives even for arbitrarily large values of the junction
capacitance.Comment: REVTeX+5 EPS figures, To appear in PRB Rapid
Current-voltage characteristics of the two-dimensional XY model with Monte Carlo dynamics
Current-voltage characteristics and the linear resistance of the
two-dimensional XY model with and without external uniform current driving are
studied by Monte Carlo simulations. We apply the standard finite-size scaling
analysis to get the dynamic critical exponent at various temperatures. From
the comparison with the resistively-shunted junction dynamics, it is concluded
that is universal in the sense that it does not depend on details of
dynamics. This comparison also leads to the quantification of the time in the
Monte Carlo dynamic simulation.Comment: 5 pages in two columns including 5 figures, to appear in PR
GaAs interfacial self-cleaning by atomic layer deposition
The reduction and removal of surface oxides from GaAs substrates by atomic layer deposition (ALD) of Al2O3 and HfO2 are studied using in situ monochromatic x-ray photoelectron spectroscopy. Using the combination of in situ deposition and analysis techniques, the interfacial "self-cleaning" is shown to be oxidation state dependent as well as metal organic precursor dependent. Thermodynamics, charge balance, and oxygen coordination drive the removal of certain species of surface oxides while allowing others to remain. These factors suggest proper selection of surface treatments and ALD precursors can result in selective interfacial bonding arrangements
Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation
The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. AsâO bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined
Small SUSY phases in string-inspired supergravity
In supersymmetric models, there are new CP violating phases which, if
unsuppressed, would give a too large neutron electric dipole moment. We examine
the possibility of small SUSY phases in string-inspired supergravity models in
which supersymmetry is broken by the auxiliary components of the dilaton and
moduli superfields. It is found that the SUSY phases can be suppressed by a
small factor governing the breakdown of the approximate Peccei Quinn symmetries
nonlinearly realized for the moduli superfields that participate in
supersymmetry breaking. In many cases, the symmetry breaking factors are
exponentially small for moderately large values of the moduli, leading to small
phase values in a natural way.Comment: 15pages, Latex, SNUTP 93-8
Numerical studies of the 2 and 3D gauge glass at low temperature
We report results from Monte Carlo simulations of the two- and
three-dimensional gauge glass at low temperature using parallel tempering Monte
Carlo. In two dimensions, we find strong evidence for a zero-temperature
transition. By means of finite-size scaling, we determine the stiffness
exponent theta = -0.39 +/- 0.03. In three dimensions, where a
finite-temperature transition is well established, we find theta = 0.27 +/-
0.01, compatible with recent results from domain-wall renormalization group
studies.Comment: 3 pages, 3 figures. Proceedings of "2002 MMM Conference", Tampa, F
String or M theory axion as a quintessence
A slow-rolling scalar field ( Quintessence) with potential energy
has been proposed as the origin of
accelerating universe at present. We investigate the effective potential of
in the framework of supergravity model including the quantum corrections
induced by generic (nonrenormalizable) couplings of to the gauge and
charged matter multiplets. It is argued that the K\"ahler potential,
superpotential and gauge kinetic functions of the underlying supergravity model
are required to be invariant under the variation of with an extremely fine
accuracy in order to provide a working quintessence potential. Applying these
results for string or -theory, we point out that the heterotic -theory or
Type I string axion can be a plausible candidate for quintessence if (i) it
does not couple to the instanton number of gauge interactions not weaker than
those of the standard model and (ii) the modulus partner of the
periodic quintessence axion has a large VEV:
. It is stressed
that such a large gives the gauge unification scale at around the
phenomenologically favored value GeV. To provide an
accelerating universe, the quintessence axion should be at near the top of its
effective potential at present, which requires a severe fine tuning of the
initial condition of and in the early universe. We discuss a late
time inflation scenario based on the modular and CP invariance of the moduli
effective potential, yielding the required initial condition in a natural
manner if the K\"ahler metric of the quintessence axion superfield receives a
sizable nonperturbative contribution.Comment: 23 pages, 3 figures, version to be published at Phys. Rev.
Phase ordering on small-world networks with nearest-neighbor edges
We investigate global phase coherence in a system of coupled oscillators on a
small-world networks constructed from a ring with nearest-neighbor edges. The
effects of both thermal noise and quenched randomness on phase ordering are
examined and compared with the global coherence in the corresponding \xy model
without quenched randomness. It is found that in the appropriate regime phase
ordering emerges at finite temperatures, even for a tiny fraction of shortcuts.
Nature of the phase transition is also discussed.Comment: 5 pages, 4 figures, Phys. Rev. E (in press
Structural Relaxation, Self Diffusion and Kinetic Heterogeneity in the Two Dimensional Lattice Coulomb Gas
We present Monte Carlo simulation results on the equilibrium relaxation
dynamics in the two dimensional lattice Coulomb gas, where finite fraction
of the lattice sites are occupied by positive charges. In the case of high
order rational values of close to the irrational number
( is the golden mean), we find that the system
exhibits, for wide range of temperatures above the first-order transition, a
glassy behavior resembling the primary relaxation of supercooled liquids.
Single particle diffusion and structural relaxation show that there exists a
breakdown of proportionality between the time scale of diffusion and that of
structural relaxation analogous to the violation of the Stokes-Einstein
relation in supercooled liquids. Suitably defined dynamic cooperativity is
calculated to exhibit the characteristic nature of dynamic heterogeneity
present in the system.Comment: 12 pages, 20 figure
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