425 research outputs found
Role of topological defects in the phase transition of modified XY model : A Monte Carlo study
Monte Carlo simulation has been performed on a classical two dimensional XY-
model with a modified form of interaction potential to investigate the role of
topological defects on the phase transition exhibited by the model. In
simulations in a restricted ensemble without defects, the system appears to
remain ordered at all temperatures. Suppression of topological defects on the
square plaquettes in the modified XY- model leads to complete elimination of
the phase transition observed in this model.Comment: 19 pages, 12 figures, Accepted for publication in Phys. Rev.
Phase Diagram of the Half-Filled Ionic Hubbard Model
We study the phase diagram of the ionic Hubbard model (IHM) at half-filling
using dynamical mean field theory (DMFT), with two impurity solvers, namely,
iterated perturbation theory (IPT) and continuous time quantum Monte Carlo
(CTQMC). The physics of the IHM is governed by the competition between the
staggered potential and the on-site Hubbard U. In both the methods we
find that for a finite and at zero temperature, anti-ferromagnetic
(AFM) order sets in beyond a threshold via a first order phase
transition below which the system is a paramagnetic band insulator. Both the
methods show a clear evidence for a transition to a half-metal phase just after
the AFM order is turned on, followed by the formation of an AFM insulator on
further increasing U. We show that the results obtained within both the methods
have good qualitative and quantitative consistency in the intermediate to
strong coupling regime. On increasing the temperature, the AFM order is lost
via a first order phase transition at a transition temperature within both the methods, for weak to intermediate values of U/t. But
in the strongly correlated regime, where the effective low energy Hamiltonian
is the Heisenberg model, IPT is unable to capture the thermal (Neel) transition
from the AFM phase to the paramagnetic phase, but the CTQMC does. As a result,
at any finite temperature T, DMFT+CTQMC shows a second phase transition (not
seen within DMFT+IPT) on increasing U beyond . At , when
the Neel temperature for the effective Heisenberg model becomes lower
than T, the AFM order is lost via a second order transition. In the
3-dimensonal parameter space of , there is a line of
tricritical points that separates the surfaces of first and second order phase
transitions.Comment: Revised versio
Correlation between incoherent phase fluctuations and disorder in YPrBaCuO epitaxial films from Nernst effect measurements
Measurements of Nernst effect, resistivity and Hall angle on epitaxial films
of YPrBaCuO(Pr-YBCO, 00.4) are
reported over a broad range of temperature and magnetic field. While the Hall
and resistivity data suggest a broad pseudogap regime in accordance with
earlier results, these first measurements of the Nernst effect on Pr-YBCO show
a large signal above the superconducting transition temperature(T). This
effect is attributed to vortex-like excitations in the phase incoherent
condensate existing above T. A correlation between disorder and the width
of the phase fluctuation regime has been established for the YBCO family of
cuprates, which suggests a T110K for disorder-free
YBaCuO.Comment: 5 pages, 6 figure
Growth of [110] LaSrMnO - YBaCuO heterostructures
YBaCuO - LaSrMnO heterostructures
of [110] orientation are grown to allow direct injection of spin polarized
holes from the LaSrMnO into the CuO
superconducting planes. The magnetic response of the structure at T
T shows both diamagnetic and ferromagnetic moments with [001] direction
as magnetic easy axis. While the superconducting transition temperature
(T) of these structures is sharp (T 2.5 K), the
critical current density (J) follows a dependence of the type with highly suppressed J ( A/cm) indicating strong pair breaking effects of the ferromagnetic
boundary.Comment: 12 pages five figure
Reflection coefficient for superresonant scattering
We investigate superresonant scattering of acoustic disturbances from a
rotating acoustic black hole in the low frequency range. We derive an
expression for the reflection coefficient, exhibiting its frequency dependence
in this regime.Comment: 7 page
Giant coherence in driven systems
We study the noise-induced currents and reliability or coherence of transport
in two different classes of rocking ratchets. For this, we consider the motion
of Brownian particles in the over damped limit in both adiabatic and
non-adiabatic regimes subjected to unbiased temporally symmetric and asymmetric
periodic driving force. In the case of a time symmetric driving, we find that
even in the presence of a spatially symmetric simple sinusoidal potential,
highly coherent transport occurs. These ratchet systems exhibit giant coherence
of transport in the regime of parameter space where unidirectional currents in
the deterministic case are observed. Outside this parameter range, i.e., when
current vanishes in the deterministic regime, coherence in transport is very
low. The transport coherence decreases as a function of temperature and is a
non-monotonic function of the amplitude of driving. The transport becomes
unreliable as we go from the adiabatic to the non-adiabatic domain of
operation.Comment: 15 pages, 9 figures, replaced by the version to appear in JSTA
Tools and Techniques of Quality Management: An Empirical Investigation of Their Impact on Performance
An investigation of quality management at an operational rather than a strategic level is described. Using a survey of senior quality personnel, data were collected on four aspects of quality: management; quality tools; documentation; and the dimensions of quality that companies measure. Regression analysis confirms suggestions in the literature that company performance is positively affected by a culture in which quality is ingrained. Moreover, it identifies positive relationships between several widely used operational practices and company performance
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