75,319 research outputs found
A Monte Carlo study of the triangular lattice gas with the first- and the second-neighbor exclusions
We formulate a Swendsen-Wang-like version of the geometric cluster algorithm.
As an application,we study the hard-core lattice gas on the triangular lattice
with the first- and the second-neighbor exclusions. The data are analyzed by
finite-size scaling, but the possible existence of logarithmic corrections is
not considered due to the limited data. We determine the critical chemical
potential as and the critical particle density as
. The thermal and magnetic exponents
and , estimated from Binder ratio and
susceptibility , strongly support the general belief that the model is in
the 4-state Potts universality class. On the other hand, the analyses of
energy-like quantities yield the thermal exponent ranging from
to . These values differ significantly from the expected value 3/2,
and thus imply the existence of logarithmic corrections.Comment: 4 figures 2 table
Interatomic collisions in two-dimensional and quasi-two-dimensional confinements with spin-orbit coupling
We investigate the low-energy scattering and bound states of two
two-component fermionic atoms in pure two-dimensional (2D) and quasi-2D
confinements with Rashba spin-orbit coupling (SOC). We find that the SOC
qualitatively changes the behavior of the 2D scattering amplitude in the
low-energy limit. For quasi-2D systems we obtain the analytic expression for
the effective-2D scattering amplitude and the algebraic equations for the
two-atom bound state energy. Based on these results, we further derive the
effective 2D interaction potential between two ultracold atoms in the quasi-2D
confinement with Rashba SOC. These results are crucial for the control of the
2D effective physics in quasi-2D geometry via the confinement intensity and the
atomic three-dimensional scattering length.Comment: 13pages, 5 figure
Influence of crystal structure on charge carrier effective masses in BiFeO
Ferroelectric-based photovoltaics have shown great promise as a source of
renewable energy, thanks to their in-built charge separation capability, yet
their efficiency is often limited by low charge carrier mobilities. In this
work, we compare the photovoltaic prospects of various phases of the
multiferroic material BiFeO by evaluating their charge carrier effective
masses using first-principles simulations. We identify a tetragonal phase with
the promising combination of a large spontaneous polarisation and relatively
light charge carriers. From a systematic study of the octahedral distortions
present in BiFeO, we explain the relationship between structure and
effective masses in terms of the changes to the orbital character and overlap
at the band edges that result from changes in the geometry. The findings in
this study provide some design principles to engineer desired effective masses
in BiFeO and similar materials through manipulation of their crystal
structures in experimentally accessible ways.Comment: 12 pages, 10 figure
Weak phase stiffness and mass divergence of superfluid in underdoped cuprates
Despite more than two decades of intensive investigations, the true nature of
high temperature (high-) superconductivity observed in the cuprates
remains elusive to the researchers. In particular, in the so-called
`underdoped' region, the overall behavior of superconductivity deviates
from the standard theoretical description pioneered by Bardeen,
Cooper and Schrieffer (BCS). Recently, the importance of phase fluctuation of
the superconducting order parameter has gained significant support from various
experiments. However, the microscopic mechanism responsible for the
surprisingly soft phase remains one of the most important unsolved puzzles.
Here, opposite to the standard BCS starting point, we propose a simple,
solvable low-energy model in the strong coupling limit, which maps the
superconductivity literally into a well-understood physics of superfluid in a
special dilute bosonic system of local pairs of doped holes. In the
prototypical material (LaSr)CuO, without use of
any free parameter, a -wave superconductivity is obtained for doping above
, below which unexpected incoherent -wave pairs dominate.
Throughout the whole underdoped region, very soft phases are found to originate
from enormous mass enhancement of the pairs. Furthermore, a striking mass
divergence is predicted that dictates the occurrence of the observed quantum
critical point. Our model produces properties of the superfluid in good
agreement with the experiments, and provides new insights into several current
puzzles. Owing to its simplicity, this model offers a paradigm of great value
in answering the long-standing challenges in underdoped cuprates
Morphology of rain water channelization in systematically varied model sandy soils
We visualize the formation of fingered flow in dry model sandy soils under
different raining conditions using a quasi-2d experimental set-up, and
systematically determine the impact of soil grain diameter and surface wetting
property on water channelization phenomenon. The model sandy soils we use are
random closely-packed glass beads with varied diameters and surface treatments.
For hydrophilic sandy soils, our experiments show that rain water infiltrates
into a shallow top layer of soil and creates a horizontal water wetting front
that grows downward homogeneously until instabilities occur to form fingered
flows. For hydrophobic sandy soils, in contrast, we observe that rain water
ponds on the top of soil surface until the hydraulic pressure is strong enough
to overcome the capillary repellency of soil and create narrow water channels
that penetrate the soil packing. Varying the raindrop impinging speed has
little influence on water channel formation. However, varying the rain rate
causes significant changes in water infiltration depth, water channel width,
and water channel separation. At a fixed raining condition, we combine the
effects of grain diameter and surface hydrophobicity into a single parameter
and determine its influence on water infiltration depth, water channel width,
and water channel separation. We also demonstrate the efficiency of several
soil water improvement methods that relate to rain water channelization
phenomenon, including pre-wetting sandy soils at different level before
rainfall, modifying soil surface flatness, and applying superabsorbent hydrogel
particles as soil modifiers
Functional Forms for the Squeeze and the Time-Displacement Operators
Using Baker-Campbell-Hausdorff relations, the squeeze and harmonic-oscillator
time-displacement operators are given in the form , where ,
, , and are explicitly determined. Applications are
discussed.Comment: 10 pages, LaTe
Characteristics of the Limit Cycle of a Reciprocating Quantum Heat Engine
When a reciprocating heat engine is started it eventually settles to a stable
mode of operation. The approach of a first principle quantum heat engine toward
this stable limit cycle is studied. The engine is based on a working medium
consisting of an ensemble of quantum systems composed of two coupled spins. A
four stroke cycle of operation is studied, with two {\em isochore} branches
where heat is transferred from the hot/cold baths and two {\em adiabats} where
work is exchanged. The dynamics is generated by a completely positive map. It
has been shown that the performance of this model resembles an engine with
intrinsic friction. The quantum conditional entropy is employed to prove the
monotonic approach to a limit cycle. Other convex measures, such as the quantum
distance display the same monotonic approach. The equations of motion of the
engine are solved for the different branches and are combined to a global
propagator that relates the state of the engine in the beginning of the cycle
to the state after one period of operation of the cycle. The eigenvalues of the
propagator define the rate of relaxation toward the limit cycle. A longitudinal
and transverse mode of approach to the limit cycle is identified. The entropy
balance is used to explore the necessary conditions which lead to a stable
limit cycle. The phenomena of friction can be identified with a zero change in
the von Neumann entropy of the working medium.Comment: 29 pages and six figure
Bulk-fragment and tube-like structures of AuN (N=2-26)
Using the relativistic all-electron density-functional calculations on the
AuN (N=2-26) in the generalized gradient approximation, combined with the
guided simulated annealing, we have found that the two- to three-dimensional
structural transition for AuN occurs between N=13 and 15, and the AuN (16<= N
<=25) prefer also the pyramid-based bulk fragment structures in addition to the
Au20. More importantly, the tubelike structures are found to be the most stable
for Au24 and Au26, offering another powerful structure competitor with other
isomers, e.g., amorphous, bulk fragment, and gold fullerene. The mechanism to
cause these unusual AuN may be attributed to the stronger s-d hybridization and
the d-d interaction enhanced by the relativistic effects.Comment: 12 pages and 3 figure
- …