13,239 research outputs found
Sticky Particles and Stochastic Flows
Gaw\c{e}dzki and Horvai have studied a model for the motion of particles
carried in a turbulent fluid and shown that in a limiting regime with low
levels of viscosity and molecular diffusivity, pairs of particles exhibit the
phenomena of stickiness when they meet. In this paper we characterise the
motion of an arbitrary number of particles in a simplified version of their
model
Compact and Broadband Microstrip-Line-Fed Modified Rhombus Slot Antenna
The printed microstrip-line-fed broadband rhombus slot antenna is investigated in this paper. With the use of the offset microstrip feed line and the corner-truncated protruded ground plane, the bandwidth enhancement and the slot size reduction for the proposed slot antenna can be obtained. The experimental results demonstrate that the impedance bandwidth for 10 dB return loss reaches 5210 MHz (108.2%, 2210-7420 MHz), which is about 2.67 times of a conventional microstrip-line-fed rhombus slot antenna. This bandwidth can provide with the wireless communication services operating in wireless local area network (WLAN) and worldwide interoperability for microwave access (WiMAX) bands. Under the use of the protruded ground plane, the slot size can be reduced by about 52%. Details of simulated and measured results are presented and discussed
Realistic many-body models for Manganese Monoxide under pressure
In materials like transition metals oxides where electronic Coulomb
correlations impede a description in terms of standard band-theories, the
application of genuine many-body techniques is inevitable. Interfacing the
realism of density-functional based methods with the virtues of Hubbard-like
Hamiltonians, requires the joint ab initio construction of transfer integrals
and interaction matrix elements (like the Hubbard U) in a localized basis set.
In this work, we employ the scheme of maximally localized Wannier functions and
the constrained random phase approximation to create effective low-energy
models for Manganese monoxide, and track their evolution under external
pressure. We find that in the low pressure antiferromagnetic phase, the
compression results in an increase of the bare Coulomb interaction for specific
orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79,
235133 (2009)], this seemingly counter-intuitive behavior is a consequence of
the delocalization of the respective Wannier functions. The change of screening
processes does not alter this tendency, and thus, the screened on-site
component of the interaction - the Hubbard U of the effective low-energy system
- increases with pressure as well. The orbital anisotropy of the effects
originates from the orientation of the orbitals vis-a-vis the deformation of
the unit-cell. Within the high pressure paramagnetic phase, on the other hand,
we find the significant increase of the Hubbard U is insensitive to the orbital
orientation and almost exclusively owing to a substantial weakening of
screening channels upon compression.Comment: 13 pages, 6 figure
Effects of CMB temperature uncertainties on cosmological parameter estimation
We estimate the effect of the experimental uncertainty in the measurement of
the temperature of the cosmic microwave background (CMB) on the extraction of
cosmological parameters from future CMB surveys. We find that even for an ideal
experiment limited only by cosmic variance up to l = 2500 for both the
temperature and polarisation measurements, the projected cosmological parameter
errors are remarkably robust against the uncertainty of 1 mK in the FIRAS
instrument's CMB temperature monopole measurement. The maximum degradation in
sensitivity is 20%, for the baryon density estimate, relative to the case in
which the monopole is known infinitely well. While this degradation is
acceptable, we note that reducing the uncertainty in the current temperature
measurement by a factor of five will bring it down to the per cent level. We
also estimate the effect of the uncertainty in the dipole temperature
measurement. Assuming the overall calibration of the data to be dominated by
the dipole error of 0.2% from FIRAS, the sensitivity degradation is
insignificant and does not exceed 10% in any parameter direction.Comment: 12 pages, 2 figures, uses iopart.cls, v2: added discussion of CMB
dipole uncertainty, version accepted by JCA
Persistent junk solutions in time-domain modeling of extreme mass ratio binaries
In the context of metric perturbation theory for non-spinning black holes,
extreme mass ratio binary (EMRB) systems are described by distributionally
forced master wave equations. Numerical solution of a master wave equation as
an initial boundary value problem requires initial data. However, because the
correct initial data for generic-orbit systems is unknown, specification of
trivial initial data is a common choice, despite being inconsistent and
resulting in a solution which is initially discontinuous in time. As is well
known, this choice leads to a "burst" of junk radiation which eventually
propagates off the computational domain. We observe another unintended
consequence of trivial initial data: development of a persistent spurious
solution, here referred to as the Jost junk solution, which contaminates the
physical solution for long times. This work studies the influence of both types
of junk on metric perturbations, waveforms, and self-force measurements, and it
demonstrates that smooth modified source terms mollify the Jost solution and
reduce junk radiation. Our concluding section discusses the applicability of
these observations to other numerical schemes and techniques used to solve
distributionally forced master wave equations.Comment: Uses revtex4, 16 pages, 9 figures, 3 tables. Document reformatted and
modified based on referee's report. Commentary added which addresses the
possible presence of persistent junk solutions in other approaches for
solving master wave equation
Unconventional superconducting phases in a correlated two-dimensional Fermi gas of nonstandard quasiparticles: a simple model
We discuss a detailed phase diagram and other microscopic characteristics on
the applied magnetic field - temperature (H_a-T) plane for a simple model of
correlated fluid represented by a two-dimensional (2D) gas of heavy
quasiparticles with masses dependent on the spin direction and the effective
field generated by the electron correlations. The consecutive transitions
between the Bardeen-Cooper-Schrieffer (BCS) and the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases are either continuous or
discontinuous, depending on the values of H_a and T. In the latter case, weak
metamagnetic transitions occur at the BCS-FFLO boundary. We single out two
different FFLO phases, as well as a reentrant behaviour of one of them at high
fields. The results are compared with those for ordinary Landau quasiparticles
in order to demonstrate the robustness of the FFLO states against the BCS state
for the case with spin-dependent masses (SDM). We believe that the mechanism of
FFLO stabilization by SDM is generic: other high-field low-temperature (HFLT)
superconducting phases benefit from SDM as well.Comment: 10 pages, 4 figure
Distribution theory for Schr\"odinger's integral equation
Much of the literature on point interactions in quantum mechanics has focused
on the differential form of Schr\"odinger's equation. This paper, in contrast,
investigates the integral form of Schr\"odinger's equation. While both forms
are known to be equivalent for smooth potentials, this is not true for
distributional potentials. Here, we assume that the potential is given by a
distribution defined on the space of discontinuous test functions.
First, by using Schr\"odinger's integral equation, we confirm a seminal
result by Kurasov, which was originally obtained in the context of
Schr\"odinger's differential equation. This hints at a possible deeper
connection between both forms of the equation. We also sketch a generalisation
of Kurasov's result to hypersurfaces.
Second, we derive a new closed-form solution to Schr\"odinger's integral
equation with a delta prime potential. This potential has attracted
considerable attention, including some controversy. Interestingly, the derived
propagator satisfies boundary conditions that were previously derived using
Schr\"odinger's differential equation.
Third, we derive boundary conditions for `super-singular' potentials given by
higher-order derivatives of the delta potential. These boundary conditions
cannot be incorporated into the normal framework of self-adjoint extensions. We
show that the boundary conditions depend on the energy of the solution, and
that probability is conserved.
This paper thereby confirms several seminal results and derives some new
ones. In sum, it shows that Schr\"odinger's integral equation is viable tool
for studying singular interactions in quantum mechanics.Comment: 23 page
Finite temperature properties of the triangular lattice t-J model, applications to NaCoO
We present a finite temperature () study of the t-J model on the
two-dimensional triangular lattice for the negative hopping , as relevant
for the electron-doped NaCoO (NCO). To understand several aspects of
this system, we study the -dependent chemical potential, specific heat,
magnetic susceptibility, and the dynamic Hall-coefficient across the entire
doping range. We show systematically, how this simplest model for strongly
correlated electrons describes a crossover as function of doping () from a
Pauli-like weakly spin-correlated metal close to the band-limit (density )
to the Curie-Weiss metallic phase () with pronounced
anti-ferromagnetic (AFM) correlations at low temperatures and Curie-Weiss type
behavior in the high-temperature regime. Upon further reduction of the doping,
a new energy scale, dominated by spin-interactions () emerges (apparent both
in specific heat and susceptibility) and we identify an effective interaction
, valid across the entire doping range. This is distinct from
Anderson's formula, as we choose here , hence the opposite sign of the
usual Nagaoka-ferromagnetic situation. This expression includes the subtle
effect of weak kinetic AFM - as encountered in the infinitely correlated
situation (). By explicit computation of the Kubo-formulae, we
address the question of practical relevance of the high-frequency expression
for the Hall coefficient . We hope to clarify some open questions
concerning the applicability of the t-J model to real experimental situations
through this study
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