50,023 research outputs found
Study of gossamer superconductivity and antiferromagnetism in the t-J-U model
The d-wave superconductivity (dSC) and antiferromagnetism are analytically
studied in a renormalized mean field theory for a two dimensional t-J model
plus an on-site repulsive Hubbard interaction . The purpose of introducing
the term is to partially impose the no double occupancy constraint by
employing the Gutzwiller approximation. The phase diagrams as functions of
doping and are studied. Using the standard value of and
in the large limit, we show that the antiferromagnetic (AF) order emerges
and coexists with the dSC in the underdoped region below the doping
. The dSC order parameter increases from zero as the doping
increases and reaches a maximum near the optimal doping . In
the small limit, only the dSC order survives while the AF order disappears.
As increased to a critical value, the AF order shows up and coexists with
the dSC in the underdoped regime. At half filing, the system is in the dSC
state for small and becomes an AF insulator for large . Within the
present mean field approach, We show that the ground state energy of the
coexistent state is always lower than that of the pure dSC state.Comment: 7 pages, 8 figure
Constraining the bulk Lorentz factor from the photosphere emission
We propose a direct and model-independent method to constrain the Lorentz
factor of a relativistically expanding object, like gamma-ray bursts. Only the
measurements, such as thermal component of the emission, the distance and the
variable time scale of the light curve, are used. If the uncertainties are
considered, we will obtain lower limits of the Lorentz factor instead. We apply
this method to GRB 090618 and get a lower limit of the Lorentz factor to be 22.
The method can be used to any relativistically moving object, such as gamma-ray
bursts, blazars, and soft gamma-ray repeaters, providing the thermal component
of the emission being observed.Comment: 10 pages, 1 figur
Coexistence of full which-path information and interference in Wheelers delayed choice experiment with photons
We present a computer simulation model that is a one-to-one copy of an
experimental realization of Wheeler's delayed choice experiment that employs a
single photon source and a Mach-Zehnder interferometer composed of a 50/50
input beam splitter and a variable output beam splitter with adjustable
reflection coefficient (V. Jacques {\sl et al.}, Phys. Rev. Lett. 100,
220402 (2008)). For , experimentally measured values of the
interference visibility and the path distinguishability , a parameter
quantifying the which-path information WPI, are found to fulfill the
complementary relation , thereby allowing to obtain partial WPI
while keeping interference with limited visibility. The simulation model that
is solely based on experimental facts, that satisfies Einstein's criterion of
local causality and that does not rely on any concept of quantum theory or of
probability theory, reproduces quantitatively the averages calculated from
quantum theory. Our results prove that it is possible to give a particle-only
description of the experiment, that one can have full WPI even if D=0, V=1 and
therefore that the relation cannot be regarded as quantifying
the notion of complementarity.Comment: Physica E, in press; see also http://www.compphys.ne
Fermi surface evolution in the antiferromagnetic state for the electron-doped t-t'-t''-J model
By use of the slave-boson mean-field approach, we have studied the
electron-doped t-t'-t''-J model in the antiferromagnetic (AF) state. It is
found that at low doping the Fermi surface (FS) pockets appear around
and , and upon increasing doping the other ones will
form around . The evolution of the FS with
doping as well as the calculated spectral weight are consistent with the
experimental results.Comment: Fig. 4 is updated, to appear in Phys. Rev.
Vapor-liquid phase separator studies
A study of porous plug use for vapor-liquid phase seperation in spaceborne cryogenic systems was conducted. The three main topics addressed were: (1) the usefulness of porous media in designs that call for variable areas and flow rates; (2) the possibility of prediction of main parameters of porous plugs for a given material; and (3) prediction of all parameters of the plug, including secondary parameters
Predicting floods in a large karst river basin by coupling PERSIANN-CCS QPEs with a physically based distributed hydrological model
In general, there are no long-term meteorological or hydrological data available for karst river basins. The lack of rainfall data is a great challenge that hinders the development of hydrological models. Quantitative precipitation estimates (QPEs) based on weather satellites offer a potential method by which rainfall data in karst areas could be obtained. Furthermore, coupling QPEs with a distributed hydrological model has the potential to improve the precision of flood predictions in large karst watersheds. Estimating precipitation from remotely sensed information using an artificial neural network-cloud classification system (PERSIANN-CCS) is a type of QPE technology based on satellites that has achieved broad research results worldwide. However, only a few studies on PERSIANN-CCS QPEs have occurred in large karst basins, and the accuracy is generally poor in terms of practical applications. This paper studied the feasibility of coupling a fully physically based distributed hydrological model, i.e., the Liuxihe model, with PERSIANN-CCS QPEs for predicting floods in a large river basin, i.e., the Liujiang karst river basin, which has a watershed area of 58 270 km-2, in southern China. The model structure and function require further refinement to suit the karst basins. For instance, the sub-basins in this paper are divided into many karst hydrology response units (KHRUs) to ensure that the model structure is adequately refined for karst areas. In addition, the convergence of the underground runoff calculation method within the original Liuxihe model is changed to suit the karst water-bearing media, and the Muskingum routing method is used in the model to calculate the underground runoff in this study. Additionally, the epikarst zone, as a distinctive structure of the KHRU, is carefully considered in the model. The result of the QPEs shows that compared with the observed precipitation measured by a rain gauge, the distribution of precipitation predicted by the PERSIANN-CCS QPEs was very similar. However, the quantity of precipitation predicted by the PERSIANN-CCS QPEs was smaller. A post-processing method is proposed to revise the products of the PERSIANN-CCS QPEs. The karst flood simulation results show that coupling the post-processed PERSIANN-CCS QPEs with the Liuxihe model has a better performance relative to the result based on the initial PERSIANN-CCS QPEs. Moreover, the performance of the coupled model largely improves with parameter re-optimization via the post-processed PERSIANN-CCS QPEs. The average values of the six evaluation indices change as follows: the Nash-Sutcliffe coefficient increases by 14 %, the correlation coefficient increases by 15 %, the process relative error decreases by 8 %, the peak flow relative error decreases by 18 %, the water balance coefficient increases by 8 %, and the peak flow time error displays a 5 h decrease. Among these parameters, the peak flow relative error shows the greatest improvement; thus, these parameters are of page1506 the greatest concern for flood prediction. The rational flood simulation results from the coupled model provide a great practical application prospect for flood prediction in large karst river basins
An MHD Model For Magnetar Giant Flares
Giant flares on soft gamma-ray repeaters that are thought to take place on
magnetars release enormous energy in a short time interval. Their power can be
explained by catastrophic instabilities occurring in the magnetic field
configuration and the subsequent magnetic reconnection. By analogy with the
coronal mass ejection (CME) events on the Sun, we develop a theoretical model
via an analytic approach for magnetar giant flares. In this model, the rotation
and/or displacement of the crust causes the field to twist and deform, leading
to flux rope formation in the magnetosphere and energy accumulation in the
related configuration. When the energy and helicity stored in the configuration
reach a threshold, the system loses its equilibrium, the flux rope is ejected
outward in a catastrophic way, and magnetic reconnection helps the catastrophe
develop to a plausible eruption. By taking SGR 1806 - 20 as an example, we
calculate the free magnetic energy released in such an eruptive process and
find that it is more than ergs, which is enough to power a giant
flare. The released free magnetic energy is converted into radiative energy,
kinetic energy and gravitational energy of the flux rope. We calculated the
light curves of the eruptive processes for the giant flares of SGR 1806 - 20,
SGR 0526-66 and SGR 1900+14, and compared them with the observational data. The
calculated light curves are in good agreement with the observed light curves of
giant flares.Comment: Accepted to Ap
The exact dynamical solution for two dust shells collapsing towards a black hole
The gravitational collapse of a star is an important issue both for general
relativity and astrophysics, which is related to the well known "frozen star"
paradox. Following the seminal work of Oppenheimer and Schneider (1939), we
present the exact solution for two dust shells collapsing towards a
pre-existing black hole. We find that the inner region of the shell is
influenced by the property of the shell, which is contrary to the result in
Newtonian theory and and the clock inside the shell becomes slower as the shell
collapses towards the pre-existing black hole. This result in principle may be
tested experimentally if a beam of light travels across the shell. We conclude
that the concept of the "frozen star" should be abandoned, since matter can
indeed cross a black hole's horizon according to the clock of an external
observer. Since matter will not accumulate around the event horizon of a black
hole, we predict that only gravitational wave radiation can be produced in the
final stage of the merging process of two coalescing black holes. Our results
also indicate that for the clock of an external observer, matter, after
crossing the event horizon, will never arrive at the "singularity" (i.e. the
exact center of the black hole.Comment: 4 pages, 3 figures, contributed talk to the Second Kolkata Conference
on Observational Evidence for Black Holes in the Universe, Feb. 2008, Editor
Sandip Chakrabart
Quantum Decoherence at Finite Temperatures
We study measures of decoherence and thermalization of a quantum system
in the presence of a quantum environment (bath) . The whole system is
prepared in a canonical thermal state at a finite temperature. Applying
perturbation theory with respect to the system-environment coupling strength,
we find that under common Hamiltonian symmetries, up to first order in the
coupling strength it is sufficient to consider the uncoupled system to predict
decoherence and thermalization measures of . This decoupling allows closed
form expressions for perturbative expansions for the measures of decoherence
and thermalization in terms of the free energies of and of . Numerical
results for both coupled and decoupled systems with up to 40 quantum spins
validate these findings.Comment: 5 pages, 3 figure
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