8,363 research outputs found
Flux-lattice melting in LaOFFeAs: first-principles prediction
We report the theoretical study of the flux-lattice melting in the novel
iron-based superconductor and
. Using the Hypernetted-Chain closure and an
efficient algorithm, we calculate the two-dimensional one-component plasma pair
distribution functions, static structure factors and direct correlation
functions at various temperatures. The Hansen-Verlet freezing criterion is
shown to be valid for vortex-liquid freezing in type-II superconductors.
Flux-lattice meting lines for and
are predicted through the combination of the density
functional theory and the mean-field substrate approach.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Non-Markovian finite-temperature two-time correlation functions of system operators: beyond the quantum regression theorem
An extremely useful evolution equation that allows systematically calculating
the two-time correlation functions (CF's) of system operators for non-Markovian
open (dissipative) quantum systems is derived. The derivation is based on
perturbative quantum master equation approach, so non-Markovian open quantum
system models that are not exactly solvable can use our derived evolution
equation to easily obtain their two-time CF's of system operators, valid to
second order in the system-environment interaction. Since the form and nature
of the Hamiltonian are not specified in our derived evolution equation, our
evolution equation is applicable for bosonic and/or fermionic environments and
can be applied to a wide range of system-environment models with any factorized
(separable) system-environment initial states (pure or mixed). When applied to
a general model of a system coupled to a finite-temperature bosonic environment
with a system coupling operator L in the system-environment interaction
Hamiltonian, the resultant evolution equation is valid for both L = L^+ and L
\neq L^+ cases, in contrast to those evolution equations valid only for L = L^+
case in the literature. The derived equation that generalizes the quantum
regression theorem (QRT) to the non-Markovian case will have broad applications
in many different branches of physics. We then give conditions on which the QRT
holds in the weak system-environment coupling case, and apply the derived
evolution equation to a problem of a two-level system (atom) coupled to a
finite-temperature bosonic environment (electromagnetic fields) with L \neq
L^+.Comment: To appear in the Journal of Chemical Physics (12 pages, 1 figure
A Deterministic and Storable Single-Photon Source Based on Quantum Memory
A single photon source is realized with a cold atomic ensemble (Rb
atoms). In the experiment, single photons, which is initially stored in an
atomic quantum memory generated by Raman scattering of a laser pulse, can be
emitted deterministically at a time-delay in control. It is shown that
production rate of single photons can be enhanced by a feedback circuit
considerably while the single-photon quality is conserved. Thus our present
single-photon source is well suitable for future large-scale realization of
quantum communication and linear optical quantum computation
Recommended from our members
Eocene–early Oligocene climate and vegetation change in southern China: Evidence from the Maoming Basin
Although the Eocene-Oligocene climate transition marks a critical point in the development of the ‘icehouse’ global climate of the present little is known about this important change in the terrestrial realm at low latitudes. Our palynological study of the Shangcun Formation shows it to be early Oligocene in age: palyno-assemblages in the lower part of the formation indicate a cool interval dominated by conifer pollen in the earliest Oligocene followed by a warmer regime in the second half of the early Oligocene. To quantify middle Eocene to late early Oligocene climate conditions at low (~ 20°N) palaeolatitudes in southern Asia several thousand leaf fossil specimens from the Maoming Basin, southern China, were subjected to a multivariate (CLAMP) analysis of leaf form. For terrestrial palaeoclimate comparisons to be valid the palaeoaltitude at which the proxy data are obtained must be known. We find that leaves preserved in the Youganwo (middle Eocene), Huangniuling (late Eocene) and Shangcun (early Oligocene) formations were likely to have been deposited well above sea level at different palaeoelevations. In the Youganwo Formation fine-grained sediments were deposited at an altitude of ~ 1.5 km, after which the basin dropped to ~ 0.5 km by the time the upper Huangniuling sediments were deposited. The basin floor then rose again by 0.5 km reaching an altitude of approximately 1 km in which the Shangcun Formation fine-grained sediments were accumulated. Within the context of these elevation changes the prevailing climates experienced by the Youganwo, Lower Huangniuling, Upper Huangniuling and Shangcun fossil floras were humid subtropical with hot summers and warm winters, but witnessed a progressive increase in rainfall seasonality. By the early Oligocene rainfall seasonality was similar to that of the modern monsoonal climate of Guangdong Province, southern China. All floras show leaf physiognomic spectra most similar to those growing under the influence of the modern Indonesia-Australia Monsoon, but with no evidence of any adaptation to today's South or East Asia Monsoon regimes. The Upper Huangniuling Flora, rich in dipterocarp plant megafossils, grew in the warmest conditions with the highest cold month mean temperature and at the lowest altitude
Shear and Bulk Viscosities of a Gluon Plasma in Perturbative QCD: Comparison of Different Treatments for the gg<->ggg Process
The leading order contribution to the shear and bulk viscosities, \eta and
\zeta, of a gluon plasma in perturbative QCD includes the gg -> gg (22)
process, gg ggg (23) process and multiple scattering processes known as the
Landau-Pomeranchuk-Migdal (LPM) effect. Complete leading order computations for
\eta and \zeta were obtained by Arnold, Moore and Yaffe (AMY) and Arnold, Dogan
and Moore (ADM), respectively, with the inelastic processes computed by an
effective g gg gluon splitting. We study how complementary calculations
with 22 and 23 processes and a simple treatment to model the LPM effect compare
with the results of AMY and ADM. We find that our results agree with theirs
within errors. By studying the contribution of the 23 process to \eta, we find
that the minimum angle \theta among the final state gluons in the fluid local
rest frame has a distribution that is peaked at \theta \sim \sqrt{\alpha_{s}},
analogous to the near collinear splitting asserted by AMY and ADM. However, the
average of \theta is much bigger than its peak value, as its distribution is
skewed with a long tail. The same \theta behavior is also seen if the 23 matrix
element is taken to the soft gluon bremsstrahlung limit in the center-of-mass
(CM) frame. This suggests that the soft gluon bremsstrahlung in the CM frame
still has some near collinear behavior in the fluid local rest frame. We also
generalize our result to a general SU(N_c) pure gauge theory and summarize the
current viscosity computations in QCD.Comment: ReVTex 4, 18 pages, 7 figures, accepted version in Phys. Rev.
Modeling and Design Algorithms for Electromagnetic Pumps
Electromagnetic (EM) induction pumps are used extensively in current and proposed nuclear power systems and industrial molten metal transfer operations. Although the Magnetohydrodynamic (MHD) theory that underlies the operation of these types of pumps has been studied extensively in the past few decades, the design of specific EM pumping systems for specific flow cases requires computational tools and expertise, which is lacking in the U.S. However, for the past two years, researchers at UNLV have been utilizing the TC-1 liquid metal loop system at UNLV and an Annular Linear Induction Pump (ALIP) to drive the liquid metal and to develop such computational tools that will allow the accurate and efficient optimization of EM pump systems for nuclear applications.
The research objectives of this task are:
• A literature review of topics pertinent to EM pump design. These topics include the equations governing the physical phenomena occurring in EM pumps and mathematical algorithms used in modeling these physical phenomena, different EM pump configurations, and the effects of materials properties on pump performance.
• Development of computational models of the TC-1 loop at UNLV.
• Evaluation of the computational models through comparison with experimental data taken on the TC-1 loop.
• A parametric study of the TC-1 loop investigating the pumping efficiency as a function of operating conditions, materials properties, and geometric parameters
- …