1,246 research outputs found
Thermodiffusion in model nanofluids by molecular dynamics simulations
In this work, a new algorithm is proposed to compute single particle
(infinite dilution) thermodiffusion using Non-Equilibrium Molecular Dynamics
simulations through the estimation of the thermophoretic force that applies on
a solute particle. This scheme is shown to provide consistent results for
simple Lennard-Jones fluids and for model nanofluids (spherical non-metallic
nanoparticles + Lennard-Jones fluid) where it appears that thermodiffusion
amplitude, as well as thermal conductivity, decrease with nanoparticles
concentration. Then, in nanofluids in the liquid state, by changing the nature
of the nanoparticle (size, mass and internal stiffness) and of the solvent
(quality and viscosity) various trends are exhibited. In all cases the single
particle thermodiffusion is positive, i.e. the nanoparticle tends to migrate
toward the cold area. The single particle thermal diffusion 2 coefficient is
shown to be independent of the size of the nanoparticle (diameter of 0.8 to 4
nm), whereas it increases with the quality of the solvent and is inversely
proportional to the viscosity of the fluid. In addition, this coefficient is
shown to be independent of the mass of the nanoparticle and to increase with
the stiffness of the nanoparticle internal bonds. Besides, for these
configurations, the mass diffusion coefficient behavior appears to be
consistent with a Stokes-Einstein like law
Relativistic coupled-cluster single-double method applied to alkali-metal atoms
A relativistic version of the coupled-cluster single-double (CCSD) method is
developed for atoms with a single valence electron. In earlier work, a
linearized version of the CCSD method (with extensions to include a dominant
class of triple excitations) led to accurate predictions for energies,
transition amplitudes, hyperfine constants, and other properties of monovalent
atoms. Further progress in high-precision atomic structure calculations for
heavy atoms calls for improvement of the linearized coupled-cluster
methodology. In the present work, equations for the single and double
excitation coefficients of the Dirac-Fock wave function, including all
non-linear coupled-cluster terms that contribute at the single-double level are
worked out. Contributions of the non-linear terms to energies, electric-dipole
matrix elements, and hyperfine constants of low-lying states in alkali-metal
atoms from Li to Cs are evaluated and the results are compared with other
calculations and with precise experiments.Comment: 12 page
Why you think Milan is larger than Modena: Neural correlates of the recognition heuristic
When ranking two alternatives by some criteria and only one of the alternatives is recognized, participants overwhelmingly adopt the strategy, termed the recognition heuristic (RH), of choosing the recognized alternative. Understanding the neural correlates underlying decisions that follow the RH could help determine whether people make judgments about the RH's applicability or simply choose the recognized alternative. We measured brain activity by using functional magnetic resonance imaging while participants indicated which of two cities they thought was larger (Experiment 1) or which city they recognized (Experiment 2). In Experiment 1, increased activation was observed within the anterior frontomedian cortex (aFMC), precuneus, and retrosplenial cortex when participants followed the RH compared to when they did not. Experiment 2 revealed that RH decisional processes cannot be reduced to recognition memory processes. As the aFMC has previously been associated with self-referential judgments, we conclude that RH decisional processes involve an assessment about the applicability of the RH
Spectroscopy of Ultracold, Trapped Cesium Feshbach Molecules
We explore the rich internal structure of Cs_2 Feshbach molecules. Pure
ultracold molecular samples are prepared in a CO_2-laser trap, and a multitude
of weakly bound states is populated by elaborate magnetic-field ramping
techniques. Our methods use different Feshbach resonances as input ports and
various internal level crossings for controlled state transfer. We populate
higher partial-wave states of up to eight units of rotational angular momentum
(l-wave states). We investigate the molecular structure by measurements of the
magnetic moments for various states. Avoided level crossings between different
molecular states are characterized through the changes in magnetic moment and
by a Landau-Zener tunneling method. Based on microwave spectroscopy, we present
a precise measurement of the magnetic-field dependent binding energy of the
weakly bound s-wave state that is responsible for the large background
scattering length of Cs. This state is of particular interest because of its
quantum-halo character.Comment: 15 pages, 12 figures, 4 table
The origin of hysteresis in resistive switching in magnetite is Joule heating
In many transition metal oxides the electrical resistance is observed to
undergo dramatic changes induced by large biases. In magnetite, FeO,
below the Verwey temperature, an electric field driven transition to a state of
lower resistance was recently found, with hysteretic current-voltage response.
We report the results of pulsed electrical conduction measurements in epitaxial
magnetite thin films. We show that while the high- to low-resistance transition
is driven by electric field, the hysteresis observed in curves results
from Joule heating in the low resistance state. The shape of the hysteresis
loop depends on pulse parameters, and reduces to a hysteresis-free "jump" of
the current provided thermal relaxation is rapid compared to the time between
voltage pulses. A simple relaxation time thermal model is proposed that
captures the essentials of the hysteresis mechanism.Comment: 7 pages, 6 figure
Lifetime Measurement of the 6s Level of Rubidium
We present a lifetime measurements of the 6s level of rubidium. We use a
time-correlated single-photon counting technique on two different samples of
rubidium atoms. A vapor cell with variable rubidium density and a sample of
atoms confined and cooled in a magneto-optical trap. The 5P_{1/2} level serves
as the resonant intermediate step for the two step excitation to the 6s level.
We detect the decay of the 6s level through the cascade fluorescence of the
5P_{3/2} level at 780 nm. The two samples have different systematic effects,
but we obtain consistent results that averaged give a lifetime of 45.57 +- 0.17
ns.Comment: 10 pages, 9 figure
Collisional relaxation of Feshbach molecules and three-body recombination in 87Rb Bose-Einstein condensates
We predict the resonance enhanced magnetic field dependence of atom-dimer
relaxation and three-body recombination rates in a Rb Bose-Einstein
condensate (BEC) close to 1007 G. Our exact treatments of three-particle
scattering explicitly include the dependence of the interactions on the atomic
Zeeman levels. The Feshbach resonance distorts the entire diatomic energy
spectrum causing interferences in both loss phenomena. Our two independent
experiments confirm the predicted recombination loss over a range of rate
constants that spans four orders of magnitude.Comment: 4 pages, 3 eps figures (updated references
Time evolution of Matrix Product States
In this work we develop several new simulation algorithms for 1D many-body
quantum mechanical systems combining the Matrix Product State variational
ansatz with Taylor, Pade and Arnoldi approximations to the evolution operator.
By comparing all methods with previous techniques based on Trotter
decompositions we demonstrate that the Arnoldi method is the best one, reaching
extremely good accuracy with moderate resources. Finally we apply this
algorithm to studying the formation of molecules in an optical lattices when
crossing a Feschbach resonance with a cloud of two-species hard-core bosons.Comment: More extensive comparison with all nearest-neighbor spin s=1/2
models. The results in this manuscript have been superseded by a more
complete work in cond-mat/061021
Power-Based Droop Control in DC Microgrids Enabling Seamless Disconnection From Upstream Grids
This paper proposes a local power-based droop controller for distributed energy resource converters in dc microgrids that are connected to upstream grids by grid-interface converters. During normal operation, the grid-interface converter imposes the microgrid bus voltage, and the proposed controller allows power flow regulation at distributed energy resource converters\u2019 output. On the other hand, during abnormal operation of the grid-interface converter (e.g., due to faults in the upstream grid), the proposed controller allows bus voltage regulation by droop control. Notably, the controller can autonomously convert from power flow control to droop control, without any need of bus voltage variation detection schemes or communication with other microgrid components, which enables seamless transitions between these two modes of operation. Considering distributed energy resource converters employing the power-based droop control, the operation modes of a single converter and of the whole microgrid are defined and investigated herein. The controller design is also introduced. Furthermore, the power sharing performance of this control approach is analyzed and compared with that of classical droop control. The experimental results from a laboratory-scale dc microgrid prototype are reported to show the final performances of the proposed power-based droop control
- …