9,008 research outputs found
Structure determination from powder data : Mogul and CASTEP
When solving the crystal structure of complex molecules from powder data, accurately locating the global minimum can be challenging, particularly where the number of internal degrees of freedom is large. The program Mogul provides a convenient means to access typical torsion angle ranges for fragments related to the molecule of interest. The impact that the application of modal torsion angle constraints has on the structure determination process of two structure solution attempts using DASH is presented. Once solved, accurate refinement of a molecular structure against powder data can also present challenges. Geometry optimisation using density functional theory in CASTEP is shown to be an effective means to locate hydrogen atom positions reliably and return a more accurate description of molecular conformation and intermolecular interactions than global optimisation and Rietveld refinement alone
Superfluidity of bosons on a deformable lattice
We study the superfluid properties of a system of interacting bosons on a
lattice which, moreover, are coupled to the vibrational modes of this lattice,
treated here in terms of Einstein phonon model. The ground state corresponds to
two correlated condensates: that of the bosons and that of the phonons. Two
competing effects determine the common collective soundwave-like mode with
sound velocity , arising from gauge symmetry breaking: i) The sound velocity
(corresponding to a weakly interacting Bose system on a rigid lattice) in
the lowest order approximation is reduced due to reduction of the repulsive
boson-boson interaction, arising from the attractive part of phonon mediated
interaction in the static limit. ii) the second order correction to the sound
velocity is enhanced as compared to the one of bosons on a rigid lattice when
the the boson-phonon interaction is switched on due to the retarded nature of
phonon mediated interaction. The overall effect is that the sound velocity is
practically unaffected by the coupling with phonons, indicating the robustness
of the superfluid state. The induction of a coherent state in the phonon
system, driven by the condensation of the bosons could be of experimental
significance, permitting spectroscopic detections of superfluid properties of
the bosons. Our results are based on an extension of the Beliaev - Popov
formalism for a weakly interacting Bose gas on a rigid lattice to that on a
deformable lattice with which it interacts.Comment: 12 pages, 14 figures, to appear in Phys. Rev.
Collective and single-particle excitations in 2D dipolar Bose gases
The Berezinskii-Kosterlitz-Thouless transition in 2D dipolar systems has been
studied recently by path integral Monte Carlo (PIMC) simulations [A. Filinov et
al., PRL 105, 070401 (2010)]. Here, we complement this analysis and study
temperature-coupling strength dependence of the density (particle-hole) and
single-particle (SP) excitation spectra both in superfluid and normal phases.
The dynamic structure factor, S(q,omega), of the longitudinal excitations is
rigorously reconstructed with full information on damping. The SP spectral
function, A(q,omega), is worked out from the one-particle Matsubara Green's
function. A stochastic optimization method is applied for reconstruction from
imaginary times. In the superfluid regime sharp energy resonances are observed
both in the density and SP excitations. The involved hybridization of both
spectra is discussed. In contrast, in the normal phase, when there is no
coupling, the density modes, beyond acoustic phonons, are significantly damped.
Our results generalize previous zero temperature analyses based on variational
many-body wavefunctions [F. Mazzanti et al., PRL 102, 110405 (2009), D. Hufnagl
et al., PRL 107, 065303 (2011)], where the underlying physics of the excitation
spectrum and the role of the condensate has not been addressed.Comment: 27 pages, 15 figures, 7 table
Conserving Gapless Mean-Field Theory for Bose-Einstein Condensates
We formulate a conserving gapless mean-field theory for Bose-Einstein
condensates on the basis of a Luttinger-Ward thermodynamic functional. It is
applied to a weakly interacting uniform gas with density and s-wave
scattering length to clarify its fundamental thermodynamic properties. It
is found that the condensation here occurs as a first-order transition. The
shift of the transition temperature from the ideal-gas result
is positive and given to the leading order by , in agreement with a couple of previous estimates. The theory is
expected to form a new theoretical basis for trapped Bose-Einstein condensates
at finite temperatures.Comment: Minor errors remove
Calibration of <i>Herschel</i> SPIRE FTS observations at different spectral resolutions
The SPIRE Fourier Transform Spectrometer on-board the Herschel Space Observatory had two standard spectral resolution modes for science observations: high resolution (HR) and low resolution (LR), which could also be performed in sequence (H+LR). A comparison of the HR and LR resolution spectra taken in this sequential mode revealed a systematic discrepancy in the continuum level. Analysing the data at different stages during standard pipeline processing demonstrates that the telescope and instrument emission affect HR and H+LR observations in a systematically different way. The origin of this difference is found to lie in the variation of both the telescope and instrument response functions, while it is triggered by fast variation of the instrument temperatures. As it is not possible to trace the evolution of the response functions using housekeeping data from the instrument subsystems, the calibration cannot be corrected analytically. Therefore, an empirical correction for LR spectra has been developed, which removes the systematic noise introduced by the variation of the response functions
Effective field theory and dispersion law of the phonons of a non-relativistic superfluid
We study the recently proposed effective field theory for the phonon of an
arbitrary non-relativistic superfluid. After computing the one-loop phonon
self-energy, we obtain the low temperature T contributions to the phonon
dispersion law at low momentum, and see that the real part of those can be
parametrized as a thermal correction to the phonon velocity. Because the
phonons are the quanta of the sound waves, at low momentum their velocity
should agree with the speed of sound. We find that our results match at order
T^4ln(T) with those predicted by Andreev and Khalatnikov for the speed of
sound, derived from the superfluid hydrodynamical equations and the phonon
kinetic theory. We get also higher order corrections of order T^4, which are
not reproduced pushing naively the kinetic theory computation. Finally, as an
application, we consider the cold Fermi gas in the unitarity limit, and find a
universal expression for the low T relative correction to the speed of sound
for these systems.Comment: 14 pages, 2 figures. References adde
Fluctuating and dissipative dynamics of dark solitons in quasi-condensates
The fluctuating and dissipative dynamics of matter-wave dark solitons within
harmonically trapped, partially condensed Bose gases is studied both
numerically and analytically. A study of the stochastic Gross-Pitaevskii
equation, which correctly accounts for density and phase fluctuations at finite
temperatures, reveals dark soliton decay times to be lognormally distributed at
each temperature, thereby characterizing the previously predicted long lived
soliton trajectories within each ensemble of numerical realizations (S.P.
Cockburn {\it et al.}, Phys. Rev. Lett. 104, 174101 (2010)). Expectation values
for the average soliton lifetimes extracted from these distributions are found
to agree well with both numerical and analytic predictions based upon the
dissipative Gross-Pitaevskii model (with the same {\it ab initio} damping).
Probing the regime for which , we find average
soliton lifetimes to scale with temperature as , in agreement
with predictions previously made for the low-temperature regime .
The model is also shown to capture the experimentally-relevant decrease in the
visibility of an oscillating soliton due to the presence of background
fluctuations.Comment: 17 pages, 14 figure
Reflection and Refraction of Bose-condensate Excitations
We investigate the transmission and reflection of Bose-condensate excitations
in the low energy limit across a potential barrier separating two condensates
with different densities. The Bogoliubov excitation in the low energy limit has
the incident angle where the perfect transmission occurs. This condition
corresponds to the Brewster's law for the electromagnetic wave. The total
internal reflection of the Bogoliubov excitation is found to occur at a large
incident angle in the low energy limit. The anomalous tunneling named by Kagan
et al. [Yu. Kagan et al., Phys. Rev. Lett., 90, 130402 (2003)] can be
understood in terms of the impedance matching. In the case of the normal
incidence, comparison with the results in Tomonaga-Luttinger liquids is made.Comment: 23 pages, 5 figure
GDASH: a grid-enabled program for structure solution from powder diffraction data
The simulated annealing approach to structure solution from powder diffraction data, as implemented in the DASH program, is easily amenable to parallelization at the individual run level. Very large scale increases in speed of execution can therefore be achieved by distributing individual DASH runs over a network of computers. The GDASH program achieves this by packaging DASH in a form that enables it to run under the Univa UD Grid MP system, which harnesses networks of existing computing resources to perform calculations
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