7,062 research outputs found
Description of hard sphere crystals and crystal-fluid interfaces: a critical comparison between density functional approaches and a phase field crystal model
In materials science the phase field crystal approach has become popular to
model crystallization processes. Phase field crystal models are in essence
Landau-Ginzburg-type models, which should be derivable from the underlying
microscopic description of the system in question. We present a study on
classical density functional theory in three stages of approximation leading to
a specific phase field crystal model, and we discuss the limits of
applicability of the models that result from these approximations. As a test
system we have chosen the three--dimensional suspension of monodisperse hard
spheres. The levels of density functional theory that we discuss are
fundamental measure theory, a second-order Taylor expansion thereof, and a
minimal phase-field crystal model. We have computed coexistence densities,
vacancy concentrations in the crystalline phase, interfacial tensions and
interfacial order parameter profiles, and we compare these quantities to
simulation results. We also suggest a procedure to fit the free parameters of
the phase field crystal model.Comment: 21 page
Off-diagonal helicity density matrix elements for heavy vector mesons inclusively produced in N-N, gamma-N, l-N interactions
Final state interactions in quark fragmentation may give origin to non zero
values of the off-diagonal element rho_(1,-1) of the helicity density matrix of
vector mesons V produced in current jets, with a large energy fraction x_E; the
value of rho_(1,-1)(V) is related to the hard constituent dynamics and tests
unusual properties of it. Some recent data on phi, K^* and D^* produced in e^+
e^- annihilations at LEP show such effects. Predictions are given here for
rho_(1,-1) of heavy mesons produced in nucleon-nucleon, gamma-nucleon and
lepton-nucleon interactions.Comment: LaTeX, 10 pages, 1 postscript figure, uses epsfig.sty. Revised
version, to be published on Phys. Lett. B. Some statements added to clarify
tex
Bailey flows and Bose-Fermi identities for the conformal coset models
We use the recently established higher-level Bailey lemma and Bose-Fermi
polynomial identities for the minimal models to demonstrate the
existence of a Bailey flow from to the coset models
where is a
positive integer and is fractional, and to obtain Bose-Fermi identities
for these models. The fermionic side of these identities is expressed in terms
of the fractional-level Cartan matrix introduced in the study of .
Relations between Bailey and renormalization group flow are discussed.Comment: 28 pages, AMS-Latex, two references adde
Linear-in-frequency optical conductivity in GdPtBi due to transitions near the triple points
The complex optical conductivity of the half-Heusler compound GdPtBi is
measured in a frequency range from 20 to 22 000 cm (2.5 meV - 2.73 eV)
at temperatures down to 10 K in zero magnetic field. We find the real part of
the conductivity, , to be almost perfectly linear in
frequency over a broad range from 50 to 800 cm ( 6 - 100 meV) for
K. This linearity strongly suggests the presence of
three-dimensional linear electronic bands with band crossings (nodes) near the
chemical potential. Band-structure calculations show the presence of triple
points, where one doubly degenerate and one nondegenerate band cross each other
in close vicinity of the chemical potential. From a comparison of our data with
the optical conductivity computed from the band structure, we conclude that the
observed nearly linear originates as a cumulative effect
from all the transitions near the triple points.Comment: 5+ pages, 5 figures, band-structure and optical-conductivity
calculations adde
Two-channel conduction in YbPtBi
We investigated transport, magnetotransport, and broadband optical properties
of the half-Heusler compound YbPtBi. Hall measurements evidence two types of
charge carriers: highly mobile electrons with a temperature-dependent
concentration and low-mobile holes; their concentration stays almost constant
within the investigated temperature range from 2.5 to 300 K. The optical
spectra (10 meV - 2.7 eV) can be naturally decomposed into contributions from
intra- and interband absorption processes, the former manifesting themselves as
two Drude bands with very different scattering rates, corresponding to the
charges with different mobilities. These results of the optical measurements
allow us to separate the contributions from electrons and holes to the total
conductivity and to implement a two-channel-conduction model for description of
the magnetotransport data. In this approach, the electron and hole mobilities
are found to be around 50000 and 10 cm/Vs at the lowest temperatures (2.5
K), respectively.Comment: 6 page
Anomalous He-Gas High-Pressure Studies on Superconducting LaO1-xFxFeAs
AC susceptibility measurements have been carried out on superconducting
LaO1-xFxFeAs for x=0.07 and 0.14 under He-gas pressures to about 0.8 GPa. Not
only do the measured values of dTc/dP differ substantially from those obtained
in previous studies using other pressure media, but the Tc(P) dependences
observed depend on the detailed pressure/temperature history of the sample. A
sizeable sensitivity of Tc(P) to shear stresses provides a possible
explanation
Exploring the Oxygen Order in Hg-1223 and Hg-1201 by 199Hg MAS NMR
We demonstrate the use of a high-resolution solid-state fast (45 kHz) magic
angle spinning (MAS) NMR for mapping the oxygen distribution in Hg-based
cuprate superconductors. We identify observed three peaks in 199Hg spectrum as
belonging to the different chemical environments in the HgO? layer with no
oxygen neighbors, single oxygen neighbor, and two oxygen neighbors. We discuss
observed differences between Hg-1201 and Hg-1223 materials.Comment: 4 pages, 2 figures included. Submitted to NATO Advanced Research
Workshop Proceedings (Miami January 2004
High-pressure spin shifts in the pseudogap regime of superconducting YBa2Cu4O8 as revealed by 17O NMR
A new NMR anvil cell design is used for measuring the influence of high
pressure on the electronic properties of the high-temperature superconductor
YBaCuO above the superconducting transition temperature . It is found that pressure increases the spin shift at all temperatures in
such a way that the pseudo-gap feature has almost disappeared at 63 kbar. This
change of the temperature dependent spin susceptibility can be explained by a
pressure induced proportional decrease (factor of two) of a temperature
dependent component, and an increase (factor of 9) of a temperature independent
component, contrary to the effects of increasing doping. The results
demonstrate that one can use anvil cell NMR to investigate the tuning of the
electronic properties of correlated electronic materials with pressure.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev.
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