208 research outputs found
The structure of fluid trifluoromethane and methylfluoride
We present hard X-ray and neutron diffraction measurements on the polar
fluorocarbons HCF3 and H3CF under supercritical conditions and for a range of
molecular densities spanning about a factor of ten. The Levesque-Weiss-Reatto
inversion scheme has been used to deduce the site-site potentials underlying
the measured partial pair distribution functions. The orientational
correlations between adjacent fluorocarbon molecules -- which are characterized
by quite large dipole moments but no tendency to form hydrogen bonds -- are
small compared to a highly polar system like fluid hydrogen chloride. In fact,
the orientational correlations in HCF3 and H3CF are found to be nearly as small
as those of fluid CF4, a fluorocarbon with no dipole moment.Comment: 11 pages, 9 figure
An electrostatically defined serial triple quantum dot charged with few electrons
A serial triple quantum dot (TQD) electrostatically defined in a GaAs/AlGaAs
heterostructure is characterized by using a nearby quantum point contact as
charge detector. Ground state stability diagrams demonstrate control in the
regime of few electrons charging the TQD. An electrostatic model is developed
to determine the ground state charge configurations of the TQD. Numerical
calculations are compared with experimental results. In addition, the tunneling
conductance through all three quantum dots in series is studied. Quantum
cellular automata processes are identified, which are where charge
reconfiguration between two dots occurs in response to the addition of an
electron in the third dot.Comment: 12 pages, 9 figure
All-electron GW calculation based on the LAPW method: application to wurtzite ZnO
We present a new, all-electron implementation of the GW approximation and
apply it to wurtzite ZnO. Eigenfunctions computed in the local-density
approximation (LDA) by the full-potential linearized augmented-plane-wave
(LAPW) or the linearized muffin-tin-orbital (LMTO) method supply the input for
generating the Green function G and the screened Coulomb interaction W. A mixed
basis is used for the expansion of W, consisting of plane waves in the
interstitial region and augmented-wavefunction products in the
augmentation-sphere regions. The frequency-dependence of the dielectric
function is computed within the random-phase approximation (RPA), without a
plasmon-pole approximation. The Zn 3d orbitals are treated as valence states
within the LDA; both core and valence states are included in the self-energy
calculation. The calculated bandgap is smaller than experiment by about 1eV, in
contrast to previously reported GW results. Self-energy corrections are
orbital-dependent, and push down the deep O 2s and Zn 3d levels by about 1eV
relative to the LDA. The d level shifts closer to experiment but the size of
shift is underestimated, suggesting that the RPA overscreens localized states.Comment: 10 pages, 3 figures, submitted to Phys. Rev.
Anomalous relaxations and chemical trends at III-V nitride non-polar surfaces
Relaxations at nonpolar surfaces of III-V compounds result from a competition
between dehybridization and charge transfer. First principles calculations for
the (110) and (100) faces of zincblende and wurtzite AlN, GaN and InN
reveal an anomalous behavior as compared with ordinary III-V semiconductors.
Additional calculations for GaAs and ZnO suggest close analogies with the
latter. We interpret our results in terms of the larger ionicity (charge
asymmetry) and bonding strength (cohesive energy) in the nitrides with respect
to other III-V compounds, both essentially due to the strong valence potential
and absence of core states in the lighter anion. The same interpretation
applies to Zn II-VI compounds.Comment: RevTeX 7 pages, 8 figures included; also available at
http://kalix.dsf.unica.it/preprints/; improved after revie
Cluster structures on quantum coordinate rings
We show that the quantum coordinate ring of the unipotent subgroup N(w) of a
symmetric Kac-Moody group G associated with a Weyl group element w has the
structure of a quantum cluster algebra. This quantum cluster structure arises
naturally from a subcategory C_w of the module category of the corresponding
preprojective algebra. An important ingredient of the proof is a system of
quantum determinantal identities which can be viewed as a q-analogue of a
T-system. In case G is a simple algebraic group of type A, D, E, we deduce from
these results that the quantum coordinate ring of an open cell of a partial
flag variety attached to G also has a cluster structure.Comment: v2: minor corrections. v3: references updated, final version to
appear in Selecta Mathematic
First principles study of strain/electronic interplay in ZnO; Stress and temperature dependence of the piezoelectric constants
We present a first-principles study of the relationship between stress,
temperature and electronic properties in piezoelectric ZnO. Our method is a
plane wave pseudopotential implementation of density functional theory and
density functional linear response within the local density approximation. We
observe marked changes in the piezoelectric and dielectric constants when the
material is distorted. This stress dependence is the result of strong, bond
length dependent, hybridization between the O and Zn electrons. Our
results indicate that fine tuning of the piezoelectric properties for specific
device applications can be achieved by control of the ZnO lattice constant, for
example by epitaxial growth on an appropriate substrate.Comment: accepted for publication in Phys. Rev.
Cluster algebras in algebraic Lie theory
We survey some recent constructions of cluster algebra structures on
coordinate rings of unipotent subgroups and unipotent cells of Kac-Moody
groups. We also review a quantized version of these results.Comment: Invited survey; to appear in Transformation Group
The liquid-vapor interface of an ionic fluid
We investigate the liquid-vapor interface of the restricted primitive model
(RPM) for an ionic fluid using a density-functional approximation based on
correlation functions of the homogeneous fluid as obtained from the
mean-spherical approximation (MSA). In the limit of a homogeneous fluid our
approach yields the well-known MSA (energy) equation of state. The ionic
interfacial density profiles, which for the RPM are identical for both species,
have a shape similar to those of simple atomic fluids in that the decay towards
the bulk values is more rapid on the vapor side than on the liquid side. This
is the opposite asymmetry of the decay to that found in earlier calculations
for the RPM based on a square-gradient theory. The width of the interface is,
for a wide range of temperatures, approximately four times the second moment
correlation length of the liquid phase. We discuss the magnitude and
temperature dependence of the surface tension, and argue that for temperatures
near the triple point the ratio of the dimensionless surface tension and
critical temperature is much smaller for the RPM than for simple atomic fluids.Comment: 6 postscript figures, submitted to Phys. Rev.
Toward polarized antiprotons: Machine development for spin-filtering experiments
The paper describes the commissioning of the experimental equipment and the
machine studies required for the first spin-filtering experiment with protons
at a beam kinetic energy of MeV in COSY. The implementation of a
low- insertion made it possible to achieve beam lifetimes of
s in the presence of a dense polarized hydrogen
storage-cell target of areal density . The developed techniques can be directly
applied to antiproton machines and allow for the determination of the
spin-dependent cross sections via spin filtering
Criticality in confined ionic fluids
A theory of a confined two dimensional electrolyte is presented. The positive
and negative ions, interacting by a potential, are constrained to move on
an interface separating two solvents with dielectric constants and
. It is shown that the Debye-H\"uckel type of theory predicts that
the this 2d Coulomb fluid should undergo a phase separation into a coexisting
liquid (high density) and gas (low density) phases. We argue, however, that the
formation of polymer-like chains of alternating positive and negative ions can
prevent this phase transition from taking place.Comment: RevTex, no figures, in press Phys. Rev.
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