2,402 research outputs found
A comparative study of CO adsorption on flat, stepped and kinked Au surfaces using density functional theory
Our ab initio calculations of CO adsorption energies on low miller index
(111), (100), stepped (211), and kinked (532) gold surfaces show a strong
dependence on local coordination with a reduction in Au atom coordination
leading to higher binding energies. We find trends in adsorption energies to be
similar to those reported in experiments and calculations for other metal
surfaces. The (532) surface provides insights into these trends because of the
availability of a large number of kink sites which naturally have the lowest
coordination (6). We also find that, for all surfaces, an increase in CO
coverage triggers a decrease in the adsorption energy. Changes in the
work-function upon CO adsorption, as well as the frequencies of the CO
vibrational modes are calculated, and their coverage dependence is reported.Comment: 18 pages, 4 figure
Axial anomaly and magnetism of nuclear and quark matter
We consider the response of the QCD ground state at finite baryon density to
a strong magnetic field B. We point out the dominant role played by the
coupling of neutral Goldstone bosons, such as pi^0, to the magnetic field via
the axial triangle anomaly. We show that, in vacuum, above a value of B ~
m_pi^2/e, a metastable object appears - the pi^0 domain wall. Because of the
axial anomaly, the wall carries a baryon number surface density proportional to
B. As a result, for B ~ 10^{19} G a stack of parallel pi^0 domain walls is
energetically more favorable than nuclear matter at the same density.
Similarly, at higher densities, somewhat weaker magnetic fields of order B ~
10^{17}-10^{18} G transform the color-superconducting ground state of QCD into
new phases containing stacks of axial isoscalar (eta or eta') domain walls. We
also show that a quark-matter state known as ``Goldstone current state,'' in
which a gradient of a Goldstone field is spontaneously generated, is
ferromagnetic due to the axial anomaly. We estimate the size of the fields
created by such a state in a typical neutron star to be of order
10^{14}-10^{15} G.Comment: 18 pages, v2: added a discussion of the energy cost of neutralizing
the domain wall charg
ZnSe/GaAs(001) heterostructures with defected interfaces: structural, thermodynamic and electronic properties
We have performed accurate \emph{ab--initio} pseudopotential calculations for
the structural and electronic properties of ZnSe/GaAs(001) heterostructures
with interface configurations accounting for charge neutrality prescriptions.
Beside the simplest configurations with atomic interdiffusion we consider also
some configurations characterized by As depletion and cation vacancies,
motivated by the recent successfull growth of ZnSe/GaAs pseudomorphic
structures with minimum stacking fault density characterized by the presence of
a defected (Zn,Ga)Se alloy in the interface region. We find that--under
particular thermodynamic conditions--some defected configurations are favoured
with respect to undefected ones with simple anion or cation mixing, and that
the calculated band offsets for some defected structures are compatible with
those measured. Although it is not possible to extract indications about the
precise interface composition and vacancy concentration, our results support
the experimental indication of (Zn,Ga)Se defected compounds in high-quality
ZnSe/GaAs(001) heterojunctions with low native stacking fault density. The
range of measured band offset suggests that different atoms at interfaces
rearrange, with possible presence of vacancies, in such a way that not only
local charges but also ionic dipoles are vanishing.Comment: 26 pages. 5 figures, revised version, in press (Physical Review B
Superconductivity in the Correlated Pyrochlore Cd_2Re_2O_7
We report the observation of superconductivity in high-quality
CdReO single crystals with room-temperature pyrochlore structure.
Resistivity and ac susceptibility measurements establish an onset transition
temperature T = 1.47 K with transition width T = 0.25
K. In applied magnetic field, the resistive transition shows a type-II
character, with an approximately linear temperature-dependence of the upper
critical field H. The bulk nature of the superconductivity is confirmed
by the specific heat jump with C = 37.9 mJ/mol-K. Using the
value extracted from normal-state specific heat data, we obtain
C/T = 1.29, close to the weak coupling BCS value. In the
normal state, a negative Hall coefficient below 100 K suggests electron-like
conduction in this material. The resistivity exhibits a quadratic T-dependence
between 2 and 60 K, i.e., +AT, indicative of Fermi-liquid
behavior. The values of the Kadowaki-Woods ratio A/ and the Wilson
ratio are comparable to that for strongly correlated materials.Comment: 4 pages, 5 figure
New Pseudo-Phase Structure for -Pu
In this paper we propose a new pseudo-phase crystal structure, based on an
orthorhombic distortion of the diamond structure, for the ground-state
-phase of plutonium. Electronic-structure calculations in the
generalized-gradient approximation give approximately the same total energy for
the two structures. Interestingly, our new pseudo-phase structure is the same
as the Pu -phase structure except with very different b/a and c/a
ratios. We show how the contraction relative to the phase, principally
in the direction, leads to an -like structure in the [0,1,1] plane.
This is an important link between two complex structures of plutonium and opens
new possibilities for exploring the very rich phase diagram of Pu through
theoretical calculations
Low Temperature Symmetry of Pyrochlore Oxide Cd2Re2O7
We report the X-ray study for the pyrochlore oxide Cd2Re2O7. Two
symmetry-lowering structural transitions were observed at Ts1=200K and
Ts2=120K. The former is of the second order from the ideal cubic pyrochlore
structure with space group Fd-3m to a tetragonally distorted structure with
I-4m2, while the latter is of the first order likely to another tetragonal
space group I4122. We discuss the feature of the lattice deformation.Comment: 4 pages, 4 figure
Intrinsic transverse momentum and the polarized Drell-Yan process
In this paper we study the cross section at leading order in for
polarized Drell-Yan scattering at measured lepton-pair transverse momentum
. We find that for a hadron with spin the quark content at leading
order is described by six distribution functions for each flavor, which depend
on both the lightcone momentum fraction , and the quark transverse momentum
\bbox{k}_T^2. These functions are illustrated for a free-quark ensemble. The
cross sections for both longitudinal and transverse polarizations are expressed
in terms of convolution integrals over the distribution functions.Comment: 25 pages, REVTEX 3.0 (3 figures included in separate LATEX file using
feynman.tex), NIKHEF-94-P1 (Revised version
Optimisation of bulk carrier loading and discharge
This report summarises progress made towards the problem submitted by Rusal Aughinish at the 93rd European Study Group with Industry. Rusal Aughinish is a company that refines alumina from bauxite. The problem presented to the study group was to review the percentage of time that the company’s inner berth was occupied and how to minimise this percent- age. A number of different approaches were taken with this aim in mind. Firstly, data supplied by Rusal Aughinish was analysed. This analysis found that there is an optimal loading rate (with respect to eliminating demurrage costs) and suggested bands of optimal ship sizes. Further to these studies, two models of Rusal Aughinish’s shipping process were developed by the group: a simulation model and an analytical model. Both models were found to replicate the shipping process reasonably well and were, hence, used to study alumina output, berth occupancy and demurrage costs
Million-atom molecular dynamics simulation by order-N electronic structure theory and parallel computation
Parallelism of tight-binding molecular dynamics simulations is presented by
means of the order-N electronic structure theory with the Wannier states,
recently developed (J. Phys. Soc. Jpn. 69,3773 (2000)). An application is
tested for silicon nanocrystals of more than millions atoms with the
transferable tight-binding Hamiltonian. The efficiency of parallelism is
perfect, 98.8 %, and the method is the most suitable to parallel computation.
The elapse time for a system of atoms is 3.0 minutes by a
computer system of 64 processors of SGI Origin 3800. The calculated results are
in good agreement with the results of the exact diagonalization, with an error
of 2 % for the lattice constant and errors less than 10 % for elastic
constants.Comment: 5 pages, 3 figure
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