5,682 research outputs found
Some Remarks on Methods of QCD Analysis of Polarized DIS Data
The results on polarized parton densities (PDFs) obtained using different
methods of QCD analysis of the present polarized DIS data are discussed. Their
dependence on the method used in the analysis, accounting or not for the
kinematic and dynamic 1/Q^2 corrections to spin structure function g_1, is
demonstrated. It is pointed out that the precise data in the preasymptotic
region require a more careful matching of the QCD predictions to the data in
this region in order to determine the polarized PDFs correctly.Comment: 14 pages, 8 figure
Observation of a temperature dependent electrical resistance minimum above the magnetic ordering temperature in GdPdSi
Results on electrical resistivity, magnetoresistance, magnetic Results on
electrical resistivity, magnetoresistance, magnetic susceptibility, heat
capacity and Gd Mossbauer measurements on a Gd-based intermetallic compound,
GdPdSi are reported. A finding of interest is that the resistivity
unexpectedly shows a well-defined minimum at about 45 K, well above the long
range magnetic ordering temperature (21 K), a feature which gets suppressed by
the application of a magnetic field. This observation in a Gd alloy presents an
interesting scenario. On the basis of our results, we propose electron
localization induced by s-f (or d-f) exchange interaction prior to long range
magnetic order as a mechanism for the electrical resistance minimum.Comment: 4 pages, 4 figure
Commensurate and modulated magnetic phases in orthorhombic A1C60
Competing magnetically ordered structures in polymerized orthorhombic A1C60
are studied. A mean-field theory for the equilibrium phases is developed using
an Ising model and a classical Heisenberg model to describe the competition
between inter- and intra-chain magnetic order in the solid. In the Ising model,
the limiting commensurate one-dimensional and three-dimensional phases are
separated by a commensurate three-sublattice state and by two sectors
containing higher-order commensurate phases. For the Heisenberg model the
quasi-1D phase is never the equilibrium state; instead the 3D commensurate
phases exhibits a transition to a continuum of coplanar spiral magnetic phases.Comment: 11 pages REVTeX 3.0 plus 4 figures appende
Phonons and Magnetic Excitations in Mott-Insulator LaTiO
The polarized Raman spectra of stoichiometric LaTiO (T K) were
measured between 6 and 300 K. In contrast to earlier report on half-metallic
LaTiO, neither strong background scattering, nor Fano shape of the
Raman lines was observed. The high frequency phonon line at 655 cm
exhibits anomalous softening below T: a signature for structural
rearrangement. The assignment of the Raman lines was done by comparison to the
calculations of lattice dynamics and the nature of structural changes upon
magnetic ordering are discussed. The broad Raman band, which appears in the
antiferromagnetic phase, is assigned to two-magnon scattering. The estimated
superexchange constant meV is in excellent agreement with the
result of neutron scattering studies.Comment: 4 pages, 5 figure
A Perturbative/Variational Approach to Quantum Lattice Hamiltonians
We propose a method to construct the ground state of local
lattice hamiltonians with the generic form , where
is a coupling constant and is a hamiltonian with a non degenerate ground
state . The method is based on the choice of an exponential ansatz
, which is a sort of generalized
lattice version of a Jastrow wave function. We combine perturbative and
variational techniques to get succesive approximations of the operator
. Perturbation theory is used to set up a variational method which
in turn produces non perturbative results. The computation with this kind of
ansatzs leads to associate to the original quantum mechanical problem a
statistical mechanical system defined in the same spatial dimension. In some
cases these statistical mechanical systems turn out to be integrable, which
allow us to obtain exact upper bounds to the energy. The general ideas of our
method are illustrated in the example of the Ising model in a transverse field.Comment: 27 pages, three .ps figures appended, DFTUZ 94-2
Long-lived space observatories for astronomy and astrophysics
NASA's plan to build and launch a fleet of long-lived space observatories that include the Hubble Space Telescope (HST), the Gamma Ray Observatory (GRO), the Advanced X Ray Astrophysics Observatory (AXAF), and the Space Infrared Telescope Facility (SIRTF) are discussed. These facilities are expected to have a profound impact on the sciences of astronomy and astrophysics. The long-lived observatories will provide new insights about astronomical and astrophysical problems that range from the presence of planets orbiting nearby stars to the large-scale distribution and evolution of matter in the universe. An important concern to NASA and the scientific community is the operation and maintenance cost of the four observatories described above. The HST cost about 160 million (1986 dollars) a year to operate and maintain. If HST is operated for 20 years, the accumulated costs will be considerably more than those required for its construction. Therefore, it is essential to plan carefully for observatory operations and maintenance before a long-lived facility is constructed. The primary goal of this report is to help NASA develop guidelines for the operations and management of these future observatories so as to achieve the best possible scientific results for the resources available. Eight recommendations are given
Parquet approach to nonlocal vertex functions and electrical conductivity of disordered electrons
A diagrammatic technique for two-particle vertex functions is used to
describe systematically the influence of spatial quantum coherence and
backscattering effects on transport properties of noninteracting electrons in a
random potential. In analogy with many-body theory we construct parquet
equations for topologically distinct {\em nonlocal} irreducible vertex
functions into which the {\em local} one-particle propagator and two-particle
vertex of the coherent-potential approximation (CPA) enter as input. To
complete the two-particle parquet equations we use an integral form of the Ward
identity and determine the one-particle self-energy from the known irreducible
vertex. In this way a conserving approximation with (Herglotz) analytic
averaged Green functions is obtained. We use the limit of high spatial
dimensions to demonstrate how nonlocal corrections to the (CPA)
solution emerge. The general parquet construction is applied to the calculation
of vertex corrections to the electrical conductivity. With the aid of the
high-dimensional asymptotics of the nonlocal irreducible vertex in the
electron-hole scattering channel we derive a mean-field approximation for the
conductivity with vertex corrections. The impact of vertex corrections onto the
electronic transport is assessed quantitatively within the proposed mean-field
description on a binary alloy.Comment: REVTeX 19 pages, 9 EPS diagrams, 6 PS figure
The ground state of the carbon atom in strong magnetic fields
The ground and a few excited states of the carbon atom in external uniform
magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for
field strengths ranging from zero up to 2.35 10^9 T. With increasing field
strength the ground state undergoes six transitions involving seven different
electronic configurations which belong to three groups with different spin
projections S_z=-1,-2,-3. For weak fields the ground state configuration arises
from the field-free 1s^2 2s^2 2p_0 2p_{-1}, S_z=-1 configuration. With
increasing field strength the ground state involves the four S_z=-2
configurations 1s^22s2p_0 2p_{-1}2p_{+1}, 1s^22s2p_0 2p_{-1}3d_{-2}, 1s^22p_0
2p_{-1}3d_{-2}4f_{-3} and 1s^22p_{-1}3d_{-2}4f_{-3}5g_{-4}, followed by the two
fully spin polarized S_z=-3 configurations 1s2p_02p_{-1}3d_{-2}4f_{-3}5g_{-4}
and 1s2p_{-1}3d_{-2}4f_{-3}5g_{-4}6h_{-5}. The last configuration forms the
ground state of the carbon atom in the high field regime \gamma>18.664. The
above series of ground state configurations is extracted from the results of
numerical calculations for more than twenty electronic configurations selected
due to some general energetical arguments.Comment: 6 figures,acc. Phys.Rev.
Aspects of Type IIB Theory on ALE Spaces
D-brane technology and strong/weak coupling duality supplement traditional
orbifold techniques by making certain background geometries more accessible. In
this spirit, we consider some of the geometric properties of the type IIB
theory on R^6 \times M where M is an `Asymptotically Locally Euclidean (ALE)'
gravitational instanton. Given the self-duality of the theory, we can extract
the geometry (both singular and resolved) seen by the weakly coupled IIB string
by studying the physics of a D1-brane probe. The construction is both amusing
and instructive, as the physics of the probe completely captures the
mathematics of the construction of ALE instantons via `HyperKahler Quotients',
as presented by Kronheimer. This relation has been noted by Douglas and Moore
for the A-series. We extend the explicit construction to the case of the D- and
E-series -- uncovering a quite beautiful structure -- and highlight how all of
the elements of the mathematical construction find their counterparts in the
physics of the type IIB D-string. We discuss the explicit ALE metrics which may
be obtained using these techniques, and comment on the role duality plays in
relating gauged linear sigma models to conformal field theories.Comment: 27 pages, three figures. Uses harvmac.tex and epsf.tex (sentences
corrected on pages 13+14, reference added, small addition to final remarks
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