45,065 research outputs found
Quasifree Knockout Of Deuterons In The â¶Li(α,αd)âŽHe Reaction At 23.6 MeV
αâd correlations in quasi-elastic scattering of 23.6-MeV α particles on the deuteron cluster of the â¶Li target were measured in and off the principal reaction plane. Despite the low c.m. energy of 14.2 MeV, the impulse approximation provides a reasonable description of the quasifree process. Computations were based on the asymptotic αâd S-state wave function and on the cluster-model wave function of â¶Li. Insensitivity of the fits to the details of the â¶Li cluster-model wave function indicates an extreme surface reaction mechanism. The full width at half-maximum of the spectator momentum distribution was found to be 48±6 MeV/c. By comparing the experimental cross section for the quasifree process at the maximum of the angular correlation ((d2Ï/dΩddΩ)=68±9 mb/srÂČ at Ξ=25°,Ξ(d)=45°) with the corresponding cross section for the free process, the probability of finding â¶Li as an αâd cluster was evaluated
Charge correlations and optical conductivity in weakly doped antiferromagnets
We investigate the dynamical charge-charge correlation function and the
optical conductivity in weakly doped antiferromagnets using Mori-Zwanzig
projection technique. The system is described by the two-dimensional t-J model.
The arising matrix elements are evaluated within a cumulant formalism which was
recently applied to investigate magnetic properties of weakly doped
antiferromagnets. Within the present approach the ground state consists of
non-interacting hole quasiparticles. Our spectra agree well with numerical
results calculated via exact diagonalization techniques. The method we employ
enables us to explain the features present in the correlation functions. We
conclude that the charge dynamics at weak doping is governed by transitions
between excited states of spin-bag quasiparticles.Comment: 5 pages, 2 figures, to appear in Europhys. Letter
Analytical ground state for the three-band Hubbard model
For the calculation of charge excitations as those observed in, e.g.,
photo-emission spectroscopy or in electron-energy loss spectroscopy, a correct
description of ground-state charge properties is essential. In strongly
correlated systems like the undoped cuprates this is a highly non-trivial
problem. In this paper we derive a non-perturbative analytical approximation
for the ground state of the three-band Hubbard model on an infinite, half
filled CuO_2 plane. By comparison with Projector Quantum Monte Carlo
calculations it is shown that the resulting expressions correctly describe the
charge properties of the ground state. Relations to other approaches are
discussed. The analytical ground state preserves size consistency and can be
generalized for other geometries, while still being both easy to interpret and
to evaluate.Comment: REVTeX, 8 pages, 6 figures, to appear in Phys. Rev.
Moduli Stabilisation in Heterotic Models with Standard Embedding
In this note we analyse the issue of moduli stabilisation in 4d models
obtained from heterotic string compactifications on manifolds with SU(3)
structure with standard embedding. In order to deal with tractable models we
first integrate out the massive fields. We argue that one can not only
integrate out the moduli fields, but along the way one has to truncate also the
corresponding matter fields. We show that the effective models obtained in this
way do not have satisfactory solutions. We also look for stabilised vacua which
take into account the presence of the matter fields. We argue that this also
fails due to a no-go theorem for Minkowski vacua in the moduli sector which we
prove in the end. The main ingredient for this no-go theorem is the constraint
on the fluxes which comes from the Bianchi identity.Comment: 20 pages, LaTeX; references adde
A Note on Flux Induced Superpotentials in String Theory
Non-vanishing fluxes in M-theory and string theory compactifications induce a
superpotential in the lower dimensional theory. Gukov has conjectured the
explicit form of this superpotential. We check this conjecture for the
heterotic string compactified on a Calabi-Yau three-fold as well as for warped
M-theory compactifications on Spin(7) holonomy manifolds, by performing a
Kaluza-Klein reduction.Comment: 19 pages, no figure
Cumulant approach to weakly doped antiferromagnets
We present a new approach to static and dynamical properties of holes and
spins in weakly doped antiferromagnets in two dimensions. The calculations are
based on a recently introduced cumulant approach to ground--state properties of
correlated electronic systems. The present method allows to evaluate hole and
spin--wave dispersion relations by considering hole or spin excitations of the
ground state. Usually, these dispersions are found from time--dependent
correlation functions. To demonstrate the ability of the approach we first
derive the dispersion relation for the lowest single hole excitation at
half--filling. However, the main purpose of this paper is to focus on the
mutual influence of mobile holes and spin waves in the weakly doped system. It
is shown that low-energy spin excitations strongly admix to the ground--state.
The coupling of spin waves and holes leads to a strong suppression of the
staggered magnetization which can not be explained by a simple rigid--band
picture for the hole quasiparticles. Also the experimentally observed doping
dependence of the spin--wave excitation energies can be understood within our
formalism.Comment: REVTEX, 25 pages, 7 figures (EPS), to be published in Phys. Rev.
String vacua with flux from freely-acting obifolds
A precise correspondence between freely-acting orbifolds (Scherk-Schwarz
compactifications) and string vacua with NSNS flux turned on is established
using T-duality.
We focus our attention to a certain non-compact Z_2 heterotic freely-acting
orbifold with N=2 supersymmetry (SUSY). The geometric properties of the T-dual
background are studied. As expected, the space is non-Kahler with the most
generic torsion compatible with SUSY. All equations of motion are satisfied,
except the Bianchi identity for the NSNS field, that is satisfied only at
leading order in derivatives, i.e. without the curvature term. We point out
that this is due to unknown corrections to the standard heterotic T-duality
rules.Comment: 13 pages, no figures; v2: references added and rearranged, version to
appear in JHE
Bounds on Dark Matter from the ``Atmospheric Neutrino Anomaly''
Bounds are derived on the cross section, flux and energy density of new
particles that may be responsible for the atmospheric neutrino anomaly. Decay of primordial
homogeneous dark matter can be excluded.Comment: 10 pages, TeX (revtex
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