2,140 research outputs found
Mossbauer neutrinos in quantum mechanics and quantum field theory
We demonstrate the correspondence between quantum mechanical and quantum
field theoretical descriptions of Mossbauer neutrino oscillations. First, we
compute the combined rate of Mossbauer neutrino emission, propagation,
and detection in quantum field theory, treating the neutrino as an internal
line of a tree level Feynman diagram. We include explicitly the effect of
homogeneous line broadening due to fluctuating electromagnetic fields in the
source and detector crystals and show that the resulting formula for
is identical to the one obtained previously (Akhmedov et al., arXiv:0802.2513)
for the case of inhomogeneous line broadening. We then proceed to a quantum
mechanical treatment of Mossbauer neutrinos and show that the oscillation,
coherence, and resonance terms from the field theoretical result can be
reproduced if the neutrino is described as a superposition of Lorentz-shaped
wave packet with appropriately chosen energies and widths. On the other hand,
the emission rate and the detection cross section, including localization and
Lamb-Mossbauer terms, cannot be predicted in quantum mechanics and have to be
put in by hand.Comment: LaTeX, 16 pages, 1 figure; v2: typos corrected; matches published
versio
A Geometric Presentation of the λ2-Modules of C(n)(q) and D(n)(q)
AbstractThe fixed-point sheaf of the λ2-module V for each of the groups Cn(q) and Dn(q) is constructed. It is shown that the 0-homology module of the sheaf is isomorphic to V. This gives a presentation of V by geometric generators and relations
Bell Inequalities in Phase Space and their Violation in Quantum Mechanics
We derive ``Bell inequalities'' in four dimensional phase space and prove the
following ``three marginal theorem'' for phase space densities
, thus settling a long standing
conjecture : ``there exist quantum states for which more than three of the
quantum probability distributions for , , and
cannot be reproduced as marginals of a positive
''. We also construct the most
general positive which reproduces
any three of the above quantum probability densities for arbitrary quantum
states. This is crucial for the construction of a maximally realistic quantum
theory.Comment: 11 pages, latex, no figure
A framework for bounding nonlocality of state discrimination
We consider the class of protocols that can be implemented by local quantum
operations and classical communication (LOCC) between two parties. In
particular, we focus on the task of discriminating a known set of quantum
states by LOCC. Building on the work in the paper "Quantum nonlocality without
entanglement" [BDF+99], we provide a framework for bounding the amount of
nonlocality in a given set of bipartite quantum states in terms of a lower
bound on the probability of error in any LOCC discrimination protocol. We apply
our framework to an orthonormal product basis known as the domino states and
obtain an alternative and simplified proof that quantifies its nonlocality. We
generalize this result for similar bases in larger dimensions, as well as the
"rotated" domino states, resolving a long-standing open question [BDF+99].Comment: 33 pages, 7 figures, 1 tabl
Exact Diagonalization of Two Quantum Models for the Damped Harmonic Oscillator
The damped harmonic oscillator is a workhorse for the study of dissipation in
quantum mechanics. However, despite its simplicity, this system has given rise
to some approximations whose validity and relation to more refined descriptions
deserve a thorough investigation. In this work, we apply a method that allows
us to diagonalize exactly the dissipative Hamiltonians that are frequently
adopted in the literature. Using this method we derive the conditions of
validity of the rotating-wave approximation (RWA) and show how this approximate
description relates to more general ones. We also show that the existence of
dissipative coherent states is intimately related to the RWA. Finally, through
the evaluation of the dynamics of the damped oscillator, we notice an important
property of the dissipative model that has not been properly accounted for in
previous works; namely, the necessity of new constraints to the application of
the factorizable initial conditions.Comment: 19 pages, 2 figures, ReVTe
Ab-initio study of BaTiO3 surfaces
We have carried out first-principles total-energy calculations of (001)
surfaces of the tetragonal and cubic phases of BaTiO3. Both BaO-terminated
(type I) and TiO2-terminated (type II) surfaces are considered, and the atomic
configurations have been fully relaxed. We found no deep-gap surface states for
any of the surfaces, in agreement with previous theoretical studies. However,
the gap is reduced for the type-II surface, especially in the cubic phase. The
surface relaxation energies are found to be substantial, i.e., many times
larger than the bulk ferroelectric well depth. Nevertheless, the influence of
the surface upon the ferroelectric order parameter is modest; we find only a
small enhancement of the ferroelectricity near the surface.Comment: 8 pages, two-column style with 4 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#pad_sur
Effect of Al substitution on the magnetocaloric properties of La(Fe SiAl)
Here we study the influence of Al doping on the magnetization, heat capacity, and entropy change of La(FeSiAl ) where x = 0, 0.048, and 0.081. When x = 0, the system shows a remarkably sharp heat capacity feature associated with spin fluctuations coincident with, but quite distinct from the latent heat spike of the first order paramagnetic to ferromagnetic phase transition. With the addition of Al the magnetic and calorimetric features become more distributed in field, suggesting that Al adds disorder to the system. For both finite x compositions studied here, the latent heat disappears and the transition can be classified as second order. Although the entropy change associated with the transition is reduced once Al is substituted for Si, the adiabatic temperature change, ΔT is still significant. In La(Fe [Al) the balance between changes in the field dependence of the heat capacity with respect to overall ΔT gain is highlighted, showing that a small amount of Al doping clearly offers some advantage for application
Mechanical versus thermodynamical melting in pressure-induced amorphization: the role of defects
We study numerically an atomistic model which is shown to exhibit a one--step
crystal--to--amorphous transition upon decompression. The amorphous phase
cannot be distinguished from the one obtained by quenching from the melt. For a
perfectly crystalline starting sample, the transition occurs at a pressure at
which a shear phonon mode destabilizes, and triggers a cascade process leading
to the amorphous state. When defects are present, the nucleation barrier is
greatly reduced and the transformation occurs very close to the extrapolation
of the melting line to low temperatures. In this last case, the transition is
not anticipated by the softening of any phonon mode. Our observations reconcile
different claims in the literature about the underlying mechanism of pressure
amorphization.Comment: 7 pages, 7 figure
Relating the Cosmological Constant and Supersymmetry Breaking in Warped Compactifications of IIB String Theory
It has been suggested that the observed value of the cosmological constant is
related to the supersymmetry breaking scale M_{susy} through the formula Lambda
\sim M_p^4 (M_{susy}/M_p)^8. We point out that a similar relation naturally
arises in the codimension two solutions of warped space-time varying
compactifications of string theory in which non-isotropic stringy moduli induce
a small but positive cosmological constant.Comment: 7 pages, LaTeX, references added and minor changes made, (v3) map
between deSitter and global cosmic brane solutions clarified, supersymmetry
breaking discussion improved and references adde
Off-Diagonal Long Range Order and Scaling in a Disordered Quantum Hall System
We have numerically studied the bosonic off-diagonal long range order,
introduced by Read to describe the ordering in ideal quantum Hall states, for
noninteracting electrons in random potentials confined to the lowest Landau
level. We find that it also describes the ordering in disordered quantum Hall
states: the proposed order parameter vanishes in the disordered
() phase and increases continuously from zero in the ordered
() phase. We study the scaling of the order parameter and
find that it is consistent with that of the one-electron Green's function.Comment: 10 pages and 4 figures, Revtex v3.0, UIUC preprint P-94-03-02
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