7,994 research outputs found
A note on the time evolution of generalized coherent states
I consider the time evolution of generalized coherent states based on
non-standard fiducial vectors, and show that only for a restricted class of
fiducial vectors does the associated classical motion determine the quantum
evolution of the states. I discuss some consequences of this for path integral
representations.Comment: 9 pages. RevTe
On the Groenewold-Van Hove problem for R^{2n}
We discuss the Groenewold-Van Hove problem for R^{2n}, and completely solve
it when n = 1. We rigorously show that there exists an obstruction to
quantizing the Poisson algebra of polynomials on R^{2n}, thereby filling a gap
in Groenewold's original proof without introducing extra hypotheses. Moreover,
when n = 1 we determine the largest Lie subalgebras of polynomials which can be
unambiguously quantized, and explicitly construct all their possible
quantizations.Comment: 15 pages, Latex. Error in the proof of Prop. 3 corrected; minor
rewritin
Effective calculation of LEED intensities using symmetry-adapted functions
The calculation of LEED intensities in a spherical-wave representation can be substantially simplified by symmetry relations. The wave field around each atom is expanded in symmetry-adapted functions where the local point symmetry of the atomic site applies. For overlayer systems with more than one atom per unit cell symmetry-adapted functions can be used when the division of the crystal into monoatomic subplanes is replaced by division into subplanes containing all symmetrically equivalent atomic positions
Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots
The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
Measurement of the Michel Parameter xi" in Polarized Muon Decay and Implications on Exotic Couplings of the Leptonic Weak Interaction
The Michel parameter xi" has been determined from a measurement of the
longitudinal polarization of positrons emitted in the decay of polarized and
depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is
consistent with the Standard Model prediction of unity, and provides an order
of magnitude improvement in the relative precision of this parameter. This
value sets new constraints on exotic couplings beyond the dominant V-A
description of the leptonic weak interaction.Comment: 15 pages, 16 figures, 3 tables; submitted to Phys. Rev.
Self-limited oxide formation in Ni(111) oxidation
The oxidation of the Ni(111) surface is studied experimentally with low
energy electron microscopy and theoretically by calculating the electron
reflectivity for realistic models of the NiO/Ni(111) surface with an ab-initio
scattering theory. Oxygen exposure at 300 K under ultrahigh-vacuum conditions
leads to the formation of a continuous NiO(111)-like film consisting of
nanosized domains. At 750 K, we observe the formation of a nano-heterogeneous
film composed primarily of NiO(111)-like surface oxide nuclei, which exhibit
virtually the same energy-dependent reflectivity as in the case of 300 K and
which are separated by oxygen-free Ni(111) terraces. The scattering theory
explains the observed normal incidence reflectivity R(E) of both the clean and
the oxidized Ni(111) surface. At low energies R(E) of the oxidized surface is
determined by a forbidden gap in the k_parallel=0 projected energy spectrum of
the bulk NiO crystal. However, for both low and high temperature oxidation a
rapid decrease of the reflectivity in approaching zero kinetic energy is
experimentally observed. This feature is shown to characterize the thickness of
the oxide layer, suggesting an average oxide thickness of two NiO layers.Comment: 10 pages (in journal format), 9 figure
From Bloch model to the rate equations II: the case of almost degenerate energy levels
Bloch equations give a quantum description of the coupling between an atom
and a driving electric force. In this article, we address the asymptotics of
these equations for high frequency electric fields, in a weakly coupled regime.
We prove the convergence towards rate equations (i.e. linear Boltzmann
equations, describing the transitions between energy levels of the atom). We
give an explicit form for the transition rates. This has already been performed
in [BFCD03] in the case when the energy levels are fixed, and for different
classes of electric fields: quasi or almost periodic, KBM, or with continuous
spectrum. Here, we extend the study to the case when energy levels are possibly
almost degenerate. However, we need to restrict to quasiperiodic forcings. The
techniques used stem from manipulations on the density matrix and the averaging
theory for ordinary differential equations. Possibly perturbed small divisor
estimates play a key role in the analysis. In the case of a finite number of
energy levels, we also precisely analyze the initial time-layer in the rate
aquation, as well as the long-time convergence towards equilibrium. We give
hints and counterexamples in the infinite dimensional case
Effect of bonding of a CO molecule on the conductance of atomic metal wires
We have measured the effect of bonding of a CO molecule on the conductance of
Au, Cu, Pt, and Ni atomic contacts at 4.2 K. When CO gas is admitted to the
metal nano contacts, a conductance feature appears in the conductance histogram
near 0.5 of the quantum unit of conductance, for all metals. For Au, the
intensity of this fractional conductance feature can be tuned with the bias
voltage, and it disappears at high bias voltage (above 200 mV). The
bonding of CO to Au appears to be weakest, and associated with monotomic Au
wire formation.Comment: 6 figure
Effect of lipemia and bilirubinemia on HIV-1 protease and reverse transcriptase genotyping and phenotyping success: a five-year analysis
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