7,217 research outputs found
Collective flow and QCD phase transition
In the first part I discuss the sensitivity of collective matter expansion in
ultrarelativistic heavy-ion collisions to the transition between quark and
hadronic matter (physics of the softest point of the Equation of State). A kink
in the centrality dependence of elliptic flow has been suggested as a signature
for the phase transition in hot QCD matter. Indeed, preliminary data of NA49
presented at this conference show first indications for the predicted kink. In
the second part I have a look at the present theories of heavy-ion reactions.
These remarks may also be seen as a critical comment to B. Mueller's summary
talk (nucl-th/9906029) presented at this conference.Comment: Write-up of QM '99 talk. Typo's correcte
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.
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
Decoherence time in self-induced decoherence
A general method for obtaining the decoherence time in self-induced
decoherence is presented. In particular, it is shown that such a time can be
computed from the poles of the resolvent or of the initial conditions in the
complex extension of the Hamiltonian's spectrum. Several decoherence times are
estimated: for microscopic systems, and
for macroscopic bodies. For the particular case of a
thermal bath, our results agree with those obtained by the einselection
(environment-induced decoherence) approach.Comment: 11 page
Highly Sensitive Centrality Dependence of Elliptic Flow -- A Novel Signature of the Phase Transition in QCD
Elliptic flow of the hot, dense system which has been created in
nucleus-nucleus collisions develops as a response to the initial azimuthal
asymmetry of the reaction region. Here it is suggested that the magnitude of
this response shows a ``kinky'' dependence on the centrality of collisions for
which the system passes through a first-order or rapid transition between
quark-gluon plasma and hadronic matter. We have studied the system Pb(158AGeV)
on Pb employing a recent version of the transport theoretical approach RQMD and
find the conjecture confirmed. The novel phase transition signature may be
observable in present and forthcoming experiments at CERN-SPS and at RHIC, the
BNL collider.Comment: Version as published in PRL 82 (1999) 2048, title chang
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
Fermi's golden rule and exponential decay as a RG fixed point
We discuss the decay of unstable states into a quasicontinuum using models of
the effective Hamiltonian type. The goal is to show that exponential decay and
the golden rule are exact in a suitable scaling limit, and that there is an
associated renormalization group (RG) with these properties as a fixed point.
The method is inspired by a limit theorem for infinitely divisible
distributions in probability theory, where there is a RG with a Cauchy
distribution, i.e. a Lorentz line shape, as a fixed point. Our method of
solving for the spectrum is well known; it does not involve a perturbation
expansion in the interaction, and needs no assumption of a weak interaction. We
use random matrices for the interaction, and show that the ensemble
fluctuations vanish in the scaling limit. Thus the limit is the same for every
model in the ensemble with probability one.Comment: 20 pages, 1 figur
Soft transverse expansion in Pb(158 AGeV) on Pb collisions: preequilibrium motion or 1st order phase transition?
Transverse expansion of centrally produced matter in Pb on Pb collisions at
beam energies around 158 AGeV appears to be rather `soft'. Two possible reasons
-- an extended preequilibrium stage and a first order phase transition from a
quark-gluon-plasma into hadronic matter -- are discussed. The softening of
transverse expansion caused by preequilibrium dynamics is estimated with the
aid of the transport model RQMD which does not contain a first order phase
transition. It is found that the anisotropy of transverse flow in non-central
reactions is very different in the preequilibrium and hydrodynamic scenarios
even if the latter are based on a strong 1st order transition.Comment: 14 pages LaTeX including 3 postscript figure
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