257 research outputs found
Resonance Effects in the Nonadiabatic Nonlinear Quantum Dimer
The quantum nonlinear dimer consisting of an electron shuttling between the
two sites and in weak interaction with vibrations, is studied numerically under
the application of a DC electric field. A field-induced resonance phenomenon
between the vibrations and the electronic oscillations is found to influence
the electronic transport greatly. For initially delocalization of the electron,
the resonance has the effect of a dramatic increase in the transport. Nonlinear
frequency mixing is identified as the main mechanism that influences transport.
A characterization of the frequency spectrum is also presented.Comment: 7 pages, 6 figure
Thermodynamic and transport properties of underdoped cuprates from ARPES data
he relationship between photoemission spectra of high-
cuprates and their thermodynamic and transport properties are discussed. The
doping dependence of the expected quasi-particle density at the Fermi level
() are compared with the electronic specific heat coefficient
and that of the spectral weight at with the in-plane
and out-of-plane superfluid density. We have estimated the electrical
resistivity of underdoped cuprates from the momentum distribution curve (MDC)
at in the nodal direction. The temperature dependence of the MDC
width is also consistent with that of the electrical resistivity.Comment: 14 pages, 4 figures, proceeding of International Symposium on
Synchrotron Radiatin Research for Spin and Electronic States in d and f
Electron Systems(SRSES2003
Analysis of particle production in ultra-relativistic heavy ion collisions within a two-source statistical model
The experimental data on hadron yields and ratios in central lead-lead and
gold-gold collisions at 158 AGeV/ (SPS) and AGeV (RHIC),
respectively, are analysed within a two-source statistical model of an ideal
hadron gas. A comparison with the standard thermal model is given. The two
sources, which can reach the chemical and thermal equilibrium separately and
may have different temperatures, particle and strangeness densities, and other
thermodynamic characteristics, represent the expanding system of colliding
heavy ions, where the hot central fireball is embedded in a larger but cooler
fireball. The volume of the central source increases with rising bombarding
energy. Results of the two-source model fit to RHIC experimental data at
midrapidity coincide with the results of the one-source thermal model fit,
indicating the formation of an extended fireball, which is three times larger
than the corresponding core at SPS.Comment: 6 pages, REVTEX
Self-trapping transition for nonlinear impurities embedded in a Cayley tree
The self-trapping transition due to a single and a dimer nonlinear impurity
embedded in a Cayley tree is studied. In particular, the effect of a perfectly
nonlinear Cayley tree is considered. A sharp self-trapping transition is
observed in each case. It is also observed that the transition is much sharper
compared to the case of one-dimensional lattices. For each system, the critical
values of for the self-trapping transitions are found to obey a
power-law behavior as a function of the connectivity of the Cayley tree.Comment: 6 pages, 7 fig
Quasi-Particle Description of Strongly Interacting Matter: Towards a Foundation
We confront our quasi-particle model for the equation of state of strongly
interacting matter with recent first-principle QCD calculations. In particular,
we test its applicability at finite baryon densities by comparing with Taylor
expansion coefficients of the pressure for two quark flavours. We outline a
chain of approximations starting from the Phi-functional approach to QCD which
motivates the quasi-particle picture.Comment: Aug 2006. 6 pp. Invited Talk given at Hot Quarks 2006, Villasimius,
Sardinia, Italy, 15-20 May 200
CutTools: a program implementing the OPP reduction method to compute one-loop amplitudes
We present a program that implements the OPP reduction method to extract the
coefficients of the one-loop scalar integrals from a user defined
(sub)-amplitude or Feynman Diagram, as well as the rational terms coming from
the 4-dimensional part of the numerator. The rational pieces coming from the
epsilon-dimensional part of the numerator are treated as an external input, and
can be computed with the help of dedicated tree-level like Feynman rules.
Possible numerical instabilities are dealt with the help of arbitrary
precision routines, that activate only when needed.Comment: Version published in JHE
Thermodynamic gauge-theory cascade
It is proposed that the cooling of a thermalized SU() gauge theory can be
formulated in terms of a cascade involving three effective theories with
successively reduced (and spontaneously broken) gauge symmetries, SU()
U(1) Z. The approach is based on the assumption that away
from a phase transition the bulk of the quantum interaction inherent to the
system is implicitly encoded in the (incomplete) classical dynamics of a
collective part made of low-energy condensed degrees of freedom. The properties
of (some of the) statistically fluctuating fields are determined by these
condensate(s). This leads to a quasi-particle description at tree-level. It
appears that radiative corrections, which are sizable at large gauge coupling,
do not change the tree-level picture qualitatively. The thermodynamic
self-consistency of the quasi-particle approach implies nonperturbative
evolution equations for the associated masses. The temperature dependence of
these masses, in turn, determine the evolution of the gauge coupling(s). The
hot gauge system approaches the behavior of an ideal gas of massless gluons at
asymptotically large temperature. A negative equation of state is possible at a
stage where the system is about to settle into the phase of the (spontaneously
broken) Z symmetry.Comment: 25 pages, 6 figures, 1 reference added, minor corrections in text,
errors in Sec. 3.2 corrected, PRD versio
Transition from a quark-gluon plasma in the presence of a sharp front
The effect of a sharp front separating the quark-gluon plasma phase from the
hadronic phase is investigated. Energy-momentum conservation and baryon number
conservation constrain the possible temperature jump across the front. If one
assumes that the temperature in the hadronic phase is 200 MeV , as
has been suggested by numerous results from relativistic ion collisions, one
can determine the corresponding temperature in the quark phase with the help of
continuity equations across the front. The calculations reveal that the quark
phase must be in a strongly supercooled state. The stability of this solution
with respect to minor modifications is investigated. In particular the effect
of an admixture of hadronic matter in the quark phase (e.g. in the form of
bubbles) is considered in detail. In the absence of admixture the transition
proceeds via a detonation transition and is accompanied by a substantial
super-cooling of the quark-gluon plasma phase. The detonation is accompanied by
less supercooling if a small fraction of bubbles is allowed. By increasing the
fraction of bubbles the supercooling becomes weaker and eventually the
transition proceeds via a smoother deflagration wave.Comment: 10 pages, manuscript in TeX, 9 figures available as Postscript files,
CERN-TH 6923/9
Influence of entrance-channel magicity and isospin on quasi-fission
The role of spherical quantum shells in the competition between fusion and
quasi-fission is studied for reactions forming heavy elements. Measurements of
fission fragment mass distributions for different reactions leading to similar
compound nuclei have been made near the fusion barrier. In general, more
quasi-fission is observed for reactions with non-magic nuclei. However, the
Ca+Pb reaction is an exception, showing strong evidence for
quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock
calculations predict fast equilibration of in the two fragments early in
the collision. This transfer of nucleons breaks the shell effect, causing this
reaction to behave more like a non-magic one in the competition between fusion
and quasi-fission. Future measurements of fission in reactions with exotic
beams should be able to test this idea with larger asymmetries.Comment: accepted for publication in Physics Letters
Electron Dynamics in NdCeCuO: Evidence for the Pseudogap State and Unconventional c-axis Response
Infrared reflectance measurements were made with light polarized along the a-
and c-axis of both superconducting and antiferromagnetic phases of electron
doped NdCeCuO. The results are compared to
characteristic features of the electromagnetic response in hole doped cuprates.
Within the CuO planes the frequency dependent scattering rate,
1/, is depressed below 650 cm; this behavior is a
hallmark of the pseudogap state. While in several hole doped compounds the
energy scales associated with the pseudogap and superconducting states are
quite close, we are able to show that in NdCeCuO
the two scales differ by more than one order of magnitude. Another feature of
the in-plane charge response is a peak in the real part of the conductivity,
, at 50-110 cm which is in sharp contrast with the
Drude-like response where is centered at . This
latter effect is similar to what is found in disordered hole doped cuprates and
is discussed in the context of carrier localization. Examination of the c-axis
conductivity gives evidence for an anomalously broad frequency range from which
the interlayer superfluid is accumulated. Compelling evidence for the pseudogap
state as well as other characteristics of the charge dynamics in
NdCeCuO signal global similarities of the cuprate
phase diagram with respect to electron and hole doping.Comment: Submitted to PR
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