19,762 research outputs found
Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric nuclear matter based on improved approximation schemes
We present Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric
nuclear matter which are based on improved approximations schemes. The
potential matrix elements have been adapted for isospin asymmetric nuclear
matter in order to account for the proton-neutron mass splitting in a more
consistent way. The proton properties are particularly sensitive to this
adaption and its consequences, whereas the neutron properties remains almost
unaffected in neutron rich matter. Although at present full Brueckner
calculations are still too complex to apply to finite nuclei, these
relativistic Brueckner results can be used as a guidance to construct a density
dependent relativistic mean field theory, which can be applied to finite
nuclei. It is found that an accurate reproduction of the
Dirac-Brueckner-Hartree-Fock equation of state requires a renormalization of
these coupling functions.Comment: 34 pages, 9 figures, submitted to Eur. Phys. J.
Quantum Cloning, Eavesdropping and Bell's inequality
We analyze various eavesdropping strategies on a quantum cryptographic
channel. We present the optimal strategy for an eavesdropper restricted to a
two-dimensional probe, interacting on-line with each transmitted signal. The
link between safety of the transmission and the violation of Bell's inequality
is discussed. We also use a quantum copying machine for eavesdropping and for
broadcasting quantum information.Comment: LaTex, 13 pages, with 6 Postscript figure
Ursell operators in statistical physics of dense systems: the role of high order operators and of exchange cycles
The purpose of this article is to discuss cluster expansions in dense quantum
systems as well as their interconnection with exchange cycles. We show in
general how the Ursell operators of order 3 or more contribute to an
exponential which corresponds to a mean-field energy involving the second
operator U2, instead of the potential itself as usual. In a first part, we
consider classical statistical mechanics and recall the relation between the
reducible part of the classical cluster integrals and the mean-field; we
introduce an alternative method to obtain the linear density contribution to
the mean-field, which is based on the notion of tree-diagrams and provides a
preview of the subsequent quantum calculations. We then proceed to study
quantum particles with Boltzmann statistics (distinguishable particles) and
show that each Ursell operator Un with n greater or equal to 3 contains a
``tree-reducible part'', which groups naturally with U2 through a linear chain
of binary interactions; this part contributes to the associated mean-field
experienced by particles in the fluid. The irreducible part, on the other hand,
corresponds to the effects associated with three (or more) particles
interacting all together at the same time. We then show that the same algebra
holds in the case of Fermi or Bose particles, and discuss physically the role
of the exchange cycles, combined with interactions. Bose condensed systems are
not considered at this stage. The similarities and differences between
Boltzmann and quantum statistics are illustrated by this approach, in contrast
with field theoretical or Green's functions methods, which do not allow a
separate study of the role of quantum statistics and dynamics.Comment: 31 pages, 7 figure
Non--Newtonian viscosity of interacting Brownian particles: comparison of theory and data
A recent first-principles approach to the non-linear rheology of dense
colloidal suspensions is evaluated and compared to simulation results of
sheared systems close to their glass transitions. The predicted scenario of a
universal transition of the structural dynamics between yielding of glasses and
non-Newtonian (shear-thinning) fluid flow appears well obeyed, and calculations
within simplified models rationalize the data over variations in shear rate and
viscosity of up to 3 decades.Comment: 6 pages, 2 figures; J. Phys. Condens. Matter to be published (Jan.
2003
Collective modes of a trapped Lieb-Liniger gas: a hydrodynamic approach
We consider a trapped repulsive one-dimensional (1D) Bose gas at very low
temperature. In order to study the collective modes of this strongly
interacting system, we use a hydrodynamic approach, where the gas is locally
described by the Lieb-Liniger model of bosons interacting via a repulsive delta
potential. Solving the corresponding linearized hydrodynamic equations, we
obtain the collective modes and concentrate more specifically on the lowest
compressional mode. This is done by finding models, approaching very closely
the exact equation of stae of the gas, for which the linearized hydrodynamic
equations are exactly solvable. Results are in excellent agreement with those
of the sum rule approach of Menotti and Stringari.Comment: Proceedings of the Laser Physics Workshop held in Hamburg (August
2003), Seminar on the Physics of Cold Trapped Atom
Quasi-Galois Symmetries of the Modular S-Matrix
The recently introduced Galois symmetries of RCFT are generalized, for the
WZW case, to `quasi-Galois symmetries'. These symmetries can be used to derive
a large number of equalities and sum rules for entries of the modular matrix S,
including some that previously had been observed empirically. In addition,
quasi-Galois symmetries allow to construct modular invariants and to relate
S-matrices as well as modular invariants at different levels. They also lead us
to an extremely plausible conjecture for the branching rules of the conformal
embeddings of g into so(dim g).Comment: 20 pages (A4), LaTe
Landau levels, response functions and magnetic oscillations from a generalized Onsager relation
A generalized semiclassical quantization condition for cyclotron orbits was
recently proposed by Gao and Niu \cite{Gao}, that goes beyond the Onsager
relation \cite{Onsager}. In addition to the integrated density of states, it
formally involves magnetic response functions of all orders in the magnetic
field. In particular, up to second order, it requires the knowledge of the
spontaneous magnetization and the magnetic susceptibility, as was early
anticipated by Roth \cite{Roth}. We study three applications of this relation
focusing on two-dimensional electrons. First, we obtain magnetic response
functions from Landau levels. Second we obtain Landau levels from response
functions. Third we study magnetic oscillations in metals and propose a proper
way to analyze Landau plots (i.e. the oscillation index as a function of
the inverse magnetic field ) in order to extract quantities such as a
zero-field phase-shift. Whereas the frequency of -oscillations depends on
the zero-field energy spectrum, the zero-field phase-shift depends on the
geometry of the cell-periodic Bloch states via two contributions: the Berry
phase and the average orbital magnetic moment on the Fermi surface. We also
quantify deviations from linearity in Landau plots (i.e. aperiodic magnetic
oscillations), as recently measured in surface states of three-dimensional
topological insulators and emphasized by Wright and McKenzie \cite{Wright}.Comment: 31 pages, 8 figures; v2: SciPost style; v3: several references added,
small corrections, typos fixed; v4: abstract changed, generalized
quantization condition called Roth-Gao-Niu; v5: minor modifications, 2
references adde
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