1,323 research outputs found
An Exact Solution to O(26) Sigma Model coupled to 2-D Gravity
By a mapping to the bosonic string theory, we present an exact solution to
the O(26) sigma model coupled to 2-D quantum gravity. In particular, we obtain
the exact gravitational dressing to the various matter operators classified by
the irreducible representations of O(26). We also derive the exact form of the
gravitationally modified beta function for the original coupling constant
. The relation between our exact solution and the asymptotic solution
given in ref[3] is discussed in various aspects.Comment: 10 pages, pupt-144
Investigation of the field-induced ferromagnetic phase transition in spin polarized neutron matter: a lowest order constrained variational approach
In this paper, the lowest order constrained variational (LOCV) method has
been used to investigate the magnetic properties of spin polarized neutron
matter in the presence of strong magnetic field at zero temperature employing
potential. Our results indicate that a ferromagnetic phase transition
is induced by a strong magnetic field with strength greater than ,
leading to a partial spin polarization of the neutron matter. It is also shown
that the equation of state of neutron matter in the presence of magnetic field
is stiffer than the case in absence of magnetic field.Comment: 23 pages, 9 figures Phys. Rev. C (2011) in pres
Quantum heat engines and nonequilibrium temperature
A pair of two-level systems initially prepared in different thermal states
and coupled to an external reversible work source, do not in general reach a
common temperature at the end of a unitary work extraction process. We define
an effective temperature for the final nonequilibrium but passive state of the
bipartite quantum system and analyse its properties.Comment: Five pages, Accepted for publication in Physical Review
First-order transitions and triple point on a random p-spin interaction model
The effects of competing quadrupolar- and spin-glass orderings are
investigated on a spin-1 Ising model with infinite-range random -spin
interactions. The model is studied through the replica approach and a phase
diagram is obtained in the limit . The phase diagram, obtained
within replica-symmetry breaking, exhibits a very unusual feature in magnetic
models: three first-order transition lines meeting at a commom triple point,
where all phases of the model coexist.Comment: 9 pages, 2 ps figures include
Disorder effects in diluted ferromagnetic semiconductors
Carrier induced ferromagnetism in diluted III-V semi-conductor is analyzed
within a two step approach. First, within a single site CPA formalism, we
calculate the element resolved averaged Green's function of the itinerant
carrier. Then using a generalized RKKY formula we evaluate the Mn-Mn long-range
exchange integrals and the Curie temperature as a function of the exchange
parameter, magnetic impurity concentration and carrier density. The effect of
the disorder (impurity scattering) appears to play a crucial role. The standard
RKKY calculation (no scattering processes), strongly underestimate the Curie
temperature and is inappropriate to describe magnetism in diluted magnetic
semi-conductors. It is also shown that an antiferromagnetic exchange favors
higher Curie temperature.Comment: tex file + 4 .eps figures are included. submited to PR
Geometrothermodynamics
We present the fundamentals of geometrothermodynamics, an approach to study
the properties of thermodynamic systems in terms of differential geometric
concepts. It is based, on the one hand, upon the well-known contact structure
of the thermodynamic phase space and, on the other hand, on the metric
structure of the space of thermodynamic equilibrium states. In order to make
these two structures compatible we introduce a Legendre invariant set of
metrics in the phase space, and demand that their pullback generates metrics on
the space of equilibrium states. We show that Weinhold's metric, which was
introduced {\it ad hoc}, is not contained within this invariant set. We propose
alternative metrics which allow us to redefine the concept of thermodynamic
length in an invariant manner and to study phase transitions in terms of
curvature singularities.Comment: Revised version, to be published in Jour. Math. Phy
Scaling of Traction Forces with Size of Cohesive Cell Colonies
To understand how the mechanical properties of tissues emerge from
interactions of multiple cells, we measure traction stresses of cohesive
colonies of 1-27 cells adherent to soft substrates. We find that traction
stresses are generally localized at the periphery of the colony and the total
traction force scales with the colony radius. For large colony sizes, the
scaling appears to approach linear, suggesting the emergence of an apparent
surface tension of order 1E-3 N/m. A simple model of the cell colony as a
contractile elastic medium coupled to the substrate captures the spatial
distribution of traction forces and the scaling of traction forces with the
colony size.Comment: 5 pages, 3 figure
Finite-size scaling considerations on the ground state microcanonical temperature in entropic sampling simulations
In this work we discuss the behavior of the microcanonical temperature
obtained by means of numerical entropic
sampling studies. It is observed that in almost all cases the slope of the
logarithm of the density of states is not infinite in the ground state,
since as expected it should be directly related to the inverse temperature
. Here we show that these finite slopes are in fact due to
finite-size effects and we propose an analytic expression for the
behavior of when . To
test this idea we use three distinct two-dimensional square lattice models
presenting second-order phase transitions. We calculated by exact means the
parameters and for the two-states Ising model and for the and
states Potts model and compared with the results obtained by entropic sampling
simulations. We found an excellent agreement between exact and numerical
values. We argue that this new set of parameters and represents an
interesting novel issue of investigation in entropic sampling studies for
different models
Microscopic Transport Theory of Nuclear Processes
We formulate a microscopic theory of the decay of a compound nucleus through
fission which generalizes earlier microscopic approaches of fission dynamics
performed in the framework of the adiabatic hypothesis. It is based on the
constrained Hartree-Fock-Bogoliubov procedure and the Generator Coordinate
Method, and requires an effective nucleon-nucleon interaction as the only input
quantity. The basic assumption is that the slow evolution of the nuclear shape
must be treated explicitely, whereas the rapidly time-dependent intrinsic
excitations can be treated by statistical approximations. More precisely, we
introduce a reference density which represents the slow evolution of the
nuclear shape by a reduced density matrix and the state of intrinsic
excitations by a canonical distribution at each given shape of the nucleus. The
shape of the nuclear density distribution is described by parameters
("generator coordinates"), not by "superabundant" degrees of freedom introduced
in addition to the complete set of nucleonic degrees of freedom. We first
derive a rigorous equation of motion for the reference density and,
subsequently, simplify this equation on the basis of the Markov approximation.
The temperature which appears in the canonical distribution is determined by
the requirement that, at each time t, the reference density should correctly
reproduce the mean excitation energy at given values of the shape parameters.
The resulting equation for the "local" temperature must be solved together with
the equations of motion obtained for the reduced density matrix.Comment: 33 pages, accepted in Nucl. Phys.
Fluctuation-dissipation ratio of a spin glass in the aging regime
We present the first experimental determination of the time autocorrelation
of magnetization in the non-stationary regime of a spin glass.
Quantitative comparison with the response, the magnetic susceptibility
, is made using a new experimental setup allowing both measurements
in the same conditions. Clearly, we observe a non-linear
fluctuation-dissipation relation between and , depending weakly on
the waiting time . Following theoretical developments on mean-field models,
and lately on short range models, it is predicted that in the limit of long
times, the relationship should become independent on . A scaling
procedure allows us to extrapolate to the limit of long waiting times.Comment: 4 pages, 3 figure
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