7,696 research outputs found
Fermionic Ising Glasses with BCS Pairing Interaction. Tricritical Behaviour
We have examined the role of the BCS pairing mechanism in the formation of
the magnetic moment and henceforth a spin glass (SG) phase by studying a
fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the
fermions. This model is obtained by using perturbation theory to trace out the
conduction electrons degrees of freedom in conventional superconducting alloys.
The model is formulated in the path integral formalism where the spin operators
are represented by bilinear combinations of Grassmann fields and it reduces to
a single site problem that can be solved within the static approximation with a
replica symmetric Ansatz. We argue that this is a valid procedure for values of
temperature above the de Almeida-Thouless instability line. The phase diagram
in the T-g plane, where g is the strength of the pairing interaction, for fixed
variance J^2/N of the random couplings J_{ij}, exhibits three regions: a normal
paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase
where there is formation of local pairs.The NP and PAIR phases are separated by
a second order transition line g=g_{c}(T) that ends at a tricritical point
T_{3}=0.9807J, g_{3}=5,8843J, from where it becomes a first order transition
line that meets the line of second order transitions at T_{c}=0.9570J that
separates the NP and the SG phases. For T<T_{c} the SG phase is separated from
the PAIR phase by a line of first order transitions.
These results agree qualitatively with experimental data in
Gd_{x}Th_{1-x}RU_{2}.Comment: 26 pages, 5 figures, to appear in The European Physical Journal
Charged Particles and the Electro-Magnetic Field in Non-Inertial Frames of Minkowski Spacetime: I. Admissible 3+1 Splittings of Minkowski Spacetime and the Non-Inertial Rest Frames
By using the 3+1 point of view and parametrized Minkowski theories we develop
the theory of {\it non-inertial} frames in Minkowski space-time. The transition
from a non-inertial frame to another one is a gauge transformation connecting
the respective notions of instantaneous 3-space (clock synchronization
convention) and of the 3-coordinates inside them. As a particular case we get
the extension of the inertial rest-frame instant form of dynamics to the
non-inertial rest-frame one. We show that every isolated system can be
described as an external decoupled non-covariant canonical center of mass
(described by frozen Jacobi data) carrying a pole-dipole structure: the
invariant mass and an effective spin. Moreover we identify the constraints
eliminating the internal 3-center of mass inside the instantaneous 3-spaces. In
the case of the isolated system of positive-energy scalar particles with
Grassmann-valued electric charges plus the electro-magnetic field we obtain
both Maxwell equations and their Hamiltonian description in non-inertial
frames. Then by means of a non-covariant decomposition we define the
non-inertial radiation gauge and we find the form of the non-covariant Coulomb
potential. We identify the coordinate-dependent relativistic inertial
potentials and we show that they have the correct Newtonian limit. In the
second paper we will study properties of Maxwell equations in non-inertial
frames like the wrap-up effect and the Faraday rotation in astrophysics. Also
the 3+1 description without coordinate-singularities of the rotating disk and
the Sagnac effect will be given, with added comments on pulsar magnetosphere
and on a relativistic extension of the Earth-fixed coordinate system.Comment: This paper and the second one are an adaptation of arXiv 0812.3057
for publication on Int.J.Geom. Methods in Modern Phys. 77
Disentangling density and temperature effects in the viscous slowing down of glassforming liquids
We present a consistent picture of the respective role of density and
temperature in the viscous slowing down of glassforming liquids and polymers.
Specifically, based in part upon a new analysis of simulation and experimental
data on liquid ortho-terphenyl, we conclude that a zeroth-order description of
the approach to the glass transition should be formulated in terms of a
temperature-driven super-Arrhenius activated behavior rather than a
density-driven congestion or jamming phenomenon. The density plays a role at a
quantitative level, but its effect on the viscosity and the structural
relaxation time can be simply described via a single parameter, an effective
interaction energy that is characteristic of the high temperature liquid
regime; as a result, density does not affect the ``fragility'' of the
glassforming system.Comment: RevTeX4, 8 pages, 8 eps figure
Spin glass freezing in Kondo lattice compounds
It is presented a theory that describes a spin glass phase at finite
temperatures in Kondo lattice systems with an additional RKKY interaction
represented by long range, random couplings among localized spins like in the
Sherrington- Kirkpatrick (SK) spin glass model. The problem is studied within
the functional integral formalism where the spin operators are represented by
bilinear combinations of fermionic (anticommuting) Grassmann variables. The
Kondo and spin glass transitions are both described with the mean field like
static ansatz that reproduces good results in the two well known limits. At
high temperatures and low values of the Kondo coupling there is a paramagnetic
(disordered) phase with vanishing Kondo and spin glass order parameters. By
lowering the temperature a second order transition line is found at Tsg to a
spin glass phase. For larger values of the Kondo coupling there is a second
order transition line at roughly Tk to a Kondo ordered state. For T<Tsg the
transition between the Kondo and spin glass phases becomes first order.Comment: 21 pages, 1 figure, to appear on Phys. Rev.
Role of the transverse field in inverse freezing in the fermionic Ising spin-glass model
We investigate the inverse freezing in the fermionic Ising spin-glass (FISG)
model in a transverse field . The grand canonical potential is
calculated in the static approximation, replica symmetry and one-step replica
symmetry breaking Parisi scheme. It is argued that the average occupation per
site is strongly affected by . As consequence, the boundary phase
is modified and, therefore, the reentrance associated with the inverse freezing
is modified too.Comment: 6 pages, 3 figures, accepted for publication in PR
Efficient and Accurate Modeling of Conformational Transitions in Proteins: The Case of c-Src Kinase
The theoretical computational modeling of large conformational transitions occurring in biomolecules still represents a challenge. Here, we present an accurate "in silico" description of the activation and deactivation mechanisms of human c-Src kinases, a fundamental process regulating several crucial cell functions. Our results clearly show that by applying an efficient and automated algorithm able to drive the molecular dynamics (MD) sampling along the pathway between the two c-Src conformational states - the active state and the inactive state - it is possible to accurately describe, at reduced computational costs, the molecular mechanism underlying these large conformational rearrangements. This procedure, combining the MD simulations with the sampling along the well-defined principal motions connecting the two conformational states, allows to provide a description well beyond the present computational limits, and it is easily applicable to different systems where the structures of both the initial and final states are known
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