3,333 research outputs found
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
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
One-step replica symmetry breaking solution for fermionic Ising spin glass in a transverse field
The fermionic Ising spin glass models in a transverse field are investigated
in a Grassmann path integral formalism. The Parisi's scheme of one-step replica
symmetry breaking (RSB) is used within the static ansatz. This formalism has
already been applied in a theory in which (Parisi's block-size parameter)
is taken as a constant (PRB 72, 014447 (2005)). Now, it is extended to consider
as a variational parameter. In this case, the results show that RSB is
present when , in which the system is driven by quantum fluctuations.Comment: 8 pages, 3 figures, accepted for publication as a Brief Report in
Phys. Rev.
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.
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