48 research outputs found
The spin glass-antiferromagnetism competition in Kondo-lattice systems in the presence of a transverse applied magnetic field
A theory is proposed to describe the competition among antiferromagnetism
(AF), spin glass (SG) and Kondo effect. The model describes two Kondo
sublattices with an intrasite Kondo interaction strength and a random
Gaussian interlattice interaction in the presence of a transverse field
. The field is introduced as a quantum mechanism to produce
spin flipping and the random coupling has average and variance . The path integral formalism with Grassmann fields is used to study
this fermionic problem, in which the disorder is treated within the framework
of the replica trick. The free energy and the order parameters are obtained
using the static ansatz. In this many parameters problem, we choose and to allow a better
comparison with the experimental findings. The obtained phase diagram has not
only the same sequence as the experimental one for
, but mainly, it also shows a qualitative agreement
concerning the behavior of the freezing temperature and the Neel temperature
which decreases until a Quantum Critical Point (QCP).Comment: 4 pages, 1 figure, accepted for publication in Physica
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
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.
Spin Glass and antiferromagnetism in Kondo lattice disordered systems
The competition between spin glass (SG), antiferromagnetism (AF) and Kondo
effect is studied here in a model which consists of two Kondo sublattices with
a gaussian random interaction between spins in differents sublattices with an
antiferromagnetic mean Jo and standard deviation J. In the present approach
there is no hopping of the conduction electrons between the sublattices and
only spins in different sublattices can interact. The problem is formulated in
the path integral formalism where the spin operators are expressed as bilinear
combinations of Grassmann fields which can be solved at mean field level within
the static approximation and the replica symmetry ansatz. The obtained phase
diagram shows the sequence of phases SG, AF and Kondo state for increasing
Kondo coupling. This sequence agrees qualitatively with experimental data of
the Ce_{2} Au_{1-x} Co_{x} Si_{3} compound.Comment: 7 pages, 1 figure, submitted to EPJ
Spin Glass and ferromagnetism in disordered Kondo lattice
The competition among spin glass (SG), ferromagnetism and Kondo effect has
been analysed in a Kondo lattice model where the inter-site coupling
between the localized magnetic moments is given by a generalized Mattis model
\cite{Mattis} which represents an interpolation between ferromagnetism and a
highly disordered spin glass. Functional integral techniques with of Grassmann
fields has been used to obtain the partition function. The static approximation
and the replica symmetric ansatz has also been used. The solution of the
problem is presented as a phase diagram temperature {\it versus} (the
strength of the intra-site interaction). If is small, for decreasing
temperature there is a second order transition from a paramagnetic to a spin
glass phase For lower temperatures, a first order transition appears where
solutions for the spin glass order parameter and the local magnetizations are
simultaneously non zero. For very low temperatures, the local magnetizations
becomes thermodinamically stables. For high , the Kondo state is
dominating. These results could be helpful to clarify the experimental
situation of .Comment: 4 pages, 1 figure, accept to be published in Physica
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
Spin Glass and Ferromagnetism in Kondo lattices compounds
The Kondo lattice model has been analyzed in the presence of a random
inter-site interaction among localized spins with non zero mean Jo and standard
deviation J. Following the same framework previously introduced by us, the
problem is formulated in the path integral formalism where the spin operators
are expressed as bilinear combinations of Grassmann fields. The static
approximation and the replica symmetry ansatz have allowed us to solve the
problem at a mean field level. The resulting phase diagram displays several
phase transitions among a ferromagnetically ordered region,a spin glass one, a
mixed phase and a Kondo state depending on Jo, J and its relation with the
Kondo interaction coupling Jk. These results could be used to address part of
the experimental data for the CeNi_{1-x}Cu_x compound, when x =< 0.8.Comment: 7 pages, 2 figures. accepted for publication in The European Physical
Journal
Quantum Critical Point in the Spin Glass-Kondo Transition in Heavy Fermion Systems
The Kondo-Spin Glass competition is studied in a theoretical model of a Kondo
lattice with an intra-site Kondo type exchange interaction treated within the
mean field approximation, an inter-site quantum Ising exchange interaction with
random couplings among localized spins and an additional transverse field in
the x direction, which represents a simple quantum mechanism of spin flipping.
We obtain two second order transition lines from the spin-glass state to the
paramagnetic one and then to the Kondo state. For a reasonable set of the
different parameters, the two second order transition lines do not intersect
and end in two distinct QCP.Comment: 20 pages; 1 figure; to appear in Physical Review