328 research outputs found

### Doniach diagram for ordered, disordered and underscreened Kondo lattices

The Doniach's diagram has been originally proposed to describe the
competition between the local Kondo effect and the intersite RKKY interactions
in cerium compounds. Here we discuss the extension of this diagram to different
variations of Kondo lattice model. We consider a) ordered cerium compounds
where the competition between magnetic order and Kondo effect plays an
important role, as $CeRh_2Si_2$, b) disordered cerium systems with competing
spin glass phase, magnetic ordered phases and a Kondo phase, as the heavy
fermion cerium alloy $CeCu_xNi_{1-x}$ and, c) uranium compounds where a
coexistence between Kondo effect and ferromagnetic order has been observed, as
UTe. We show that all these cases can be described by a generalized Doniach
phase diagram.Comment: Presented in the Latin American Workshop on Magnetism and Magnetic
Materials (LAW3M) Rio de Janeiro, Brazil, August 12-16, 2007. Proceedings to
be published in JMM

### A theoretical study of the cluster glass-Kondo-magnetic disordered alloys

The physics of disordered alloys, such as typically the well known case of
CeNi1-xCux alloys, showing an interplay among the Kondo effect, the spin glass
state and a magnetic order, has been studied firstly within an average
description like in the Sherrington-Kirkpatrick model. Recently, a theoretical
model (PRB 74, 014427 (2006)) involving a more local description of the
intersite interaction has been proposed to describe the phase diagram of
CeNi1-xCux. This alloy is an example of the complex interplay between Kondo
effect and frustration in which there is in particular the onset of a
cluster-glass state. Although the model given in Ref. PRB 74, 014427 (2006) has
reproduced the different phases relatively well, it is not able to describe the
cluster-glass state. We study here the competition between the Kondo effect and
a cluster glass phase within a Kondo Lattice model with an inter-cluster random
Gaussian interaction. The inter-cluster term is treated within the cluster
mean-field theory for spin glasses, while, inside the cluster, an exact
diagonalisation is performed including inter-site ferromagnetic and intra-site
Kondo interactions. The cluster glass order parameters and the Kondo
correlation function are obtained for different values of the cluster size, the
intra-cluster ferromagnetic coupling and the Kondo intra-site coupling. We
obtain, for instance, that the increase of the Kondo coupling tends to destroy
the cluster glass phase.Comment: 6 pages, 2 figures, Accepted for publication in Physica

### 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.

### A van Hemmen-Kondo model for disordered strongly correlated electron systems

We present here a theoretical model in order to describe the competition
between the Kondo effect and the spin glass behavior. The spin glass part of
the starting Hamiltonian contains Ising spins with an intersite exchange
interaction given by the local van Hemmen model, while the Kondo effect is
described as usual by the intrasite exchange $J_K$. We obtain, for large $J_K$
values, a Kondo phase and, for smaller $J_K$ values, a succession, with
decreasingComment: 14 pages, 4 figures, accepted for publication in Phys. Rev.

### 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 $J_{K}$ and a random
Gaussian interlattice interaction in the presence of a transverse field
$\Gamma$. The $\Gamma$ field is introduced as a quantum mechanism to produce
spin flipping and the random coupling has average $-2J_0/N$ and variance $32
J^{2}/N$. 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 $J_0/J
\approx (J_{K}/J)^{2}$ and $\Gamma/J \approx (J_{K}/J)^{2}$ to allow a better
comparison with the experimental findings. The obtained phase diagram has not
only the same sequence as the experimental one for
$Ce_{2}Au_{1-x}Co_{x}Si_{3}$, 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

### 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

### Quantum critical point in the spin glass-antiferromagnetism competition in Kondo-lattice systems

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 $J_{K}$ and an
interlattice quantum Ising interaction in the presence of a transverse field
$\Gamma$. The interlattice coupling is a random Gaussian distributed variable
(with average $-2J_0/N$ and variance $32 J^{2}/N$) while the $\Gamma$ field is
introduced as a quantum mechanism to produce spin flipping. The path integral
formalism is used to study this fermionic problem where the spin operators are
represented by bilinear combinations of Grassmann fields. The disorder is
treated within the framework of the replica trick. The free energy and the
order parameters of the problem are obtained by using the static ansatz and by
choosing both $J_0/J$ and $\Gamma/J \approx (J_k/J)^2$ to allow, as previously,
a better comparison with the experimental findings.
The results indicate the presence of a SG solution at low $J_K/J$ and for
temperature $T<T_{f}$ ($T_{f}$ is the freezing temperature). When $J_K/J$ is
increased, a mixed phase AF+SG appears, then an AF solution and finally a Kondo
state is obtained for high values of $J_{K}/J$. Moreover, the behaviors of the
freezing and Neel temperatures are also affected by the relationship between
$J_{K}$ and the transverse field $\Gamma$. The first one presents a slight
decrease while the second one decreases towards a Quantum Critical Point (QCP).
The obtained phase diagram has the same sequence as the experimental one for
$Ce_{2}Au_{1-x}Co_{x}Si_{3}$, if $J_{K}$ is assumed to increase with $x$, and
in addition, it also shows a qualitative agreement concerning the behavior of
the freezing and the Neel temperatures.Comment: 11 pages, 3 figures, accepted for publication in J. Phys.

### Effect of anisotropy in the $S=1$ underscreened Kondo lattice

We study the effect of crystal field anisotropy in the underscreened $S=1$
Kondo lattice model. Starting from the two orbital Anderson lattice model and
including a local anisotropy term, we show, through Schrieffer-Wolff
transformation, that local anisotropy is equivalent to an anisotropic Kondo
interaction ($J_{\parallel} \neq{J_{\perp}}$). The competition and coexistence
between ferromagnetism and Kondo effect in this effective model is studied
within a generalized mean-field approximation. Several regimes are obtained,
depending on the parameters, exhibiting or not coexistence of magnetic order
and Kondo effect. Particularly, we show that a re-entrant Kondo phase at low
temperature can be obtained. We are also able to describe phases where the
Kondo temperature is smaller than the Curie temperature ($T_K<T_C$). We propose
that some aspects of uranium and neptunium compounds that present coexistence
of Kondo effect and ferromagnetism, can be understood within this model.Comment: 7 pages, 3 figure

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