S. G. Magalhaes

Spin-glass freezing in Kondo-lattice compounds

A theory is presented that describes a spin-glass phase at finite temperatures in Kondo-lattice systems with an additional Ruderman‐Kittel‐Kasuya‐Yosida interaction represented by long range, random couplings among localized spins as 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.

Theory of the Kondo lattice : competition between Kondo effect and magnetic order

We present here a review of some theoretical features of the Kondo lattice problem, which can describe anomalous experimental properties of strongly correlated electron systems with mainly cerium, ytterbium or uranium. In such Kondo systems, there exist both a Kondo effect on each site and strong intersite magnetic interactions. We present firstly the general case of the Kondo lattice and the mean-field approximation. We then discuss the case of the “underscreened” Kondo lattice with a strong coexistence between the Kondo effect and a ferromagnetic order, as for example observed in uranium compounds such as UTe. We also discuss the Kondo–spin glass competition with eventually an additional magnetically ordered phase (ferromagnetic or antiferromagnetic one) and we can account for phase diagrams of some cerium and uranium disordered alloys, such as CeNi1 − x Cu x . Finally, the introduction of a transverse magnetic field in the Kondo–spin glass problem yields a quantum critical point and allows a better description of the phase diagrams of disordered alloys.

Spin glass and ferromagnetism in disordered cerium compounds

The competition between spin glass, ferromagnetism and Kondo effect is analysed here in a Kondo lattice model with an inter-site random coupling

The Ising spin glass in a transverse field revisited. Results of two fermionic models

J_{ij}

Quantum critical point in the spin glass–antiferromagnetism competition for fermionic Ising models

between the localized magnetic moments given by a generalization of the Mattis model which represents an interpolation between ferromagnetism and a highly disordered spin glass. Functional integral techniques with Grassmann fields have been used to obtain the partition function. The static approximation and the replica symmetric ansatz have also been used. The solution of the problem is presented as a phase diagram giving

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

T/{J}

Van Hemmen-Kondo model for disordered strongly correlated electron systems

{\it versus}

Role of the transverse field in inverse freezing in the fermionic Ising spin-glass model

J_K/J

Superconductivity in a two dimensional extended Hubbard model

where

Spin glass and antiferromagnetism in Kondo-lattice disordered system

T