Fermionic Ising Glasses with BCS Pairing Interaction. Tricritical Behaviour

TL;DR

This paper investigates how BCS pairing influences the formation of spin glass phases in a fermionic model, revealing a complex phase diagram with tricritical points and transitions, aligning with experimental observations in certain alloys.

Contribution

It introduces a fermionic Sherrington-Kirkpatrick model with BCS interaction and analyzes its phase diagram, highlighting tricritical behavior and phase transitions.

Findings

Identifies a phase diagram with paramagnetic, spin glass, and pairing phases.

Discovers a tricritical point where transition order changes.

Results qualitatively match experimental data in specific alloys.

Abstract

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}.