arXiv subjects

P. Fazekas

Publications and source records attributed to P. Fazekas.

21 records · Page 2Linked to original sources

arXiv1993

Correlated Mean Field Ansatz for the Kondo Necklace

We study the ground state phase diagram of the pseudospin model introduced by Doniach to describe the essential physics of Kondo lattices. We use variational trial states which augment the usual mean field solution by incorporating various intersite correlations. A composite spin correlation describing the antiparallel alignment of fluctuating triplets is found to be particularly favourable for large Kondo couplings. With this trial state, the magnetic--to--Kondo transition is suppressed and the strong coupling ground state is ordered with strongly reduced moments. The relevance of the findings is discussed.

cond-mat↗

arXiv1993

Multi-Channel Kondo Necklace

A multi--channel generalization of Doniach's Kondo necklace model is formulated, and its phase diagram studied in the mean--field approximation. Our intention is to introduce the possible simplest model which displays some of the features expected from the overscreened Kondo lattice. The $N$ conduction electron channels are represented by $N$ sets of pseudospins $\vt_{j}$, $j=1, ... , N$, which are all antiferromagnetically coupled to a periodic array of $|\vs|=1/2$ spins. Exploiting permutation symmetry in the channel index $j$ allows us to write down the self--consistency equation for general $N$. For $N>2$, we find that the critical temperature is rising with increasing Kondo interaction; we interpret this effect by pointing out that the Kondo coupling creates the composite pseudospin objects which undergo an ordering transition. The relevance of our findings to the underlying fermionic multi--channel problem is discussed.

cond-mat↗

arXiv1993

Underscreened Kondo Necklace

It has been suggested recently by Gan, Coleman, and Andrei that studying the underscreened Kondo problem may help to understand the nature of magnetism in heavy fermion systems. Motivated by Doniach's work on the S=1/2 Kondo necklace, we introduce the underscreened Kondo necklace models with S>1/2. The underscreened Kondo necklace is the simplest lattice model on which the competition between Kondo spin compensation, and magnetic ordering due to an RKKY-type interaction can be examined. We used the mean-field approximation to determine the phase diagram, and found that the low-temperature phase is always an x-y antiferromagnet. This contention is further supported by the derivation of the exact form of the effective hamiltonian in the limit of very large Kondo coupling: it is found to be an antiferromagnetic x-y model for the residual (S-1/2)-spins. In general, the degree of moment compensation depends on both the Kondo coupling, and on S.

cond-mat↗