Tunable "Doniach Phase Diagram" for strongly-correlated nanoclusters

Exact diagonalization calculations reveal that the energy spacing $\Delta$ in the conduction band tunes the interplay between the {\it local} Kondo and {\it non local} RKKY interactions, giving rise to a "Doniach phase diagram" for a nanocluster with regions of prevailing Kondo or RKKY correlations. The parity of the total number of electrons alters the competition between the Kondo and RKKY correlations. This interplay may be relevant to experimental realizations of small rings or quantum dots with tunable magnetic properties. Below a critical value V$_c$ of the hybridization the susceptibility exhibits a low-T exponential activation behavior determined by the interplay of the spin gap and $\Delta$.Comment: 4 pages, 5 figure

A nonplanar Peierls-Nabarro model and its applications to dislocation cross-slip

A novel semidiscrete Peierls-Nabarro model is introduced which can be used to study dislocation spreading at more than one slip planes, such as dislocation cross-slip and junctions. The strength of the model, when combined with ab initio calculations for the energetics, is that it produces essentiallyan atomistic simulation for dislocation core properties without suffering from the uncertainties associated with empirical potentials. Therefore, this method is particularly useful in providing insight into alloy design when empirical potentials are not available or not reliable for such multi-element systems. As an example, we study dislocation cross-slip and constriction process in two contrasting fcc metals, Al and Ag. We find that the screw dislocation in Al can cross-slip spontaneously in contrast with that in Ag, where the screw dislocation splits into two partials, which cannot cross-slip without first being constricted. The response of the dislocation to an external stress is examined in detail. The dislocation constriction energy and the critical stress for cross-slip are determined, and from the latter, we estimate the cross-slip energy barrier for straight screw dislocations.Comment: Submitted for the Proceedings of Multiscale Modelling of Materials (London, 2002