Magnetic transition in a correlated band insulator

The effect of on-site electron-electron repulsion U in a band insulator is explored for a bilayer Hubbard Hamiltonian with opposite sign hopping in the two sheets. The ground-state phase diagram is determined at half-filling in the plane of U and the interplanar hybridization V through a computation of the antiferromagnetic (AF) structure factor, local moments, single-particle and spin wave spectra, and spin correlations. Unlike the case of the ionic Hubbard model, no evidence is found for a metallic phase intervening between the Mott and band insulators. Instead, upon increase of U at large V, the behavior of the local moments and of single-particle spectra give quantitative evidence of a crossover to a Mott insulator state preceding the onset of magnetic order. Our conclusions generalize those of single-site dynamical mean-field theory, and show that including interlayer correlations results in an increase of the single-particle gap with U. DOI: 10.1103/PhysRevB.87.12514

Kondo screening and Magnetism at Interfaces

The nature of magnetic order and transport properties near surfaces is a topic of great current interest. Here we model metal-insulator interfaces with a multi-layer system governed by a tight-binding Hamiltonian in which the interaction is non-zero on one set of adjacent planes and zero on another. As the interface hybridization is tuned, magnetic and metallic properties undergo an evolution that reflects the competition between anti-ferromagnetism and (Kondo) singlet formation in a scenario similar to that occurring in heavy-fermion materials. For a few-layer system at intermediate hybridization, a Kondo insulating phase results where magnetic order and conductivity are suppressed in all layers. As more insulating layers are added, magnetic order is restored in all correlated layers except that at the interface. Residual signs of Kondo physics are however evident in the bulk as a substantial reduction of the order parameter in the 2-3 layers immediately adjacent to the interfacial one. We find no signature of long range magnetic order in the metalli

CONTRIBUTION A L'EMPLOI DE B. THURINGIENSIS BERLINER POUR LA PRÉSERVATION DE LA CIRE D'ABEILLE CONTRE GALLERIA MELLONELLA L.

International audienc