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Center for Nonlinear Studies |
Kondo Insulators or more appropriately, heavy-fermion semiconductors are materials that have high temperature properties that resemble those of heavy-fermion metals. However, at low temperatures, the materials show evidence for the existence of a small gap at the Fermi-energy. Compared with normal semiconductors which have gaps of the order of eV, the gaps found in heavy fermion semiconductors are of the order of meV. The smallness of the gap is attributed to electronic correlations. The gap has been found to be temperature dependent. The gap has been probed with thermodynamic, transport, optical and inelastic neutron scattering experiments. Since heavy fermion materials are frequently found to be in the vicinity of magnetic instabilities, mostly to antiferromagnetic ordering, heavy fermion semiconductors may also be expected to be in the proximity of antiferromagnetic quantum critical points. Due to the smallness of the gap, one can expect that the normal paramagnetic phase of the semiconductors may show precursor fluctuations similar to the anti-paramagnons found in metals as precursors to antiferromagnetic phase transitions. However, for excitation energies below the gap energies, these excitations are expected to become sharp, due to the absence of a decay channel into low-energy electron-hole pairs. Such sharp in gap branches of magnetic excitations have been reported in SmB6 and YbB12, but have not been found in cerium systems. The relation of these in-gap magnetic excitations to the proximity of magnetic instabilities has not been established. Recently, similar spin-exciton excitations have been reported in Kondo Insulators in which the gap exhibits nodes. The evolution of the gap in these Kondo semi-metals and the relation to magnetic ordering is discussed theoretically on the basis of an Anderson lattice Model.
Host: Tomasz Durakiewicz