Thesis presented September 29, 2006

Abstract:
SmB6 is one typical example of the exciting physics which can result from a quantum equilibrium between two valence configurations. The first configuration of Samarium (Sm²⁺) corresponds to an insulating and non magnetic state whereas the second one (Sm³⁺) would theoretically give a magnetic and metallic ground state. Specific heat measurements under pressure evidenced a new long range magnetic ordering for pressures higher than p_c=10GPa. On another hand, transport measurements measured for the first time in good conditions of hydrostaticity found a reliable and reproducible critical pressure for the insulator to metal transition equal to p_c. The phase diagram is now well known and the observation for the first time of a magnetic anomaly in the high pressure resistivity curves certifies that the onset of the magnetic phase really coincide with the closure of the gap. This change is discussed in a general frame taking into account the Kondo lattice temperature as a key parameter for the renormalization of the wave function from one integer configuration to the other whereas the valence itself is still intermediate. This general idea seems to be valid also for other systems studied in this dissertation like SmS or TmSe and could even be valid for more general cases.
In the same time, resistivity measurements under uniaxial stress were undertaken. The result is a strong anisotropy effect observed on the pressure dependence of the residual resistivity in the compound SmB₆. The comparison with the transport under hydrostatic conditions enables us to consider a new idea for the nature of the gap, considering only one anisotropic gap.

Keywords:
Intermediate valence, Kondo lattice, Quantum phase transition, Magnetism, Insulator to metal transition, High pressure, micro calorimetry, Transport, Nuclear Forward Scattering

On-line thesis.