We have performed small-angle neutron scattering (SANS) of the flux line lattice (FLL) in a Fe doped NbSe_2 sample which presents a large peak effect in the critical current. The scattered intensity and the width of the Bragg peaks of the equilibrium
FLL indicate an ordered structure in the peak effect zone. The history dependence in the FLL structure has been studied using field cooled and zero field cooled procedures, and each state shows the same intensity of Bragg scattering and good orientational order. These results strongly suggest that the peak effect is unrelated to a bulk disordering transition, and confirm the role of a heterogeneous distribution of screening current.
The spectacular metal-to-insulator transition of V2O3 can be progressively suppressed in thin film samples. Evidence for phase separation was observed using microbridges as a mesoscopic probe of transport properties where the same film possesses doma
ins that exhibit a metal-to-insulator transition with clear first order features or remain metallic down to low temperatures. A simple model consisting of two parallel resistors can be used to quantify a phase coexistence scenario explaining the measured macroscopic transport properties. The interaction between film and substrate is the most plausible candidate to explain this extended phase coexistence as shown by a correlation between the transport properties and the structural data.
The electronic transport properties and the magnetic susceptibility were measured in detail in $YBaCo_2O_{5.45}$. Close to the so-called metal-insulator transition, strong effects of resistance relaxation, a clear thermal hysteresis and a sudden incr
ease of the resistance noise are observed. This is likely due to the first order character of the transition and to the underlying phases coexistence. Despite these out of equilibrium features, a positive and linear magneto-resistance is also observed, possibly linked to the heterogeneity of the state. From a magnetic point of view, the paramagnetic to ordered magnetic state transition is observed using non linear susceptibilty. This transition shows the characteristics of a continuous transition, and time dependent effects can be linked with the dynamics of magnetic domains in presence of disorder. Thus, when focusing on the order of the transitions, the electronic one and the magnetic one can not be directly associated.
