We explore the flux-jump regime in type-II Pb thin films with a periodic array of antidots by means of magneto-optical measurements. A direct visualization of the magnetic flux distribution allows us to identify a rich morphology of flux penetration patterns. We determine the phase boundary H * ͑T͒ between dendritic penetration at low temperatures and a smooth flux invasion at high temperatures and fields. For the whole range of fields and temperatures studied, guided vortex motion along the principal axes of the square pinning array is clearly observed. In particular, the branching process of the dendrite expansion is fully governed by the underlying pinning topology. A comparative study between macroscopic techniques and direct local visualization sheds light onto the puzzling T-and H-independent magnetic response observed at low temperatures and fields. Finally, we find that the distribution of avalanche sizes at low temperatures can be described by a power law with exponent ϳ 0.9͑1͒.

'vVe show ev id ence th<'lt a structural martensitic transition is related to significant changes in the electronic structure, as revealed in thermodynamic measurements made in high-magnetic fields. The magnetic field dependence is considered unusual as many influential investigations of m artensitic transitions have emphasized that the structural transitions are primarily lat tice dynamical a nd are driven by the entropy due to the phonons. We provide a theoretical framework which c<'ln be lls ed to describ e the effect of m<'lgnetic field on the lattice dynamics in which the field dependence originates from the dielectric constant.