Size-induced entropy-jump depression at the first-order melting transition line of nanocrystalline vortex matter?

Y. FASANO

El Escorial

Workshop; Invited talk at the XVth International Workshop on Vortex Matter; 2015

Universidades Autónoma y Complutense de Madrid

The persistence of the first-order solidification and solid order-disorder transition lines in the phase diagram of nanocrystalline Bi2Sr2CaCu2O8+y vortex matter is detected down to a system size of less than hundred vortices. The temperature-location of the vortex solidification transition line HFOT is not altered by decreasing the number of vortices down to less than hundred, although there is a depletion of the entropy-jump of the transition with respect to macroscopic vortex matter. Direct imaging of nanocrystalline vortex matter indicates this entropy-depletion is due to the nucleation of a more disordered vortex solid structure in the case of micron-sized samples due to the register of vortex lines with the sample edge. The solid order-disorder phase transition field HSP moves upward on decreasing the system size due to the increase of the surface-to-volume ratio of vortices entailing a decrease on the average vortex binding energy. The onset of irreversible magnetic response is also affected by confinement with a noticeable enlargement of the irreversible vortex region above HSP on decreasing the number of vortices. Precise and highly-sensitive to bulk currents ac magnetization techniques proved to be mandatory in order to obtain this information.E-mail: Yanina.Fasano@gmail.com