Mechanical and squid measurements on NB thin films: Learning form a conventional superconductor

摘要

Vibrating reed, torque and SQUID measurements on Nb networks of strand widths between 200 nm and 550 nm as well as on Nb thin films of thickness 120 nm and 1.2 #mu# m have been performed. Our results indicate that in samples with thickness approx 10 #xi# GL a flux line lattice exists in the complete field range B_(c1) <= B_a <=B_(cu), with B_(cu) (T, #theta# chemical bounds 0 deg ) chemical bounds B_(c3)(T) the surface superconductivity critical field, and B_(cu) (T, #theta# chemical bounds 90 deg ) chemical bounds B_(c2) (T), being #theta# the angle between field and main surface. When the transversal correlation length R_c is of the order of the strand width, the flux line lattice undergoes a dimensional crossover which increases its pinning strength. Anomalies in the magnetization are obseved at fields which are consistent with the matching fields for formation and splitting of ovrtex chains. Magnetization measurements reveal also the existence of a "second magnetization peak" (SMP) well below the upper critical field, similar to those measured in Bi_2Sr_2CaO_8 (Bi2212) crystals and other nonconventional superconductors. We argure that the SMP's measured in Nb and also in Bi2212 crystals are due to thermomagnetic flux jump instability and are not related to an enhancement of the critical current density produced by phase transitions of the flux-line lattice.