We report on recent results of point-contact spectroscopy measurements in Mg^{11}B_2 polycrystals irradiated at different neutron fluences up to 1.4 x 10^{20} cm^{-2}. The point contacts were made by putting a small drop of Ag paint -- acting as the counterelectrode -- on the cleaved surface of the samples. The gap amplitudes were extracted from the experimental conductance curves, showing Andreev-reflection features, through a two-band Blonder-Tinkham-Klapwijk fit and reported as a function of the Andreev critical temperature of the junctions, TcA. The resulting Delta_sigma(TcA) and Delta_pi(TcA) curves show a clear merging of the gaps when TcA ~ 9K that perfectly confirms the findings of specific-heat measurements in the same samples. Anomalous contacts with TcA > Tc (being Tc the bulk critical temperature) were often obtained, particularly in samples irradiated at very high fluences. Their fit gave a different dependence of Delta_pi on TcA. The possible origin of these contacts is discussed in terms of local current-induced annealing and/or nanoscale inhomogeneities observed by STM in the most irradiated samples.
@article{arxiv.cond-mat/0607515,
title = {Point-contact spectroscopy in neutron-irradiated Mg^{11}B_2},
author = {D. Daghero and A. Calzolari and G. A. Ummarino and M. Tortello and R. S. Gonnelli and V. A. Stepanov and C. Tarantini and P. Manfrinetti and E. Lehmann},
journal= {arXiv preprint arXiv:cond-mat/0607515},
year = {2009}
}
Comments
12 pages, 10 figures, revtex4. One figure removed; Figs. 4 and 8 changed. Text changed (particularly Sect. V)