Physics – Condensed Matter – Superconductivity
Scientific paper
2006-07-20
Phys. Rev. B 74, 174519 (2006)
Physics
Condensed Matter
Superconductivity
12 pages, 10 figures, revtex4. One figure removed; Figs. 4 and 8 changed. Text changed (particularly Sect. V)
Scientific paper
10.1103/PhysRevB.74.174519
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.
Calzolari Arrigo
Daghero Dario
Gonnelli Renato S.
Lehmann E.
Manfrinetti P.
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