Point-contact spectroscopy in neutron-irradiated Mg^{11}B_2

TL;DR

This study investigates how neutron irradiation affects the superconducting gaps in Mg^{11}B_2 using point-contact spectroscopy, revealing gap merging at low critical temperatures and discussing anomalous contact phenomena.

Contribution

It provides new experimental data on the evolution of superconducting gaps in irradiated Mg^{11}B_2 and discusses the origins of anomalous contacts in such conditions.

Findings

Gaps merge at TcA ~ 9K confirming specific-heat results

Irradiation causes a reduction in superconducting gaps

Anomalous contacts often occur at high irradiation fluences

Abstract

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.