Point-contact spectroscopy in neutron-irradiated Mg¹¹B₂

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D. Daghero, A. Calzolari, G. A. Ummarino, M. Tortello, and R. S. Gonnelli
Dipartimento di Fisica and CNISM, Politecnico di Torino, 10129 Torino, Italy

V. A. Stepanov
P.N. Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia

C. Tarantini
CNR-INFM-LAMIA and Dipartimento di Fisica, Università di Genova, 16146 Genova, Italy

P. Manfrinetti
CNR-INFM-LAMIA and Dipartimento di Chimica e Chimica Industriale, Università di Genova, 16146 Genova, Italy

E. Lehmann
Department of Spallation Neutron Source SINQ, Paul Scherrer Institut, CH-5232 Villigen, Switzerland

Received 19 July 2006; revised 13 September 2006; published 28 November 2006

Wereport on recent results of point-contact spectroscopy measurements in Mg¹¹B₂polycrystals irradiated at different neutron fluences up to Phi =1.4×10²⁰ cm⁻². Thepoint contacts were made by putting a small drop ofAg paint—acting as the counterelectrode—on the cleaved surface of thesamples. The gap amplitudes were extracted from the experimental conductancecurves, showing Andreev-reflection features, through a two-band Blonder-Tinkham-Klapwijk fit andreported as a function of the Andreev critical temperature ofthe junctions, T cA . The resulting Delta (T) and Delta (T) curves showa clear merging of the gaps when T9 K that perfectlyconfirms the findings of specific-heat measurements in the same samples.Anomalous contacts with T>T_c (being T_c the bulk critical temperature)were often obtained, particularly in samples irradiated at very highfluences. Their fit gave a different dependence of Delta onT cA . The possible origin of these contacts is discussed interms of local current-induced annealing and/or nanoscale inhomogeneities observed bySTM in the most irradiated samples.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.74.174519
DOI:	10.1103/PhysRevB.74.174519
PACS:	74.45.+c; 74.70.Ad; 74.62.Dh 74.45.+c Proximity effects; Andreev effect; SN and SNS junctions 74.70.Ad Superconducting metals; alloys and binary compounds (including A15, MgB₂, etc.) 74.62.Dh Effects of crystal defects, doping and substitution on superconducting transition temperature YEAR: 2006
KEYWORDS:	magnesium compounds, type II superconductors, point contact spectroscopy, neutron effects, electric admittance, superconducting transition temperature, specific heat, annealing, scanning tunnelling microscopy

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