The Magnetic-field Dependence of the Gaps in a Two-band Superconductor: A Point-contact Study of MgB_2 Single Crystals

TL;DR

This study uses directional point-contact spectroscopy on MgB_2 single crystals in magnetic fields to measure the two superconducting gaps and tests the two-band model's predictions, confirming the pi band is in the moderate dirty limit.

Contribution

First direct experimental validation of the two-band model in MgB_2, showing the pi band is in the moderate dirty limit through gap measurements under magnetic fields.

Findings

Measured Delta_sigma and Delta_pi as functions of magnetic field.

Confirmed the pi band is in the moderate dirty limit.

Provided quantitative comparison with theoretical predictions.

Abstract

We present the results of directional point-contact measurements in MgB_2 single crystals, in magnetic fields up to 9 T parallel to the c axis. By fitting the conductance curves of our point contacts -- showing clear Andreev-reflection features -- with a generalized Blonder-Tinkham-Klapwijk model, we were able to extract the values of the two gaps Delta_sigma and Delta_pi. The comparison of the resulting Delta_sigma(B) and Delta_pi(B) curves to the theoretical predictions of the two-band model in dirty limit, recently appeared in literature, allows the first direct test of this model and gives a clear and quantitative proof that the pi band, even in the best single crystals, is in the moderate dirty limit.