MgB2 is known as a multiband superconductor, where two distinct bands take part in the superconductivity with different gap sizes. According to the classical Bardeen-Stephen theory, a flux-flow resistivity in a conventional s-wave superconductor increases linearly with increasing the magnetic field and reaches a normal resistivity at an upper critical field Bc2. In MgB2, on the other hand, it rises more steeply than expected by the Bardeen-Stephen theory at low fields before slowly approaching the normal resistivity at Bc2, as revealed by microwave surface impedance measurements [1]. This unusual field dependence has been interpreted in terms of the multiband property that the superconductivity with the smaller gap is suppressed at a field much lower than Bc2. One may expect that this unusual behavior of the flux-flow resistivity should be also detected by a dc resistivity measurement in the fast-flow regime where pinning is ineffective. However, to our knowledge, it has not been reported. This is probably because the large dc current is needed to realize the ideal flux flow in the bulk crystals, which generates large Joule heating.
In this work, we measure the field dependence of the dc vortex-flow resistivity in the fast-flow regime in an epitaxially grown MgB2 thin film [2] with a thickness of about 100 nm by employing pulsed-current measurements. The current-voltage (I-V) characteristics show a crossover from nonlinear to linear behavior with increasing I, indicating a change from pinning-dominated to driving(viscous)-force dominated flow. The slopes extracted from the linear part of the I-V curves show an unusual field dependence similar to that observed by microwave surface impedance measurements. These results indicate that multiband properties in MgB2 can be also detected by dc measurements in an epitaxially grown thin film.
[1] A. Shibata, M. Matsumoto, K. Izawa, Y. Matsuda, S. Lee, and S. Tajima, Phys. Rev. B 68, 060501(R) (2003).
[2] H. Shishido, T. Yoshida, and T. Ishida, Appl. Phys. Exp. 8, 113101 (2015)
Keywords: multiband superconductivity, MgB2, epitaxial thin film, vortex flow