Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

Abstract

Abstract. We report on the microwave surface resistance of two polycrystalline Mg 11 B2 samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near Tc and for magnetic fields greater than 0.5Hc2(T ) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account. PACS

Depinning frequency in a heavily neutron-irradiated MgB2 sample

“…These results have been reported and discussed in Ref. [18]. In order to fit the data, we have determined the B profile inside the sample due to the critical state and calculated a proper averaged value of R s (B) over the whole sample.…”

“…The lines are the best-fit curves obtained, as explained in Ref. [18], with µ 0 H c2 = 1.71 T, ω 0 /ω = 0.67 and the field dependence of the critical current density reported in Ref. [25].…”

“…Preliminary results obtained in this sample have shown that the mw losses can be justified in the framework of standard models for vortex dynamics [18]. Here, we report the results obtained in a wide range of temperatures (4.2 K ÷ T c ), from which we determine the temperature and the magnetic-field dependencies of the depinning frequency.…”

“…Since the first studies on MgB 2 , different authors have highlighted several anomalies in the field-induced variations of the mw surface impedance, especially at low temperatures and magnetic fields much lower than the upper critical field [13,14,15,16,17,18]. These studies have established that the standard theories are inadequate to describe the fluxon dynamics in the two-gap MgB 2 , in wide ranges of temperatures and magnetic fields.…”

Fluxon dynamics in Li–Al codoped by microwave surface resistance measurements

“…It has already been shown that the in-field microwave (mw) response of two-gap MgB 2 superconductor cannot be accounted for by standard models for fluxon dynamics [1][2][3]. The anomalies have been ascribed to the unusual structure of fluxons, related to the different field dependence of the two superconducting gaps [4,5].…”