Field-induced suppression of the π-band superconductivity and magnetic hysteresis in the microwave surface resistance of MgB₂at temperatures nearT_c

Abstract

We report on the magnetic-field-induced variations of the microwave surface resistance, R s , in a polycrystalline MgB 2 sample, at different values of temperature. We have detected a magnetic hysteresis in R s , which exhibits an unexpected plateau on decreasing the DC magnetic field below a certain value. In particular, at temperatures near T c the hysteresis manifests itself only through the presence of the plateau. Although we do not quantitatively justify the anomalous shape of the magnetic hysteresis, we show that the results obtained in the reversible region of the R s (H) curve can be quite well accounted for by supposing that, in this range of magnetic field, the π gap is almost suppressed by the applied field and, consequently, all the π-band charge carriers are quasiparticles. On this hypothesis, we have calculated R s (H) supposing that fluxons assume a conventional (single core) structure and the flux dynamics can be described in the framework of conventional models. From the fitting of the experimental results, we determine the values of H π c2 (T) at temperatures near T c. In our opinion, the most important result of our investigation is that, at least at temperatures near T c , the value of the applied field that separates the reversible and irreversible regions of the R s (H) curves is just H π c2 (T); a qualitative discussion of the possible reasons for this finding is given.

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

“…By applying the same arguments of Ref. [7] to the results obtained in the sample with x ¼ 0:1, we have deduced a linear dependence of H p c2 ðTÞ for T P 29 K, which is well described by the law H p c2 ðTÞ ¼ ð0:57 À 0:017 TÞ T.…”

“…By fitting the data, we have shown that, at least in the temperature range in which the hysteresis manifests itself only through the presence of the plateau, the values of H p c2 ðTÞ coincide with the values of H 0 at which the plateau in the decreasing-field branch of R s ðH 0 Þ starts. The result we obtained in [7] allows to determine the temperature dependence of H p c2 at temperatures near T c . By applying the same arguments of Ref.…”

“…The anomalies have been ascribed to the unusual structure of fluxons, related to the different field dependence of the two superconducting gaps [4,5]. Support to this hypothesis has been given from investigation of a strongly neutron-irradiated MgB 2 sample [6], in which the two gaps merged into a single value, and in a two-gap sample only at applied magnetic fields higher than the field at which the smaller gap is almost suppressed [7] (i.e. when fluxons assume a more conventional shape).…”

Dielectric properties of myoglobin at 10 GHz by microwave cavity perturbation measurements

“…A more quantitative analysis of the data can be performed by assuming that we are observing a relaxation process characterized by a single relaxation time (Debye approximation) 1 whose temperature dependence is given by the Arrhenius law. In this case: ε (ω, T ) = Δε (ωτ 0 e E/RT )/(1 + (ωτ 0 e E/RT ) 2 ) ,…”

“…A copper cavity, of cylindrical shape with golden-plated walls, is tuned in the TE 011 mode resonating at 9.6 GHz; the sample is located along the axis of the cavity, where the mw electric field has a node. The cavity is placed inside a cryostat which allows to operate in the temperature range 2.2-350 K. Further details of the apparatus are reported elsewhere [1]. As demonstrated by Zhai et al, by measuring the quality factor of the resonant cavity one can measure the dielectric properties of the sample even though it is placed in a pure mw magnetic field [16].…”