M. Bonura

Critical-state effects on microwave losses in type-II superconductors

Abstract.We discuss the microwave energy losses in superconductors in the critical state. The field-induced variations of the surface resistance are determined, in the framework of the Coffey and Clem model, by taking into account the distribution of the vortex magnetic field inside the sample. It is shown that the effects of the critical state cannot generally be disregarded to account for the experimental data. Results obtained in bulk niobium at low temperatures are quantitatively justified.

Magnetic hysteresis in the microwave surface resistance of Nb samples in the critical state

Abstract.We discuss the hysteretic behavior of the field-induced variations of the microwave surface resistance in superconductors in the critical state. Measurements have been performed in a bulk sample of Nb and a powdered one at different values of the temperature. We discuss a model, based on the Coffey and Clem theory, in which we take into account the flux distribution inside the sample, due to the critical state. The experimental results are quite well justified in the framework of our model. We show that by fitting the experimental data it is possible to determine the value of the critical current density and its field dependence.

Depinning frequency in a heavily neutron-irradiated MgB2 sample

Abstract The magnetic-field-induced variations of the microwave surface resistance have been investigated in a heavily neutron-irradiated MgB 2 sample, in which the irradiation has caused the merging of the two gaps into a single value. The experimental results have been analyzed in the framework of the Coffey and Clem model. By fitting the experimental data, we have determined the field dependence of the depinning frequency, ω 0 , at different values of the temperature. Although the pinning is not particularly effective, the value of ω 0 obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors.

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

Abstract.We report on the microwave surface resistance of two polycrystalline Mg11 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.

Microwave properties of Ba

Abstract.We report on field-induced variations of the microwave surface resistance at 9.6 GHz of Ba0.6K0.4BiO3 crystals. Energy losses have been investigated as a function of the static magnetic field in the range of temperatures 4.2 K

\mathsf{_{0.6}}

\div T_c

K

. By analyzing the experimental results in the framework of the Coffey and Clem model we determine the temperature dependence of the first-penetration field, upper critical field and depinning frequency. The results show that the pinning energy of this bismuthate superconductor is weaker than those of cuprates.

\mathsf{_{0.4}}

We report the successful manufacture and characterization of a microwave resonant cylindrical cavity made of bulk MgB2 superconductor (Tc?38.5?K), which has been produced by the reactive liquid Mg infiltration technique. The quality factor of the cavity for the TE011 mode, resonating at 9.79?GHz, has been measured as a function of the temperature. At T = 4.2?K, the unloaded quality factor is ?2.2?105; it remains of the order of ? 105 up to T~30?K. We discuss the potential performance improvements of microwave cavities built from bulk MgB2 materials produced by reactive liquid Mg infiltration.

BiO

We report experimental results of the field-induced variations of the microwave surface resistance in samples of MgB2, produced by different methods. By sweeping the DC magnetic field at increasing and decreasing values, we have detected a magnetic hysteresis that can be ascribed to the different magnetic induction, due to the critical state of the fluxon lattice. The hysteresis observed in the bulk samples has an unusual shape, which cannot be justified in the framework of the critical-state models.

\mathsf{_3}

Field-induced variations of the microwave surface resistance, Rs(H), have been investigated in high-density ceramic MgB2. At low temperatures, several peculiarities of the Rs(H) curves cannot be justified in the framework of models reported in the literature. We suggest that they are ascribable to the unconventional vortex structure in MgB2, related to the presence of two gaps. On the contrary, the results near Tc can be accounted for by the Coffey and Clem model, with fluxons moving in the flux-flow regime, provided that the anisotropy of the upper critical field is taken into due account.