A. Agliolo Gallitto

Harmonic emission at microwave frequencies in YBa2Cu3O7 single crystals near Tc

Abstract Nonlinear microwave emission by YBa 2 Cu 3 O 7 single crystals is investigated at temperatures close to T c . Second- and third-harmonic signals are studied as a function of the temperature and input power, for different values of the external magnetic field. The results are discussed in the framework of a phenomenological theory based on the two-fluid model which assumes that both the microwave and static fields perturb the partial concentrations of the normal and condensate fluids. It is suggested that, at temperatures very close to T c , this mechanism is the only one responsible for the harmonic emission by YBa 2 Cu 3 O 7 single crystals in the Meissner state. It is also effective in the mixed state where the nonlinearity mechanism suggested by Coffey may play a role.

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.

Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB2 samples

The real Rs(T) and imaginary Xs(T) parts of the surface impedance Zs(T) = Rs(T) + iXs(T) in polycrystalline MgB2 samples of different density with the critical temperature Tc ≈ 38 K are measured at the frequency of 9.4 GHz and in the temperature range 5 ≤ T < 200 K. The normal skin-effect condition Rs(T) = Xs(T) at T ≥ Tc holds only for the samples of the highest density with roughness sizes not more than 0.1 μm. For such samples extrapolation T → 0 of the linear at T < Tc/2 temperature dependences λL(T) = Xs(T)/ωμ0 and Rs(T) results in values of the London penetration depth λL(0) ≈ 600 A and residual surface resistance Rres ≈ 0.8 mΩ. In the entire temperature range the dependences Rs(T) and Xs(T) are well described by the modified two-fluid model.

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.

Effects of Weak Links in the Nonlinear Microwave Response of MgB2 Superconductor

We report experimental results of second-harmonic (SH) response at microwave frequency in several ceramic MgB2 samples, prepared by different methods. The SH signal has been investigated as a function of the temperature and DC magnetic field. The investigation has been carried out at low magnetic fields, where nonlinear processes arising from motion of Abrikosov fluxons are ineffective. We show that the low-temperature SH emission is ascribable to processes involving weak links. Comparison among the peculiarities of the SH signal radiated by the different samples shows that the presence of weak links strongly depends on the sample preparation method, as well as on the purity and morphology of the components used for the preparation.

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.