S. Fricano

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.

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}}

Abstract The field-induced variations of the microwave surface resistance are investigated in two different samples of powdered MgB 2 . The experimental results can be justified in the framework of 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. Assuming the angular dependence of the upper critical field expected from the anisotropic Ginzburg–Landau theory, we determine the anisotropy factor by fitting the experimental data of the field dependence of the microwave surface resistance. We show that the anisotropy factor is constant in a range of temperatures of about 3 K below T c and it takes on different values for the two samples, even though they have similar shape.

BiO

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.

\mathsf{_3}

We report experimental results on microwave surface resistance in MgB2 superconductors. The results have been obtained using the cavity perturbation method. Two samples have been investigated: one was made up of commercial Alfa-Aesar powder and the other one was obtained by direct reaction from elements. The temperature and dc magnetic field dependence of the surface resistance has been investigated down to 4.2 K and up to 10 kOe, respectively. Our results show that microwave losses are strongly affected by the magnetic field even by a relatively weak one. In particular, an unusually enhanced field dependence of the low-T residual surface resistance has been detected in both samples.

crystals

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

Microwave surface resistance and upper-critical-field anisotropy of MgB2 superconductor

div T_c

Fluxon dynamics by microwave surface resistance measurements in MgB2

. 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.