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We report on the microwave (mw) properties of a cylindrical MgB2 rod prepared by the reactive liquid Mg infiltration technology. The MgB2 rod, 94.3 mm long, is used as inner conductor of a coaxial cavity having a Cu tube as external conductor. By analyzing the resonance curves of the cavity in the different resonant modes and at different temperatures, we have determined the temperature dependence of the mw surface resistance, RsRs, of the MgB2 material, at fixed frequencies, and the frequency dependence of RsRs, at fixed temperatures. Our results show that the Rs(f)Rs(f) curves follow a fnfn law, where n decreases on increasing the temperature, starting from n≈2n≈2, at T=4.2K, down to n≈0.7n≈0.7 at T...TcT...Tc. The double-gap nature of MgB2 manifests itself in the presence of a wide low-T tail in the Rs(T)Rs(T) curves, which can be ascribed to the quasiparticles thermally excited through the ππ gap even at relatively low temperatures. [ABSTRACT FROM AUTHOR]

Abstract: We report on the microwave surface resistance of melt-textured Y1.6Ba2.3Cu3.3O7−x samples, doped with different amount of B2O3 and, subsequently, irradiated by thermal neutrons at the fluence of 1.476×1017 cm−2. The microwave surface resistance has been measured as a function of temperature and DC magnetic field. The experimental results are quantitatively discussed in the framework of the Coffey and Clem theory, properly adapted to take into account the d-wave nature of cuprate superconductors. By fitting the experimental data at zero DC field, we have highlighted the effects of the induced defects in the general properties of the samples, including the intergranular region. The analysis of the results obtained at high DC fields allowed us to investigate the fluxon dynamics and deduce the depinning frequency; in particular, we show that the addition of B2O3 up to 0.1wt.% increases the effectiveness of the defects to hinder the fluxon motion induced by the microwave current. [Copyright &y& Elsevier]

Abstract: The magnetic-field-induced variations of the microwave surface resistance, , have been investigated in ceramic Mg1−x (LiAl) x B2, with x in the range . The measurements have been performed on increasing and decreasing the DC magnetic field, , at fixed temperatures. At low temperatures, we have observed a magnetic hysteresis in the curves in all the investigated samples. On increasing the temperature, the range of in which the hysteretic behavior is visible shrinks; however, in the sample with it is present up to temperatures close to . We show that the field dependence of can be quantitatively justified taking into account the critical-state effects on the fluxon lattice only in the sample with . On the contrary, in the samples with the hysteresis exhibits an unusual shape, similar to that observed in others two-gap MgB2 samples, which cannot be justified in the framework of the critical-state models. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic-field-induced variations of the microwave surface resistance have been investigated in a heavily neutron-irradiated MgB2 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, , at different values of the temperature. Although the pinning is not particularly effective, the value of obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors. [Copyright &y& Elsevier]

Abstract: Nonlinear microwave response of different ceramic MgB2 samples has been investigated by the technique of second-harmonic emission. The second-harmonic signal has been investigated as a function of temperature, DC magnetic field and input microwave power. The attention has mainly been devoted to the response at low magnetic fields, where nonlinear processes arising from motion of Abrikosov fluxons are ineffective. The results show that different mechanisms are responsible for the nonlinear response in the different ranges of temperature. At low temperatures, the nonlinear response is due to processes involving weak links. At temperatures close to T c, a further contribution to the harmonic emission is present; it can be ascribed to modulation of the order parameter by the microwave field and gives rise to a peak in the temperature dependence of the harmonic signal. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

We report on transient effects in the microwave second-order response of two ceramic MgB2 samples. The time evolution of the second-harmonic signal is investigated for about 500 s after the sample has been exposed to a variation of the dc magnetic field. We suggest that during the first seconds the response is determined by diffusive motion of fluxons, while in the time scale of minutes it is ruled by magnetic relaxation over the surface barrier. © 2001 Elsevier Science. All rights reserved. [Copyright &y& Elsevier]

Transient effects in the microwave second-order response of YBaCuO powder are investigated. The time evolution of the second-harmonic signal has been measured for about 300 s after the sample had been exposed to variations of the DC magnetic field. We show that in different time scales the transient response has different origin. In the time scale of milliseconds the transient response of samples in the critical state is ascribable to processes of flux redistribution induced by the switching on/off of the microwave field. At longer times, the time evolution of the second-harmonic signal can be ascribed to motion of fluxons induced by the variation of the DC magnetic field. In particular, diffusive motion of fluxons determines the response in the first 10 s after the stop of the magnetic field variation; magnetic relaxation over the surface barrier determines the response in the time scale of minutes. [Copyright &y& Elsevier]

The field-induced variations of the microwave surface resistance are investigated in two different samples of powdered MgB2. 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 Tc and it takes on different values for the two samples, even though they have similar shape. [Copyright &y& Elsevier]

We report experimental results of second-harmonic (SH) generation in superconductors in the critical state, exposed to intense pulsed microwave fields. The SH signal exhibits time evolution, within the pulse width, and magnetic hysteresis. We show that the two effects are strictly related to each other. The peculiarities of both the time evolution and the hysteresis depend on the nature of the investigated superconductor. However, in all the samples a correlation occurs between the hysteresis amplitude and the ratio of the decay rates of the SH signals, detected when the static magnetic field has been reached at increasing and decreasing values. The transient effects are ascribed to processes of flux redistribution, which come into play when the fluxon lattice is driven by high frequency em fields. We infer that the hysteretic behaviour is due to the fact that, when the static field is reached at decreasing or increasing values, the redistribution processes occur with different rates. [Copyright &y& Elsevier]

We report experimental results on microwave (mw) second-harmonic (SH) emission in Ba0.6K0.4BiO3 crystal in the critical state, exposed to intense pulsed mw fields. The SH signals show transient effects and magnetic hysteresis. Our results suggest that history and memory effects are present in the dynamics of vortices driven by pulsed mw fields. [Copyright &y& Elsevier]