Search

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., 4 pages, 4 embedded figures, to be published on Eur. Phys. J. B

We report on the microwave surface resistance of melt-textured Y_1.6Ba_2.3Cu_3.3O_7-x samples, doped with different amount of B_2O_3 and, subsequently, irradiated by thermal neutrons at the fluence of 1.476 \times 10^17 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 have shown that the addition of B_2O_3 up to 0.1 wt% increases the effectiveness of the defects to hinder the fluxon motion induced by the microwave current., Comment: 9 pages, 8 embedded figures, accepted for publication in Physica C

We report on transient effects in the microwave second-harmonic response of different type of superconductors in the mixed state. The samples have contemporarily been exposed to a dc magnetic field, varying with a constant rate of 60 Oe/s, and a pulsed microwave magnetic field. The time evolution of the signal radiated at the second-harmonic frequency of the driving field has been measured for about 500 s from the instant in which the dc-field sweep has been stopped, with sampling time of ∼0.3 s. We show that the second-harmonic signal exhibits two relaxation regimes; an initial exponential decay, which endures roughly 10 s, and a logarithmic decay in the time scale of minutes. Evidence is given that the decay in the time scale of minutes is ruled by magnetic relaxation over the surface barrier.

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 $\div T_c$ . 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.

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., Comment: 4 pages, 2 figures (Proceedings of the Third European Conference on Vortex Matter in Superconductors, to be published in Physica C)

We report a set of experimental results on microwave third-harmonic emission by MgB 2 superconductor. We have investigated third-harmonic signals as a function of temperature, input power level and intensity of the static magnetic field. We show that different mechanisms are responsible for third-harmonic emission at temperatures near T c and far below T c , similarly to what has been obtained in other high- T c superconductors.

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.

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.

We report experimental results on the second-order response of superconductors in the critical state exposed to intense pulsed microwave (mw) fields. The second-harmonic power radiated by the samples is investigated during the time interval in which the mw pulse endures, for mw pulses ≈1 ms long. The second-harmonic signal exhibits a time decay whose features depend on the investigated sample, the input power level, and the way in which the value of the static magnetic field has been reached. The transient is ascribed to processes of magnetic flux redistribution, which come into play when fluxons in the critical state are driven by electromagnetic (em) fields of high frequency.

We report experimental results on microwave second harmonic emission in different superconductors in the critical state, exposed to intense pulsed mw magnetic fields. We show that the second order response of some superconducting samples shows a magnetic hysteretic behavior. We suggest that this behavior is related to non-equilibrium phenomena induced by the mw pulses. In particular, it may be ascribed to the different time response of the fluxons in the reverse and direct critical state.

We report a detailed investigation of harmonic generation at microwave frequencies in Ba 0.6 K 0.4 BiO 3 crystals. Second- and third-harmonic signals are investigated as a function of temperature, external magnetic field and input power level, at temperatures close to T c . The experimental data are discussed in the framework of a phenomenological theory, based on the two-fluid model, which assumes that the microwave and the static fields, penetrating the sample within the penetration depth, perturb the partial concentrations of the normal and condensate fluids. We show that the enhanced nonlinear effects detected near T c in Ba 0.6 K 0.4 BiO 3 and YBa 2 Cu 3 O 7 crystals may have the same origin. However, in order to account for the experimental data of Ba 0.6 K 0.4 BiO 3 , one needs to involve a distribution of T c over the crystal.

Microwave second harmonic (SH) emission by Ba 0.6 K 0.4 BiO 3 (BKBO) crystals is investigated at low temperatures. A comparison with results obtained in ceramic YBa 2 Cu 3 O 7 (YBCO) is performed. Though some differences have been evidenced, in both compounds SH emission may be ascribed to the same mechanisms.

We report a set of experimental results on microwave third-harmonic emission by Ba 0.6 K 0.4 BiO 3 crystals. Third-harmonic signals are investigated as a function of temperature, input power level and intensity of the static magnetic field. The experimental data are discussed in the framework of models previously reported in the literature.

We report results of microwave second-harmonic generation in ceramic samples of MgB2, prepared by different methods. The SH signal has been investigated as a function of the temperature and the static magnetic field. The results are discussed in the framework of models reported in the literature. We show that the peculiarities of the SH signal are related to the specific properties of the sample. A comparison with the results obtained in ceramic and crystalline YBa2Cu3O7 shows that the second-harmonic emission in MgB2 is weaker than that observed in ceramic YBa2CuO7. (c) 2006 Wiley Periodicals, Inc.

Nonlinear microwave emission by Ba0.6K0.4BiO3 crystals is investigated. Second- and third-harmonic signals are studied as a function of the temperature and the applied magnetic field at different values of the input power level. The experimental data are discussed in the framework of models previously reported in the literature. We show that there are several mechanisms responsible for harmonic emission in Ba0.6K0.4BiO3 crystals: they play a different role depending on the temperature and the intensity of the applied field. We suggest that harmonic emission at temperatures far from Tc is due to nonlinear processes in weak links or flux motion, while nonlinearity near Tc is ascribable to the time variation of the order parameter induced by the microwave field.

The microwave surface impedance Z{sub s} of a YBa{sub 2}Cu{sub 3}O{sub 7} single crystal is investigated as a function of the magnetic field H{sub 0} at different temperatures close to T{sub c}. It is shown that Z{sub s} depends on H{sub 0} for applied fields higher as well as lower than H{sub c1}. A model is discussed in which the field dependence of Z{sub s} below H{sub c1} is ascribed to the field-induced variations, within the penetration depth, of the partial concentrations of both normal and condensate electrons. The field dependence of Z{sub s} above H{sub c1} is explained using the Coffey and Clem model. {copyright} {ital 1997} {ital The American Physical Society}

We report a set of experimental results on third harmonic generation at microwave frequencies in YBa 2 Cu 3 O 7 single crystals near T c . The harmonic signal is investigated as a function of the intensity and orientation of the dc magnetic field. We show that investigation of nonlinear microwave emission may afford an alternative way to study the upper critical field and crystal anisotropy.

The non-linear response near Tc and the complex electromagnetic conductivity of YBa2Cu3O7 single crystals at microwave frequencies are explained in the framework of the two-fluid model with the additional hypothesis that the e.m. field modulates the partial concentrations of both normal and superconducting fluids.

We report a set of experimental results on second- and third-harmonic generation at microwave frequencies in YBa2Cu3O7 single crystals near T c. The investigation of the harmonic signals as a function of the intensity and orientation of the d.c. magnetic field yields information on the upper critical field and crystal anisotropy.

We discuss experimental results obtained using a tunable cylindrical coaxial cavity constituted by an outer Cu cylinder and an inner Pb-BSCCO wire. We have used this device for investigating the microwave response of the superconducting wire, both in the linear and nonlinear regimes. In particular, by tuning the different modes of the cavity to make them resonant at exactly harmonic frequencies, we have detected the power emitted by the superconducting inner wire at the second- and third-harmonic frequency of the driving field. The results obtained in the nonlinear regime, whether for the microwave surface impedance or the harmonic emission, are qualitatively accounted for considering intergrain fluxon dynamics. The use of this kind of device can be of strong interest to investigate and characterise wires of large dimensions to be used for implementing superconducting-based microwave devices., Comment: 14 pages, 6 embedded figures, accepted for publication in Supercond. Sci. Technol

We report on the microwave properties of a resonant cylindrical cavity made of bulk MgB2 superconductor, produced by the reactive liquid Mg infiltration process. The frequency response of the cavity has been measured in the range 5-13GHz. Among the various modes, the TE011, resonating at 9.79GHz, exhibits the highest quality factor. For this mode, we have determined the temperature dependence of the quality factor, obtaining values of the order of 10^5 in the temperature range 4.2-30K. The values of the surface resistance deduced from the measurements of the quality factor agree quite well with those independently measured in a small sample of MgB2 extracted from the same specimen from which the cavity has been obtained., 12 pages, 5 eps figures, submitted to Physica C

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 2.2x10^5; it remains of the order of 10^5 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., Comment: 7 pages, 2 embedded figures, accepted for publication in Supercond. Sci. Technol

We discuss the microwave second-harmonic generation in high-density bulk MgB2, prepared by the reactive liquid Mg infiltration technology. The intensity of the harmonic signal has been investigated as a function of temperature and amplitudes of the DC and microwave magnetic fields. 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 magnetic fields, penetrating in the surface layers of the sample, weakly perturb the partial concentrations of the normal and superconducting fluids. We show that, in order to account for the experimental results, it is essential to suppose that in MgB2 the densities of the normal and condensed fluids linearly depend on the temperature., 6 pages, 4 embedded figures, submitted to Eur. Phys. J. B

The microwave response of three high-density bulk MgB2 samples has been investigated in the linear and nonlinear regimes. The three samples, characterized by different mean size of grains, have been obtained by reactive infiltration of liquid Mg in powdered B preforms. The linear response has been studied by measuring the microwave surface impedance; the nonlinear response by detecting the power radiated by the sample at the second-harmonic frequency of the driving field. Our results suggest that bulk MgB2 prepared by the liquid Mg infiltration technique is particularly promising for manufacturing resonant cavities operating at microwave frequencies., 4 pages, 2 embedded figures; Proceedings of 7th EUCAS Conference (11-15 September 2005, Vienna - Austria)

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 seconds after the stop of the magnetic field variation; magnetic relaxation over the surface barrier determines the response in the time scale of minutes., Comment: 16 pages, 7 figures, submited to Physica C

We report experimental results on microwave second harmonic emission in different types of conventional and high-Tc superconductors in the critical state. We show that the peculiarities of the second harmonic signal strongly depend on the type of superconductors.

We show that the nonlinear response near T c of YBCO crystals at microwave frequencies can be explained in the framework of the two-fluid model with the additional hypothesis that the electromagnetic field modulates the partial concentrations of both normal and superconducting fluids.

Experimental data on second-harmonic generation at microwave frequencies in field-cooled ceramic YBa2Cu3O7 and polycrystalline niobium are reported. Evidence is given that the signals are radiated by the component of the induced magnetization oscillating at the second-harmonic frequency of the driving field. It is shown that the experimental data are satisfactorily accounted for if the assumption is made that a rectification process of the microwave field is operated by superconductors in the critical state.

The fluorescence emission by a two-level spin system strongly saturated by a resonant field is investigated at microwave frequencies. The measurements are performed on samples of Al2O3+0.065% of Cr2O3 near the second harmonic (SH) frequency of the driving field. The on-resonance SH spectrum shows the three-peaked structure where the relative distance of the sideband components increases on increasing the field intensity.

The authors report saturation effects at microwave frequencies of a paramagnetic two-level spin system by double-quantum transitions. The saturation factor and the power radiated at the second harmonic frequency are calculated by using the density matrix formalism. It is shown that the saturation factor (by double-quantum transitions) of Cr3+ in Al2O3 is field and frequency dependent and presents singularities at particular values of the field. The saturation factor can be orders of magnitude higher than in a Kramer's doublet.

The a.c. magnetic susceptibility of high-Tc superconductors is markedly nonlinear. In particular, it contains a quadratic component which accounts for the second-harmonic (SH) generation observed in these materials at all the temperatures below Tc. SH spectra of oxide superconductors display several spin-glass features. It is also suggested that SH data can be conveniently used for a characterization of high-Tc superconductors.

The authors have observed that second-harmonic (SH) waves are generated in amorphous SiO2 by the concurrent effects of intense electric and magnetic microwave fields and of a DC magnetic field. The effect is ascribed to the motion of paired electrons between charged defects. They solve the equation of motion of such electrons for terms varying at the SH frequency of the driving fields and show that the SH waves are due to the induced electric polarisation. The concentration of charged defects in amorphous SiO2 can be much increased by straining it in an inert atmosphere.

The second-harmonic (SH) response of zero-field-cooled ceramic YBa2Cu3O7 to a pulsed field has been investigated at microwave frequencies. The SH spectrum comprises a non-resonant low-field signal which is irreversibly suppressed if the field exceeds a threshold value. It is due to the magnetization induced by the microwave magnetic field and oscillations at the SH frequency. The temperature, angular and magnetic field dependence of this signal is reported at different input power levels. The results are explained by supposing that induced currents in the superconductors form loops involving Josephson junctions.

Relaxation of the microwave second order response of YBa2Cu3O7 and Ba0.6K0.4BiO3 crystals in the critical state is studied. The samples are exposed to static and pulsed microwave magnetic fields. The second harmonic signals decay during the time in which the microwave pulse endures. The decay times depend on the supercon-ductor investigated and on the way the value of the static field has been reached., Comment: 2 pages, 1 figure

The nonlinear response of YBa2Cu3O7 samples to intense pulsed microwave fields is discussed. The main results on second-harmonic and third-harmonic emission by ceramic and single crystal samples are reported and the mechanisms which give rise to the nonlinear emission are analyzed.