0000000000172801
AUTHOR
Giovanni Giunchi
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 the purity and morphology of …
Microwave harmonic emission in MgB2 superconductor: Comparison with YBA2CU3O7
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.
Near-Tc second-harmonic emission in high-density bulk MgB2 at microwave frequency
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 densit…
Microwave response of bulk MgB2 samples of different granularity
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.
Superconducting Microwave Cavity Made of Bulk MgB2
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.
Surface-barrier effects in the microwave second-harmonic response of superconductors in the mixed state
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 …
Frequency dependence of the microwave surface resistance of MgB2 by coaxial cavity resonator
Abstract 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, R s , of the MgB2 material, at fixed frequencies, and the frequency dependence of R s , at fixed temperatures. Our results show that the R s ( f ) curves follow a f n law, where n decreases on increasing the temperature, starting from n ≈ 2 , at T = 4.2 K , d…
Fluxon dynamics by microwave surface resistance measurements in MgB2
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.
Microwave Response of Coaxial Cavities Made of Bulk Magnesium Diboride
We report on the microwave (mw) properties of coaxial cavities built by using bulk MgB2 superconductor prepared by reactive liquid Mg infiltration technology. We have assembled a homogeneous cavity, by using an outer MgB2 cylinder and an inner MgB2 rod, and a hybrid cavity by using an outer copper cylinder and the same MgB2 rod as inner conductor. By the analysis of the resonance curves, in the different resonant modes, we have determined the microwave surface resistance, Rs, of the MgB2 materials as a function of the temperature and the frequency, in the absence of DC magnetic fields. At T = 4.2 K and f = 2.5 GHz, by a mw pulsed technique, we have determined the quality factor of the homog…
Microwave response of a cylindrical cavity made of bulk MgB2 superconductor
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 whic…