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Register a new userMicrowave Power, DC Magnetic Field, Frequency and Temperature Dependence of the Surface Resistance of MgB2
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The microwave power, dc magnetic field, frequency and temperature dependence of the surface resistance of MgB2 films and powder samples were studied. Sample quality is relatively easy to identify by a number of characteristics, the most clear being the breakdown in the omega squared law for poor quality samples. Analysis of the experimental data suggests the most attractive procedure for high quality film growth for technical applications.
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We report on the microwave surface resistance of two polycrystalline Mg11B2 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.
The magnetic-field-induced variations of the microwave surface resistance, R_s, have been investigated in ceramic Mg_{1-x}(LiAl)_xB_2, with x in the range 0.1 - 0.4. The measurements have been performed on increasing and decreasing the DC magnetic field, H_0, at fixed temperatures. At low temperatures, we have observed a magnetic hysteresis in the R_s(H_0) curves in all the investigated samples. On increasing the temperature, the range of H_0 in which the hysteretic behavior is visible shrinks; however, in the sample with x = 0.1 it is present up to temperatures close to T_c. We show that the field dependence of R_s can be quantitatively justified taking into account the critical-state effects on the fluxon lattice only in the sample with x = 0.4. On the contrary, in the samples with x < 0.4 the hysteresis exhibits an unusual shape, similar to that observed in others two-gap MgB_2 samples, which cannot be justified in the framework of the critical-state models.
We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa$_2$Cu$_3$O$_{7-delta}$ films on SrTiO$_3$ substrates. BaZrO$_3$ crystallites were included in the films using composite targets containing BaZrO$_3$ inclusions with mean grain size smaller than 1 $mu$m. X-ray diffraction showed single epitaxial relationship between BaZrO$_3$ and YBa$_2$Cu$_3$O$_{7-delta}$. The effective surface resistance was measured at 47.7 GHz for 60$< T <$90 K and 0$< mu_0H <$0.8 T. The magnetic field had a very different effect on pristine YBa$_2$Cu$_3$O$_{7-delta}$ and YBa$_2$Cu$_3$O$_{7-delta}$/BaZrO$_3$, while for $mu_0H=$0 only a reduction of $T_c$ in the YBa$_2$Cu$_3$O$_{7-delta}$/BaZrO$_3$ film was observed, consistent with dc measurements. At low enough $T$, in moderate fields YBa$_2$Cu$_3$O$_{7-delta}$/BaZrO$_3$ exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO$_3$ inclusions determine a strong reduction of the field-dependent surface resistance. From the analysis of the data in the framework of simple models for the microwave surface impedance in the mixed state we argue that BaZrO$_3$ inclusions determine very steep pinning potentials.
We report on the magnetic-field-induced variations of the microwave surface resistance, R_s, in a polycrystalline MgB_2 sample, at different values of temperature. We have detected a magnetic hysteresis in R_s, which exhibits an unexpected plateau on decreasing the DC magnetic field below a certain value. In particular, at temperatures near T_c the hysteresis manifests itself only through the presence of the plateau. Although we do not quantitatively justify the anomalous shape of the magnetic hysteresis, we show that the results obtained in the reversible region of the R_s(H) curve can be quite well accounted for by supposing that, in this range of magnetic field, the pi-gap is almost suppressed by the applied field and, consequently, all the pi-band charge carriers are quasiparticles. On this hypothesis, we have calculated R_s(H) supposing that fluxons assume a conventional (single core) structure and the flux dynamics can be described in the framework of conventional models. From the fitting of the experimental results, we determine the values of H_{c2}^pi(T) at temperatures near T_c. In our opinion, the most important result of our investigation is that, at least at temperatures near T_c, the value of the applied field that separates the reversible and irreversible regions of the R_s(H) curves is just H_{c2}^pi(T); a qualitative discussion of the possible reason of this finding is given.
We study the temperature dependence of the resistivity as a function of magnetic field in superconducting transition (Tconset - TcR=0) region for different Bi2Sr2CaCu2O8+{delta} superconducting samples being synthesized using sol-gel method. The superconducting transition temperature (TcR=0) of the studied samples is increased from 32 K to 82K by simply increasing the final sintering temperature with an improved grains morphology. On the other hand, broadening of transition is increased substantially with decrease in sintering temperature; this is because Tconset is not affected much with grains morphology. Further broadening of the superconducting transition is seen under magnetic field, which is being explained on the basis of thermally activated flux flow (TAFF) below superconducting transition temperature (Tc). TAFF activation energy (U0) is calculated using the resistive broadening of samples in the presence of magnetic field. Temperature dependence of TAFF activation energy revealed linear temperature dependence for all the samples. Further, magnetic field dependence is found to obey power law for all the samples and the negative exponent is increased with increase in sintering temperature or the improved grains morphology for different Bi-2212 samples. We believe that the sintering temperature and the ensuing role of grain morphology is yet a key issue to be addressed in case of cuprate superconductors.
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