Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

Physics of Condensed Matter (Impact Factor: 1.35). 04/2008; 63(2):165-177. DOI: 10.1140/epjb/e2008-00231-9
Source: arXiv


We report on the microwave surface resistance of two
polycrystalline Mg11 B2 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.

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    ABSTRACT: We report experimental results of the field-induced variations of the microwave surface resistance in samples of MgB2, produced by different methods. By sweeping the DC magnetic field at increasing and decreasing values, we have detected a magnetic hysteresis that can be ascribed to the different magnetic induction, due to the critical state of the fluxon lattice. The hysteresis observed in the bulk samples has an unusual shape, which cannot be justified in the framework of the critical-state models.
    Journal of Physics Conference Series 10/2007; 97(1). DOI:10.1088/1742-6596/97/1/012207
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    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, omega_0, at different values of the temperature. Although the pinning is not particularly effective, the value of omega_0 obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors.
    Physica C Superconductivity 10/2008; 468(24). DOI:10.1016/j.physc.2008.09.002 · 0.94 Impact Factor
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    ABSTRACT: 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.
    Superconductor Science and Technology 04/2009; 22(5). DOI:10.1088/0953-2048/22/5/055010 · 2.33 Impact Factor
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