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Fluxon dynamics by microwave surface resistance measurements in MgB2  [PDF]
A. Agliolo Gallitto,M. Bonura,S. Fricano,M. Li Vigni,G. Giunchi
Physics , 2003, DOI: 10.1016/j.physc.2003.09.103
Abstract: 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 Power, DC Magnetic Field, Frequency and Temperature Dependence of the Surface Resistance of MgB2  [PDF]
A. A. Zhukov,A. Purnell,Y. Miyoshi,Y. Bugoslavsky,Z. Lockman,A. Berenov,J. L. MacManus-Driscoll,L. F. Cohen,H. Y. Zhai,H. M. Christen,M. P. Paranthaman,D. H. Lowndes,M. H. Jo,M. G. Blamire,Ling Hao,J. C. Gallop
Physics , 2001,
Abstract: 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.
Microwave Surface Resistance and Upper-Critical-Field Anisotropy of MgB2 Superconductor  [PDF]
A. Agliolo Gallitto,S. Fricano,M. Li Vigni
Physics , 2002, DOI: 10.1016/S0921-4534(02)02281-5
Abstract: 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.
Energy gap, penetration depth and surface resistance of MgB2 thin films determined by microwave resonator mesurements  [PDF]
B. B. Jin,N. Klein,W. N. Kang,Hyeong-Jin Kim,Eun-Mi Choi,Sung-Ik Lee
Physics , 2001, DOI: 10.1103/PhysRevB.66.104521
Abstract: We have measured the temperature dependence of the microwave surface impedance Zs = Rs + i\Omega\mu_0 \Lamda of two c-axis oriented MgB2 films at a frequency \Omega /(2\pi) of 17.9 GHz employing a dielectric (sapphire) resonator technique. The temperature dependence of the magnetic field penetration depth \Lamda can be well fitted from 5 K close to Tc by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kTc to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 110 nm and 115 nm were determined from the fit. Our results clearly indicates the s-wave character of the order parameter. The temperature dependence of the surface resistance Rs below Tc/2 is consistent with the low value of the energy gap. The surface resistance below 8 K was found to be below the resolution limit of 100 microOhm of our measurement technique.
Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field  [PDF]
M. Bonura,A. Agliolo Gallitto,M. Li Vigni,C. Ferdeghini,C. Tarantini
Physics , 2007, DOI: 10.1140/epjb/e2008-00231-9
Abstract: 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.
Fluxon dynamics in Li-Al codoped MgB2 by microwave surface resistance measurements  [PDF]
M. Bonura,A. Agliolo Gallitto,M. Li Vigni,M. Monni
Physics , 2009, DOI: 10.1016/j.physc.2010.02.029
Abstract: 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.
Perspectives of superconducting MgB2 for microwave applications  [PDF]
Matthias A. Hein
Physics , 2002,
Abstract: We discuss the temperature, frequency, and power-dependent surface resistance of the boride superconductor MgB2 in relation to possible applications for passive microwave devices. The data available in the literature are compared with results for polycrystalline Nb3Sn and epitaxial YBa2Cu3O7-x, which are representative of the classical and cuprate superconductors. MgB2 displays all specific features that make superconductors attractive for high-performance devices, even though the fabrication technology is not yet mature. We attempt to identify promising areas of applications, as well as material requirements, which could further promote the attractiveness of the new superconductor in this field.
Microwave performance of high-density bulk MgB2  [PDF]
A. T. Findikoglu,A. Serquis,L. Civale,X. Z. Liao,Y. T. Zhu,M. E. Hawley,F. M. Mueller,V. F. Nesterenko,Y. Gu
Physics , 2003, DOI: 10.1063/1.1590739
Abstract: We have performed microwave measurements on superconducting hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave surface resistance decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 nm to 110 nm through surface-polishing and ion-milling. The surface resistance was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low surface resistance (~0.8 mOhms at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.
Microwave Residual Surface Resistance of Superconductors  [PDF]
Mario Rabinowitz
Physics , 2003,
Abstract: Two distinct models account for the microwave residual surface resistance of superconducting cavities with equally good agreement with the measured temperature and frequency dependence. In presenting his phonon-generation model, Passow claimed that Rabinowitz' fluxoid power-loss model of residual resistance does not fit the experimental data, whereas his does. In fact, the two models have essentially the same temperature and frequency dependence. Furthermore, Passow's phonon-generation model cannot explain the observed sensitivity to details of sample preparation and history, while the fluxoid model can.
Microwave Response of MgB2/Al2O3 Superconducting Thin Films by Microstrip Resonator Technique
Microwave Response of MgB2/A12O3 Superconducting Thin Films by Microstrip Resonator Technique

SHI Li-Bin,ZHENG Yan,REN Jun-Yuan,LI Ming-Biao,ZHANG Feng-Yun,LI Bo-Xin,DONG Hai-Kuan,
史力斌
,郑岩,任骏原,李明标,张凤云,李波欣,董海宽

中国物理快报 , 2007,
Abstract: Double-sided superconducting MgB2 thin films are deposited onto c-A120a substrates by the hybrid physical chemical vapour deposition method. The microwave response of MgB2/A12O3 is investigated by microstrip resonator technique. A grain-size model is introduced to the theory of microstrip resonators to analyse microwave properties of the films. We obtain effective penetration depth of the films at OK (λe0 = 463nm) and surface resistance (R8 = 1.52 mΩ at 11 K and 8. 73 GHz) by analysing the resonant frequency and unload quality factor of the microstrip resonator, which suggests that the impurities and disorders of grain boundaries of MgB2/A12 Oa result in increasing penetration depth and surface resistance of the films.
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