Search results for "Surface resistance"

Modelling bulk surface resistance by MODIS data and assessment of MOD16A2 evapotranspiration product in an irrigation district of Southern Italy

2016

Abstract In this study, accurate estimates of daily actual evapotranspiration, ET a , were obtained based on the direct Penman–Monteith application, in which the bulk surface resistance term was computed by considering, as main input, daily remotely sensed Land Surface Temperature (LST). In particular, Eddy Covariance measurements of ET a , associated to LST obtained by MODIS time series (MOD11A2) characterized by 8-day resolution, allowed to calibrate a simple bulk surface resistance model, based on two-years of data observations collected in a quite homogeneous irrigation district of Sicily, where olive grove is the main crop. The model was then validated by an independent database collec…

Electromagnetic response of LaO_0.94F_0.06FeAs: AC susceptibility and microwave surface resistance

2009

We discuss on the electromagnetic response of a polycrystalline sample of LaO_0.94F_0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and high-frequency responses have been investigated by measuring the AC susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At low as well as high DC magnetic fields, the susceptibility strongly depends on the amplitude of the AC driving field, highlighting enhanced nonlinear effects. The field dependence of the AC susceptibility exhibits a magnetic hysteresis that can be justified considering the intragrain-field-penetration effects on the intergrain critical current density. The microwave surface resistance exhibits a cloc…

Frequency dependence of the microwave surface resistance of MgB2 by coaxial cavity resonator

2014

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 in Li–Al codoped MgB2 by microwave surface resistance measurements

2010

The magnetic-field-induced variations of the microwave surface resistance, Rs, 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, H0, at fixed temperatures. At low temperatures, we have observed a magnetic hysteresis in the Rs(H0) curves in all the investigated samples. On increasing the temperature, the range of H0 in which the hysteretic behavior is visible shrinks; however, in the sample with x=0.1 it is present up to temperatures close to Tc. We show that the field dependence of Rs can be quantitatively justified taking into account the critical-state effects on th…

Anomalous hysteretic behaviour of the microwave surface resistance in MgB2 superconductor

2007

Effect of boron doping in the microwave surface resistance of neutron irradiated melt-textured Y1.6Ba2.3Cu3.3O7−x samples

2012

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.…

Depinning frequency in a heavily neutron-irradiated MgB2 sample

2008

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.

Microwave response of a cylindrical cavity made of bulk MgB2 superconductor

2008

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…

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

2008

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 ob…

Microwave Surface Resistance and Upper-Critical-Field Anisotropy of MgB2 Superconductor

2002

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 di…