Temperature Dependence Of Surface Resistance Research Articles

Effect of nonlinearity, magnetic and nonmagnetic impurities, and spin-orbit scattering on the nonlocal microwave response of ad-wave superconductor

By using linear response theory the low-temperature microwave response of a nonlocal and nonlinear d-wave superconductor with magnetic and nonmagnetic impurities is calculated. We will show that for the local, linear, and pure sample, penetration depth, Δλ(T), and conductivity, Δσ1(T), vary linearly with temperature, consequently the resistance, ΔR(T), would change linearly with temperature in agreement with experimental results and for the nonlocal, nonlinear sample the linear temperature dependences ΔR(T) change to quadratic function. For impure samples the nonlocality and nonlinearity effects are completely hidden by impurities and the temperature dependences Δλ(T) and Δσ1(T) are determined by temperature interval namely the ranges of T < T* and T* ≪ T ≪ Tc which T* is determined by nonmagnetic impurity concentration and the strength of impurity scattering. For T < T*, ΔR(T) varies as T2, on the other hand for, T* ≪ T ≪ Tc, ΔR(T) varies linearly with temperature. We will also show that the temperature dependence of surface resistance is unaffected by spin-orbit interaction and magnetic impurities.

Microwave characterization of laser ablated Y 1Ba 2Cu 3O 7− x thin films

In the present study we report the measurements of microwave surface resistance ( R s) of YBCO thin films on LaAlO 3 substrate as a function of temperature, thickness and magnetic field by microstrip resonator technique. The T c( R = 0) of the films is 90 K and J c > 10 6 A/cm 2 at 77 K. The microwave surface resistance has been measured for films of various thicknesses. The value of R s has been found to be initially decreased with increasing film thickness due to increase in number of defects. A minimum microwave surface resistance has been obtained for film thickness of about 300 nm. The increase of R s with film thickness above 300 nm is possibly due to degradation of the film microstructure as observed with Atomic Force Microscopy. Temperature dependence of surface resistance has been studied for best quality films. The field induced variations of surface resistance are also investigated by applying dc magnetic field perpendicular to stripline structure and surface of the film. A general linear and square field dependence of R s at low and high value of fields has been observed with critical field value of 0.4 T which confirms the microwave dissipation induced by flux flow in these resonators at 10 GHz frequency. The hysteresis of R s in dc field observed for field value above critical field shows the higher value of surface resistance in decreasing field than in increasing field which is in agreement with one state critical model and is a characteristic of homogeneous superconductors.

Preparation of Tl-2212 and Tl-1223 superconductor thin films and their microwave surface resistance

Epitaxial Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub y/ and Tl(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ thin films have been prepared on CeO/sub 2/ buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the Tl-2212 films showed well connected and smaller sized grains of 1 /spl mu/m or less. In the case of Tl-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm/sup 2/ at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm /spl times/ 10 mm. The temperature dependence of surface resistance of Tl-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the Tl-1223 exhibits slightly higher values resulting from an inferior surface morphology.

Microwave properties of MgB/sub 2/ thin films

The microwave surface impedance Z/sub s/=R/sub s/+j/spl omega//spl mu//sub 0//spl lambda/ of MgB/sub 2/ thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth /spl lambda/ measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to T/sub c/ by the standard BCS integral expression assuming the reduced energy gap /spl Delta/(0)/kT/sub c/ to be as low as 1.0-1.1 assuming /spl lambda/(0)=100--110 nm. These results clearly indicate the s-wave nature of the order parameter. Similar good fits were achieved by an anisotropic one gap and an isotropic two-gap model. Second, the temperature dependence of surface resistance R/sub s/, as measured with a rutile puck, indicates an exponential behavior below about T/sub c//2 with a reduced energy gap being consistent with the one determined from the /spl lambda/ data. The R/sub s/ value at 4.2 K was found to be as low as 19 /spl mu//spl Omega/ at 7.2 GHz, which is comparable with that of a high-quality high temperature thin films of YBa/sub 2/Cu/sub 3/O/sub 7/. A higher-order mode at 17.9 GHz was employed to investigate the frequency f dependence of R/sub s//spl alpha/f/sup n(T)/. Our results revealed an decrease of n with increasing temperature ranging from n=2 below 8 K to n=1 close to T/sub c/. Finally, the microwave power handling of MgB/sub 2/ films was deduced and compared with values for YBa/sub 2/Cu/sub 3/O/sub 7/ films. We found that the power handling of MgB/sub 2/ is comparable or even better than that of YBa/sub 2/Cu/sub 3/O/sub 7/ films for temperature below 30 K.

Surface impedance measurements of Nb/(Cu–Mn) artificial multilayers

Surface impedance measurements of Nb/Cu and Nb/(Cu–Mn) multilayers, produced by a co-sputtering technique, have been performed in the microwave region using an inverted microstrip resonator. The aim of the work is to study the electromagnetic response of layered, proximity coupled superconductors, doped with magnetic impurities, and the role of pair breaking and gaplessness. Various samples with constant thickness layering and different Mn doping have been studied. The temperature dependence of surface resistance and penetration depth has been analysed.

Study of the Linear Microwave Absorption in YBa2Cu3O7—δ Ceramic Samples for Low Level of Incident Power

The temperature dependence (35 K ≤ T < 105 K) of direct microwave absorption is studied as a function of incident microwave power up to 30 mW, for highly compacted ceramic samples of YBa 2 Cu 3 O 7-δ . It is observed that the microwave absorption, under zero dc field, reveals a strong cavity resonance frequency shift as a function of the incident power for temperatures in the superconducting transition region. The absorption presents a linear behavior with the incident microwave power in both normal and superconducting states. From these measurements the temperature dependence of surface resistance is also obtained.

Thickness dependence of electrodynamic properties of TlCaBaCuO thin films

The thickness dependence of electrodynamic properties of Tl2Ca2Ba2Cu3Ox films has been examined after correction for geometrical effects. The surface resistance increases for very thin or thick films as is consistent with the presence of a poorly superconducting layer (phase inclusions or morphology breakdown) as was detected with magnetization measurements. The temperature dependence of surface resistance has been examined at low temperatures and fitted to a multigap model. There was reasonable agreement for moderate thickness films, suggesting why such films have been and will be excellent for applications, while the behavior of other thicknesses may be dominated by grain boundaries.