Search

Studying the advanced dielectric resonator techniques, the microwave surface impedance has been determined for c-axis oriented MgB2 thin film is discussed. The residual surface resistance represents an important figure-of-merit for progress in MgB2 thin film preparation is concluded.

The equation describing the response of high-Tc superconducting thin film under microwave current irf is solved using a perturbation method. This assumes that the film consists of a network of superconducting grains connected by grain-boundary weak links. The derived analytic expressions for film's microwave surface impedance show its dependence on a rf magnetic field in low and intermediated field limit.

Abstract: We present here the temperature dependence of the real part of the microwave conductivity of MgB2 thin film measured at 7.2GHz. A pronounced peak occurs at a temperature around 0.6 times the critical temperature, same position as it did at 17.9GHz [Jin et. al., Phys. Rev. Lett. 91 (2003) 127006]. This frequency independence of the peak position confirms our previous conclusion, i.e., the peak comes from the coherence peak from the π band contribution. Some discussions are given to the measurement and extraction procedures on the microwave conductivity. We conclude that it is crucial to measure the real and imaginary parts of the surface impedance simultaneously at the same frequency to get an accurate temperature dependence of the microwave conductivity through the superconducting transition. [Copyright &y& Elsevier]

The microwave surface impedance Z[sub s] = R[sub s] + jωμ[sub 0]λ of MgB[sub 2] thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth A 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 Δ(0)/kT[sub c] to be as low as 1.0-1.1 assuming λ(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 futile puck, indicates an exponential behavior below about T[sub c]/2 with a reduced energy gap being consistent with the one determined from the λ data. The Rs value at 4.2 K was found to be as low as 19 μΩ 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]αf[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. [ABSTRACT FROM AUTHOR]

Using a dielectric resonator technique the magnetic field penetration depth λ and the energy gap Δ of two MgB2 thin films were determined by measuring changes of resonance frequency with temperature. The exponential temperature dependence of λ is revealed at low temperatures (T, Tc is the critical temperature of the thin film). It indicates that a finite energy gap exists for thermal excitation of quasiparticles. From our measurements values of Δ/kTc=1.25 and λ0=290 nm for sample A and Δ/kTc=1.20 and λ0=160 nm for sample B give the best fit to the experimental results. The corresponding values for the energy gap Δ are 3.4 meV for sample A and 3.8 meV for sample B. In the high temperature region (T>Tc/2) the temperature dependence of λ can be well fitted by λ0/[1−(T/Tc)2]1/2. Our results strongly support the absence of nodes in the gap and a possible multigap scenario. [Copyright &y& Elsevier]

The coherence peak is observed in the temperature dependence of the real part of the microwave conductivity (σ1) of c-axis oriented MgB2 thin film, which is extracted from microwave surface impedance (Zs) measurements using a sapphire resonator at 17.9 GHz. The peak appears at a temperature around 0.6 times the critical temperature (Tc), which is greatly shifted downwards in temperature compared with conventional superconductors for which the peak occurs around 0.9Tc. Calculations based on the two-band model suggest that this peak corresponds to the smaller of the two energy gaps. [Copyright &y& Elsevier]