Sheng Luo

Microwave Study of

was studied by a sapphire dielectric cavity at 9.315 GHz. The cavity, which has a quality factor of 45000 in room temperature with -mode, is specially designed for the measurement of small samples with the sapphire cylinder having a small hole in the center. Thin film samples with dimension of 1‐2 mm can be put in the middle of the hole, supported by a very thin sapphire rod. The cavity is sealed in a vacuum chamber soaked in the liquid and the temperature of the thin sapphire rod (hence the sample) can be controlled from 1.6 K to 60 K with a stability about 1 mK. Temperature dependence of transmission response and Q-factors were measured by a network analyser (Agilent N5230C). The results showed a clear signature of multi-gap superconductivity. No evidences of existence of node in the energy gap were found as the normalized change in the surface reactance and the corresponding normalized change in the in-plane penetration depth have flat dependence at low temperatures.

{\rm FeSe}_{0.3}{\rm Te}_{0.7}

Double-side superconducting MgB2 films have been grown on double-side polished sapphire substrates by using hybrid physical-chemical vapor deposition method. The zero resistance temperatures TC(0)>37K and the critical current densities JC(5K,0T)>3×106A∕cm2 have been achieved for both sides. In the first 8.73G MgB2 microstrip resonator fabricated by a double-side film, an unloaded Q value of 3400 at 11K was obtained corresponding to surface resistance of 1.2mΩ for the MgB2∕Al2O3 interface. By simply using a sapphire substrate with two polished holes, superconducting connection has been achieved for the double-side MgB2 film. The results show potential applications for the superconducting MgB2 films in microwave and electronic devices.

Thin Film by

Integration of a high-temperature superconducting (HTS) narrowband bandpass filter and a HTS bandstop filter onto a single piece double-sided YBCO thin film with 0.5mm-thick LaAlO3 substrate has been carried out. The 8-pole bandpass filter was designed with a fractional bandwidth about 0.8%. A pair of transmission zeros was implemented, showing typical quasi-elliptical function characteristics. The bandstop filter has a high stopband attenuation (~70dB) to fulfill the special requirement of potential application in communications. Integration of the two filters was successfully achieved.

{\rm TE}_{011}

Results of X-band microwave surface impedance measurements of FeSe1− x Te x very thin film are reported. The effective surface resistance shows appearance of peak at T ≤ Tc when plotted as a function of temperature. The authors suggests that the most well-reasoned explanation can be based on the idea of the changing orientation of the microwave magnetic field at a S–N phase transition near the surface of a very thin film. The magnetic penetration depth exhibits a power-law behavior of δλ L (T) ∝ CT n , with an exponent n ≈ 2.4 at low temperatures, which is noticeably higher than in the published results on FeSe1 − x Te x single crystal. However the temperature dependence of the superfluid conductivity remains very different from the behavior described by the BCS theory. Experimental results are fitted very well by a two-gap model with Δ1/kTc = 0.43 and Δ2/kTc = 1.22, thus supporting s ±-wave symmetry. The rapid increase of the quasiparticle scattering time is obtained from the microwave impedance measurements.

-Mode Sapphire Dielectric Resonator

Abstract Through scaling the dc I–V isotherms of Bi2212 whisker in magnetic field H‖c, the vortex glass–liquid transition is found in Bi2212 whisker whereas it does not appear in Bi2212 crystal slab [R. Behr, J. Kotzler, G. Nakielski, M. Baumann, M. Kaufmann, J. Kowalewski, Physica C 235 (1994) 2669]. The critical exponents are ν=0.5, z=4.0. We found that the critical exponents are influenced by the planar defects of the sample. The result was compared to those of single crystals and films [Behr et al., 1994; H. Yamasaki, K. Endo, S. Kosaka, M. Umeda, S. Yoshida, K. Kajimura, Phys. Rev. B 50 (17) (1994) 12959].

Double-side superconducting MgB2 films for microwave and electronic applications

Abstract Superconducting MgB 2 wires were prepared by exposing boron fiber to magnesium powder in high temperature. Pulse current method has been used to study the V – I characteristics systematically in a large current range. Using a scaling function, all the R – T curves in different field converge into a single universal curve. From the V – I curves, we find a much sharp N–S transition and in high field a reasonable large J c is found.

A COMPACT INTEGRATION OF HIGH-TEMPERATURE SUPERCONDUCTING BANDPASS AND BANDSTOP FILTERS

A modified Ambegaokar-Halperin, thermal-fluctuation model has been developed to describe the c-axis V-I characteristics and low-current ohmic resistance of highly anisotropic superconductors in a magnetic field parallel to the c-axis. The model assumes loss of phase coherence across the CuO-planes associated with the correlated motion of pancake vortices in the liquid state. The predicted V-I characteristics in the current-induced transition from the superconducting to the resistive state are in good agreement with measurements on a 2212-BSCCO single crystal as a function of temperature and field, provided the effect of the interlayer capacitance is taken into account. The measurements are consistent with a flux pancake correlation length within the CuO-planes varying as xi(0)( T/ T-0 - 1)(upsilon), where xi(0) = 1.57 +/- 0.08 mum and upsilon = 0.50 +/- 0.01. Our measurements imply a current-dependent interlayer resistance above and below T-c.

Unusual microwave response and bulk conductivity of very thin FeSe0.3Te0.7 films as a function of temperature

Magnetic relaxation and I?V characteristics of YBCO films with a thickness ranging from 2.4 to 200 nm have been studied. Magnetic relaxation measurements show strong quantum tunnelling effects at temperatures as high as Tcr = 8.5?15 K in ultra-thin films (2.4?40 nm), whereas in 200 nm film it is dominated by thermal activation down to 4.2 K. The high Tcr of ultra-thin films is interpreted using Blatters dissipative tunnelling theory. The strong quantum fluctuations in the ultra-thin films are explained by their large normal state resistivity and higher activation energy compared to thicker films.

The vortex glass to liquid transition in Bi2212 whisker

This present work is aimed at obtaining bulk (i.e. intrinsic) microwave properties R_s(T) and X_s(T) of FeSe_1-xTe_x using our experimental data and thus to obtain temperature dependences of the penetration depth λ_L(T) and whole complex conductivity σ₁(T)-i σ₂(T). The results obtained for our thin epitaxial film and single crystal of the same compound are compared.

Electric transport study of superconducting MgB2 wire

The microwave properties of YBa2Cu3O7-x/LaAlO3 (YBCO/LAO) and YBa2Cu3O7-x/MgO (YBCO/MgO) thin films have been investigated by microstrip resonator technique. The penetration depth λ0(0K) and the microwave surface resistance Rs can be obtained by analyzing the experimental data of the temperature dependence of resonant frequency, unload quality factor of the microstrip resonator. The stress in the two thin films can be obtained from their XRD patterns. The result shows that the stronger stress in YBCO/LAO thing film makes it have larger microwave surface resistance at lower temperature.