Dong-Ho Wu

DC magnetic field dependence of the surface impedance in superconducting parallel plate transmission line resonators

The authors have measured the real and imaginary parts of the surface impedance of cuprate superconducting films in the parallel plate resonator geometry at 11 GHz as a function of perpendicular DC magnetic field. Above a temperature-dependent crossover field, a linear increase of the surface resistance and reactance with field, up to 4 kG, is found. The authors extract the microscopic vortex dynamical parameters-the viscosity and the pinning-potential restoring force constant-along with their temperature dependences, using the low-temperature and low-field limit of the unified model of M.W. Coffey and J.R. Clem (1991). It is found that the pinning frequency is above 40 GHz for temperatures below 60 K. Consequently, a complete understanding of the field dependence of the microwave surface impedance of the cuprates must include both vortex pinning and viscosity. The vortex viscosity is temperature dependent and, in the context of the Bardeen-Stephen model, is consistent with a temperature-dependent normal-state resistivity below T/sub c/.<<ETX>>

Magnetic Penetration Depth Measurements in Cuprate Superconductors

We examine recent results on measurements of the magnetic penetration depth in cuprate superconductors, with particular emphasis on our results obtained with the microstrip and parallel-plate resonator techniques. The results show that the magnetic penetration depth temperature dependence in YBa2Cu3O7−δ is not consistent with a simple scaled weak-coupled BCS temperature dependence. We present three classes of interpretations of these measurements, and how they relate to other experimental studies of the cuprates. Finally, these interpretations are discussed in terms of some theories of cuprate superconductivity.

Scanned perturbation technique for imaging electromagnetic standing wave patterns of microwave cavities

We have developed a method to measure the electric field standing wave distributions in a microwave resonator using a scanned perturbation technique. Fast and reliable solutions to the Helmholtz equation (and to the Schrodinger equation for two-dimensional systems) with arbitrarily shaped boundaries are obtained. We use a pin perturbation to image primarily the microwave electric field amplitude, and we demonstrate the ability to image broken time-reversal symmetry standing wave patterns produced with a magnetized ferrite in the cavity. The whole cavity, including areas very close to the walls, can be imaged using this technique with high spatial resolution over a broad range of frequencies.

Oxidation and reduction during fabrication of high quality Nd/sub 1.85/Ce/sub 0.15/CuO/sub 4-y/ superconducting thin films

Using pulsed-laser deposition and N/sub 2/O reactive gas, the authors have fabricated very high quality c-axis n-type Nd/sub 1.85/Ce/sub 0.15/CuO/sub 4-y/ (NCCO) oxide epitaxial superconducting thin films on different substrates. The film shows a superconducting transition temperature T/sub c/ (R=0 Omega ) of 22.4 K and a transition width of 0.2 K from AC susceptibility measurements. The critical current density J/sub c/ is 8*10/sup 5/ A/cm/sup 2/ at 4.2 K in zero magnetic field. The microwave surface resistance measured at 9.6 GHz shows a value of 3 m Omega at 4.2 K in 500-nm-thick NCCO film, the best result reported so far for NCCO thin films. The oxygen deficiency is necessary to achieve the superconductivity in NCCO, and the oxygen reduction during and after film deposition is critical. The oxidation and reduction processes are studied systematically for various substrate temperatures, atmospheres, and annealing durations.<<ETX>>

Pairing symmetry and electrodynamics of superconducting YBa2Cu3O7−δ, Nd1.85Ce0.15Ce0.15CuO4, and Nb

We experimentally investigate the pairing symmetry and electrodynamics of YBa2Cu3O7−δ (YBCO), Nd1.852Ce0.152CuO4 (NCCO), and Nb by examining the temperature dependence of the penetration depthλ(T) and surface resistanceRs(T) in a comparative manner. Using the measuredλ(T) andRs(T), we extract the complex conductivityσ=σ1−iσ2 for each sample, and the quasiparticle scattering timeΤ(T) for theab-plane andc-axis in YBCO. While NCCO and Nb show a strong resemblance in their electrodynamic properties, the electrodynamic properties of YBCO are very distinctive from the others. The results suggest that NCCO may have a BCSs-wave-like pairing state, while YBCO possibly has an unconventional pairing state. We compare the results on YBCO with thed-wave pairing scenario, as well as with other possible theoretical models.

Measurements of the Frequency Dependent Microwave Fluctuation Conductivity of Cuprate Thin Film Superconductors

We describe our measurements of the fluctuation lifetime in high-Tc superconducting thin films above Tc. The paper is laid out as follows. Section 1 discusses our our motivation and broadband experimental technique, while sections 2 and 3 present the experimental method in detail. Section 4 is a brief discussion of the electromagnetic fields used in our experiment. Section 5 presents and discusses the data on fluctuation conductivity, and summarizes our observations regarding the nature of fluctuations above Tc in the cuprate superconductors. Finally, section 6 serves as a conclusion.

Superconducting Nd/sub 1.85/Ce/sub 0.15/CuO/sub 4-y/ thin films and heterostructures on sapphire

Superconducting Nd/sub 1.85/Ce/sub 0.15/CuO/sub 4-y/ (NCCO) thin films have been made on yttria-stabilized zirconia (YSZ) buffered sapphire. The films are epitaxially grown and highly in-plane oriented. X-ray diffraction shows the c-axis of the film normal to the surface of the substrate. The width of rocking curve is 0.2/spl deg/ and the RBS channeling yield is 9%, indicating high crystallinity of the film. Cross-sectional transmission electron microscopy images reveal a sharp interface between NCCO and YSZ. The microwave surface resistance of NCCO films on YSZ buffered sapphire at 9.6 GHz was measured and a value of 80 /spl mu//spl Omega/ (at 4.2 K in zero DC magnetic field) was obtained, which is comparable to Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) films at the same reduced temperature. A trilayer structure of YBCO/SrTiO/sub 3//NCCO on YSZ buffered sapphire has been fabricated with all layers oriented, in which both the YBCO and NCCO layers are superconducting.<<ETX>>

Temperature dependence of the surface impedance of Nd1.85Ce0.15CuO4−y and YBa2Cu3O7−δ

Abstract We have measured the surface impedance of YBa 2 Cu 3 O 7−δ (YBCO) single crystals, Nd 1.85 Ce 0.15 CuO 4−y (NCCO) thin films and single crystals, and Nb single crystals at 9.6 GHz using a superconducting cavity perturbation technique. Our results on Nb are in good agreement with s-wave BCS theory. The magnetic penetration depth in the ab-plane in YBCO single crystals exhibits a linear temperature dependence at low temperatures, and is consistent with clean 2D d-wave superconductivity. Both thin films and single crystals of the electron-doped cuprate NCCO show detailed agreement with the predictions of single-gap isotropic s-wave BCS electrodynamics over the measured temperature range up to T c . 1–3