Jian Mao

Deposition and reduction of Nd1.85Ce0.15CuO4−y superconducting thin films

Superconducting thin films of Nd1.85Ce0.15CuO4−y (NCCO) were grown at 720–820 °C by pulsed laser deposition in N2O atmosphere. The reduction subsequent to the deposition was found to be critical, and was studied systematically for various temperature, atmosphere, and duration. An ac susceptibility technique was used to stringently characterize the film quality. Very high quality films were made with optimized reduction conditions which showed a Tc(R=0) of 22.4 K, with a transition width of 0.2 K and a Jc (4.2 K) of 8×105/cm2 at zero field. Microwave surface resistance Rs was measured at 9.6 GHz and a value of ∼3 mΩ was obtained at 4.2 K in a 5000 A thick NCCO film.

Structural characterization and microwave loss of Nd1.85Ce0.15CuO4−y superconducting thin films on yttria‐stabilized zirconia buffered sapphire

We have grown superconducting Nd1.85Ce0.15CuO4−y (NCCO) thin films on (1102) sapphire using a yttria‐stabilized zirconia (YSZ) buffer layer, which has been demonstrated to be the best material for the growth of n‐type superconducting NCCO thin films. The films are c‐axis oriented, epitaxially grown with a small mosaic spread of 0.2° and a Rutherford backscattering spectroscopy channeling yield of ∼9%. 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 is only 80 μΩ at 4.2 K in zero magnetic field, which is comparable to Y1Ba2Cu3O7−y films at similar reduced temperature, as a consequence of the decrease of structural imperfection in the film. The temperature dependence of the surface resistance and magnetic penetration depth in these films further confirms the s‐wave BCS nature of NCCO.

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>>

Consequences of d-wave superconductivity for high frequency applications of cuprate superconductors

A number of recent experiments suggest that the superconducting ground state wave function in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) has d-wave symmetry. However little attention has been given to the consequences of d-wave pairing symmetry on applications of the cuprates. An intrinsic finite residual loss, approximately one order of magnitude below the lowest losses measured in YBCO thin films at 10 GHz, is one important consequence. In addition, an unusual sensitivity to disorder, an intrinsic non-linear power dependence, and unique mid-gap states associated with specific film textures and possibly twin boundaries, are also expected. We present our own attempts to identify these unique properties of d-wave superconductors, and discuss how these results may dictate the ultimate limitations of the cuprates in high frequency applications.<<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.

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

Surface impedance measurements of cuprate superconductors: Nd1.85.85Ce0.15CuO4−δ—A case study

We present results on the temperature dependence of the surface impedance of high-quality thin films and single crystals of the electron-doped Nd1.85Ce0.15CuO4−δ cuprate superconductor. Surprisingly, these measurements are consistent with the behavior of ans-wave BCS superconductor. We also briefly review some highlights of surface impedance measurements on hole-doped cuprate superconductors, and contrast those with the results on Nd,1.85Ce0.15CuO4−δ.

Electrodynamics of oxide superconductors

We present a brief review of results on the surface impedance of cuprate superconductors, focusing mainly on YBa2Cu3O7-(delta ) (YBCO) and evidence of d-wave superconductivity in that material. We then discuss our recent results on Ba-K-Bi-O thin films, and the effects of DC electric fields on the surface impedance of YBCO films. A summary of our data on high quality thin films and single crystals of the electron-doped Nd1.85Ce0.15CuO4-(delta ) (NCCO) cuprate superconductor follows. Surprisingly, the measurements on NCCO are consistent with the behavior of an s-wave BCS superconductor, in striking contrast to recent results on YBCO. Finally we discuss some of the interesting potential implications of d-wave superconductivity for microwave applications of the cuprates.