H. Piel

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique

We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceRsof 1×1 cm2 superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE01δ oscillation mode the highly reproducible quality factor of about 105 results in a sensitivity of ±50μΩ forRsmeasurements. The temperature dependence ofRscan be measured up to values as high as 1 Ω. We have investigated several YBa2Cu3O7 thin films prepared by high oxygen pressure d.c. sputtering on LaAlO3 and NdGaO3. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivityρ(T) withρ(300K)/ρ(100K) values of about 3.7. Those films show, besides the initial fall-off just belowTc, a further strong decrease ofRsat low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForT≪Tc/2 these films exhibit an exp (−αTc/T) dependence ofRswithα-values around 0.4. These observations may be explained by a superconducting energy gap with 2Δ/kTc≈0.8 for charge carriers localized in the CuO chains for YBa2Cu3O7.

Survey of microwave surface impedance data of high-T c superconductors—Evidence for nonpairing charge carriers

The microwave surface impedance of the high-Tc oxide superconductors has been measured at many laboratories around the world. A survey of their data between 100 MHz and 150 GHz for polycrystalline as well as single crystalline samples is given, focusing on YBa2Cu3O7−δ. In comparison to the classical superconductors, these results reveal a very similar temperature dependence of the surface impedance close toTc but an anomalous high residual surface resistance at lower temperatures. Both features can be explained by the assumption that oxide superconductors contain a significant number of nonpairing charge carriers. Within the framework of a properly extended two-fluid model, this is shown by analysis of our best thin-film data. Moreover, the enhanced losses in polycrystalline material especially for superposed magnetostatic fields result, to a large extent, from the deeper penetration depth. The possible origin of the nonpairing charge carriers and their impact on the applicability of the oxide superconductors is briefly discussed.

Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy

Ya. G. Ponomarev, S. A. Kuzmichev, M. G. Mikheev, M. V. Sudakova, S. N. Tchesnokov, N. Z. Timergaleev, A. V. Yarigin, M. V. Lomonosov Moscow State University, Department of Physics, 119899 Moscow, Russia, E. G. Maksimov, S. I. Krasnosvobodtsev and A. V. Varlashkin, P. N. Lebedev Physics Institute, RAS, Moscow, Russia, M. A. Hein, G. Müller, H. Piel, Bergische Universität Wuppertal, Fachbereich Physik, D-42097 Wuppertal, Germany, L. G. Sevastyanova, O. V. Kravchenko, K. P. Burdina, B. M. Bulychev, M. V. Lomonosov Moscow State University, Department of Chemistry, 119899 Moscow, Russia.

Evidence for strong electron-phonon interaction from inelastic tunneling of cooper pairs in c-direction in bi2sr2cacu2o8 break junctions

A reproducible fine structure at subgap voltages in the I(U)-characteristics of Bi2Sr2CaCu2O8 break junctions has been observed and investigated. The structure is detectable only in the presence of an a.c. Josephson current. The position of the dips, composing the structure in the dI/dU-characteristics, is independent of the gap parameter Δ, the temperature T and the geometry of the contacts. The overall form of the fine structure is in good agreement with the Raman scattering spectra of the phonon modes in this material. We attribute this structure to an inelastic (phonon assisted) tunneling of Cooper pairs, which is accompanied by the emission of coherent Raman-active optical phonons at resonance voltages Ures=ℏωphon/2e. These results hint for strong electron–phonon interaction in this material.

Surface resistance and nonlinear dynamic microwave losses of epitaxial HTS films

Many applications of high-temperature superconductor (HTS) films in microwave devices require high power-handling capabilities. Therefore we have studied the temperature and power dependence of the surface resistance R/sub s/(T,B/sub s/) of unpatterned HTS films with a very sensitive dielectric resonator technique at 19GHz. R/sub s/-values as low as 120/spl plusmn/15 /spl mu//spl Omega/ (100/spl plusmn/15 /spl mu//spl Omega/) and 760/spl plusmn/80 /spl mu//spl Omega/ (610/spl plusmn/30 /spl mu//spl Omega/) at 4.2 and 77K were obtained for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (Tl/sub 2/Ba/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8/) films. In contrast to the small scattering in R/sub s/ at low fields, the high-field behavior varied strongly. Maximum surface magnetic fields (B/sub s/) between 1 and 15 mT (2-28 mT) were achieved at 77K (4.2K). Different loss mechanisms, like defects, heating, and intrinsic phenomena, could be identified. Besides time-domain measurements in the pulsed mode, the dynamic nature of the nonlinearities was investigated by two-tone intermodulation measurements. The very high sensitivity of this technique was applied to proof the existence of fast nonlinearities (t<100 ps) in high quality films.

Two-gap structure in Yb(Y)Ba2Cu3O7−x single crystals

Tunneling effects in YbBa2Cu3O7−x (YbBCO) and YbBa2Cu3O7−x (YBCO) single crystals have been investigated using an improved technique of break junction preparation. A two-gap structure with BCS-like density of states has been observed. The value of the “large” gap is in the range of 24–28 meV for YBCO (Tc=85–90 K) and 29–30 meV for YbBCO (Tc=87–90 K). A variation of the value of the “small” gap between 0.5 and 11 meV has been detected, even for the same crystal. As a possible explanation, two-band superconductivity, surface properties, or strong gap anisotropy will be discussed.

Conditions for and limitations of high-power handling capabilities of planar YBa2Cu3O7-x filters

The conditions for the design of compact high-power HTS filters are deduced from basic considerations, and related to the microwave properties of the HTS films and the substrates. Potential mechanisms limiting the linear microwave response of HTS films are analyzed systematically. Magnetic and thermal field breakdowns are found to confine the power handling capability to comparable field levels. While magnetic limitation is related to flux penetration at the lower critical field Bc1, thermally induced quenches occur dependent on the presence of defects and on the thermal conductivity of the substrate. The detrimental role of film inhomogeneities for the linear microwave response is confirmed experimentally for microwave fields at 19 GHz and for dc fields at 87 GHz.

Josephson effect and single-particle tunneling in YBa2Cu3O7−x and YbBa2Cu3O7−x single-crystal break junctions

Abstract We have studied the current-voltage characteristics of break junctions in single crystals of YBa 2 Cu 3 O 7− x and YbBa 2 Cu 3 O 7− x at helium temperature. For a YBa 2 Cu 3 O 7− x Josephson junction Fiske resonances in low magnetic fields have been observed in the I ( V ) characteristics, thus proving the tunneling nature of the contact. The reduced velocity of the electromagnetic wave in the contact area has been calculated to be c / c 0 ∼ 0.018 ( c is the Swihart velocity). The ratio of the barrier thickness d 1 to the relative dielectric constant of the barrier ϵ was found to be d I / ϵ ∼ 0.09 nm. In the quasiparticle tunneling regime, the YbBa 2 Cu 3 O 7− x break junctions showed in some cases I ( V ) characteristics with a relatively small excess current at “sub-gap” voltages and a steep rise of the current at the “gap” voltage V g = 2 Δ / e . The “BCS-like” form of the observed I ( V ) characteristics seems to be in favor of s wave superconductivity.

Electromagnetic properties of electrophoretic YBa2Cu3O7?? films

In order to apply highTc superconductivity to general microwave cavities and to shielding devices for quasistationary magnetic fields, we have developed an electrophoretic coating technique. This work reports about the continuation of our experiments. More than 60 samples of electrophoretic layers deposited on silver substrates were fabricated by systematically varying the processing parameters. The r.f. surface resistance measured at 21.5 GHz and 77 K at low excitation fields is found to be a sensitive measure of the quality of the samples and falls rapidly with increasing average grain size. Textured electrophoretic layers of large grain size show the smallest sensitivity ofRs against the r.f. surface magnetic fieldHs. The magnetic sensitivitydRs/dHs is found to be in direct proportion toRs (77 K). After optimizing the sintering procedure the electrodes of the prototype cavity for a compact hydrogen maser were coated. The cavity was successfully operated at 1.42 GHz, and a surface resistance of 1 mΩ was achieved at 77 K. This compares to 4 mΩ for copper at the same temperature.

A Superconducting Nb3Sn Coated Multicell Accelerating Cavity

We report on the first results, obtained with a five cell Nb3Sn coated accelerating structure of 3 GHz. A uniform layer of Nb3Sn was formed by processing a niobium structure in a tin atmosphere at 1170°C. At 4.2 K a resonator Q of 7·109 was measured. This corresponds to a residual surface resistance of 27 n¿ which is the lowest ever achieved with a Nb3Sn cavity in the GHz range. The rf losses of this resonator are by a factor of 55 lower than those of an equivalent niobium cavity at 4.2 K. The maximum accelerating field at this temperature was 4.0 MV/m. It is similar to the quench field of the niobium structure before coating. The spatial distribution of the rf losses was measured at different field levels using the temperature mapping technique. We describe the characteristic features of the Nb3Sn layer, we report on residual losses specific to Nb3Sn coated niobium structures and we discuss the observed field limitations.