G. Muller

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.

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.

Fundamental limits of the linear microwave power response of epitaxial Y-Ba-Cu-O films

The microwave field dependent surface resistance R/sub s/(B/sub hf/) of unpatterned epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ films was measured at 19 GHz between 4.2 and 80 K with a Nb-shielded sapphire resonator. In addition, the dc field dependence R/sub s/(B/sub dc/) was investigated at 87 GHz and 4.2 K with a Cu host cavity. Maximum microwave field amplitudes B/sub cr//sup hf//spl ges/27, 23 and 15 mT were obtained at 4.2, 50 and 77 K with no detectable degradation of R/sub s/ at lower fields. Similarly, R/sub s/ stayed constant at 4.2 K up to maximum dc fields, B/sub cr//sup dc//spl les/40 mT. There is increasing evidence from both data sets that the maximum fields B/sub cr//sup hf/ and B/sub cr//sup dc/ are fundamentally limited by the lower critical field B/sub c1/(/spl lambda//sub L/) at a given, quality-dependent, penetration depth /spl lambda//sub L/. The R/sub s/(B/sub hf/) performance obtained for the best YBa/sub 2/Cu/sub 3/O/sub 7/ films comes already close to this limit. Microwave field limitation in less optimized films is expected to be caused by local heating at non-superconducting defects.

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.

Large-Area YBa2Cu3O7 − x Films on Sapphire with Excellent Microwave Power Handling Capability

Crack-free thick YBa2Cu3O7 − x films are prepared on CeO2 buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces (“peak-to-valley” roughness <10 nm), high critical currents (JC > 2 MA/cm2 at 77 K and 0 T), and low microwave surface resistances (Rs(77K) ≍.4mω and Rs(4.2K) ≍.110μω at 19.15GHz) comparable to the best values reported in the literature for YBCO films on structurally better matched substrates. These thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (BHF) up to 54, 37, and 17.4 mT at 4.2, 50, and 77 K, respectively, which for the lower temperatures were limited by the available power of the 25-W HF amplifier. The high-power performance, which to our knowledge belongs to the best reported so far for unpatterned YBa2Cu3O7 − x films, was achieved without any degradation of the samples despite frequent thermal cycling.

Quasiparticle tunneling in the c-direction in stacks of Bi2Sr2CaCu2O8+δ S–I–S junctions and the symmetry of the superconducting order parameter

Abstract A clear gap structure has been observed in the current–voltage characteristics (CVC) of stacks of S–I–S junctions obtained in Bi 2 Sr 2 CaCu 2 O 8+ δ (BSCCO) crystals using a break junction technique. Due to the high resistance of the stacks, the range of bias voltages could be extended well above the gap voltage V gn without significant overheating of the contacts. For samples with a critical temperature T c =84–89 K and 2 Δ / kT c =6.7±0.3, the value of V gn corresponding to a steep rise of the quasiparticle current I qp is well-described by the relation V gn = n (2 Δ / e ) with a gap parameter Δ (4.2 K)=(25±1) meV. The presence of a sharp gap feature in characteristics of stacks points to an essential contribution of s-wave pairing to the symmetry of the order parameter in BSCCO.

Electrophoretic deposition of highly textured YBa2Cu3O7−x thick films

Abstract We have developed an electrophoretic technique for the deposition of polycrystalline YBa 2 Cu 3 O 7−x layers of several tens of microns in thickness on arbitrary shaped silver substrates. Applying a magnetic field H of 8 T to the suspension, a high degree of c -axis texturing parallel to H and perpendicular to the surface has been achieved for such layers as shown by x-ray diffraction. After sintering for about 140 h at 920°C in pure oxygen, the textured thick films provide much better superconducting properties than the untextured ones and than bulk samples. Magnetic shielding measurements yield narrower transition curves and higher T c values due to improved intergrain contacts. The surface resistance R s has been measured on coated silver discs of 24 mm diameter in a copper host cavity at 22 GHz between 4.2 K and 300 K. For the best sample, R s (100 K) is 0.33 Ω which corresponds to a resistivity of 130 μΩcm. Below T c ≈ 92±1K, Rs drops at first steeply and then gradually to 18 mΩ at 77 K and less than 3 mΩ at 4.2 K. Since these values are lower than R s of pure copper, microwave applications of high T c superconductors like cavities with high quality factors are within reach now.

PAIR AND QUASIPARTICLE STATES OF YBA2CU3O7-X FILMS DEDUCED FROM THE SURFACE IMPEDANCE AND A COMPARISON WITH NB3SN

Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb3Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance Rs and penetration depth λ of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance Rres(19 GHz)∼90 μΩ constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting Rres from Rs(T) revealed activated behavior with a reduced energy gap Δ0/kBTc∼0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T≤0.5 ·Tc, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of λ(T) at T≥0.5 · Tc was related to qp scattering, which also caused an extremal conductivity σ1(T). A shoulder in λ(T) at T=(0.6–0.7) · Tc confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of λ(B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.

Nb/sub 3/Sn films on sapphire. A promising alternative for superconductive microwave technology

Phase-pure, large grained Nb/sub 3/Sn films on sapphire substrates have been prepared by a two-step process. The average grain size increased with the film thickness. Transport properties like the penetration depth, mean free path, and critical current density have been investigated in relation to the microstructure of the films. Measurements of the DC- and HF-magnetic field dependent surface impedance were performed. Nonlinear surface resistance occurred at field levels above B/sub s/=25 mT, and was related to extrinsic mechanisms. The relevance of weak coupling at grain boundaries decreased with increasing grain size. Thus heating at local defects was concluded to be the dominant limiting mechanism in large grained films. An intrinsic field limit of B/sub cl/=140 mT was found. The polycrystalline films showed a much lower residual resistance and comparable power handling compared to high-quality epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//-films.

Nonlinear microwave response of Nb/sub 3/Sn films: a case study of granular superconductors

We have investigated the microwave magnetic field dependent surface resistance, R/sub s/(B/sub s/), of Nb/sub 3/Sn films of different film thickness and on different substrates at 19 GHz. The film thickness controls the grain size of the polycrystalline films. It provides a tool to study the role of grain boundaries on the nonlinear microwave response of granular superconductors. We find a non-monotonic dependence of the onset field, B/sub on/, of nonlinear surface resistance on film thickness, with a maximum of 25 mT at 1.2 /spl mu/m. This behaviour reveals that magnetic losses at grain boundaries dominate in thin films, and microwave heating in thick films. The universal scaling behaviour of B/sub on/ with temperature, its dependence on the thermal conductivity of the substrate, and the good agreement between the quadratic field dependence of R/sub s/ and a weakly-coupled grain model indicate heating at grain boundaries as a possible mechanism. The correlation between the nonlinear microwave response and microstructure of the granular films indicates possible improvements and intrinsic limitations.