D. Wehler

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.

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.

Microwave conductivity of YBa2Cu3O6.9 single crystals near Tc

Abstract Measurements of the temperature dependencies of the surface resistance and the resonance frequency shift of some YBa 2 Cu 3 O 6.9 single crystals have been performed at 3 GHz. A narrow peak in the real part of the conductivity σ 1 was observed near T c . The experimental data fit to calculations based on the strong coupling theory assuming a narrow T c distribution in the sample. We conclude that the conductivity peak in our measurements, as well as in earlier experiments on high- T c materials in the microwave range, is not the so-called coherence peak.

Temperature-induced transition from tunneling to metallic behaviour of Bi2−xPbxSr2CanCun+1Oy (x=0.4, n=1 and n=2) break junctions

Abstract A distinct gap feature has been observed in polycrystalline Bi 2−x Pb x Sr 2 Ca n Cu n+1 O y ( x =0.4, n =1 and n =2) break junctions. The gap values varied from junction to junction displaying different temperature dependences but the same critical temperature. The observed effect can be interpreted as an indication for an intrinsic proximity effect associated with the layered structure of BiSrCaCuO:Pb superconductors and the variable properties of the fracture surface. The properties of the surface barrier affect also the current flow across the contact. We observed a temperature-induced transition from a current-voltage characteristic with tunnelling behavior to one with metallic behaviour. The modification of the I ( V )-characteristic with temperature is explained by the presence of a localized state in the barrier.

A new method of YBa2Cu3O7−x single-crystal growth in Al2O3 crucibles with layered protective covers

Abstract A new method of YBa 2 Cu 3 O 7− x (YBCO) crystal growth is presented, which uses Al 2 O 3 crucibles with protective covers. The protective cover includes at least two ceramic layers, which are composed of (Y 2 O 3 + Y 2 BaCuO 5 ) mixtures sintered under proper conditions. The layer nearest to the Al 2 O 3 crucible wall is enriched by Y 2 O 3 and protects the crystallization zone against Al into pollution. The layer nearest to the Y B C O melt consists of a pure Y 2 BaCuO 5 phase and provides the introduction of Y the reaction volume via partial dissolution. Large crystals of 3.2 × 3.2 × 0.2 mm 3 of high purity with low residual microwave losses have been grown by this technique from YBa 2 Cu 3 O 7− x + Ba 2.8 Cu 7.2 O x flux. This method can be extended for other single crystals and melted material. The protective covers may be prepared also on crucibles of other materials, if they have a good thermal stability and can hold several thermal cycles.

Evidence for electron-phonon interaction in Bi2Sr2CaCu2O8+x from tunneling measurements

Abstract Tunneling effects in Bi2Sr2CaCu2O8+x single crystals with Tc=80÷90K have been investigated using a break junction technique. A reproducible fine structure has been observed in the tunneling spectrum at voltages V > Vg=2Δ/e, which shifts with the gap feature to lower voltages with increasing temperature. This structure correlates with the phonon density of states, as determined by inelastic neutron scattering, and indicates the importance of electron-phonon interaction for the superconductivity in BiSrCaCuO.

Single Crystal Break Junctions

Tunnelling effects in BiSrCaCuO (2212) single crystals and polycrystalline BiSrCaCuO:Pb (2212 and 2223) samples have been investigated using an improved technique of break junction preparation. A reproducible, nearly vertical rise of the current at the gap voltages has been found. The shape of I-V curves, the value of the superconducting gap and the temperature dependence of the gap parameter point to an essential influence of the proximity effect. A temperature induced transition from a contact with tunnelling behaviour to a contact with direct linkage has been detected.

Tunneling spectroscopy of Bi(Pb)-Sr-Ca-Cu-O superconductors

Abstract The tunneling characteristics for break junctions in polycrystalline Bi 1.7 Pb 0.4 Sr 2−X Ca 2+Y Cu 3+Z O 10+δ samples fabricated by direct mixing of oxides-carbonates (1) and via Bi(Pb)-Ca − O + Sr-Ca-Cu-O precursors (2) have been investigated. Long time annealing of the samples (1) causes decomposition in the form of the intergrowth faults (2212 - in 2223 - phase), which leads to the formation of the natural superlattice of the type S 1 -S 2 -S 1 -S 2 -.. and to an emergence of a reproducible vertical rise of current in the I-V characteristics for break junctions at voltages V ≅ ± 2Δ/e. A record value of the characteristic voltage V c = I c R n = (31 ± 4) m V at T=4.2K has been detected. Evidence for an important role of the proximity effect in the samples (1) has been obtained. For the samples (2) the tunneling characteristics were strongly dependent on X, Y and Z. The samples with X=0.2, Y=Z=0 consist of an almost pure 2223-phase with a large gap parameter Δ=(31 ± 1)meV (T=4.2K). For these samples the subharmonic structure in the I-V characteristics, caused by the self-detection of Josephson radiation was observed. From I-V and dI/dV - V characteristics the tunneling density of states has been calculated. The advantage of the method of precursor synthesis for the effective modification of superconducting properties of Bi(Pb)-Sr-Ca-Cu-O has been demonstrated.

Geometrical resonance effects in HTSC break junctions

A periodic structure has been observed in the tunneling characteristics of HTSC break junctions. This structure is explained in terms of the electron-hole interference effect in the surface layer, which causes the formation of bound states in the normal regions N of the SNINS-type junctions. Good qualitative agreement has been found between experimental data and the predictions of the Arnolds proximity model. Several parameters of the SNINS-type structures have been derived from studies of Fiske resonances in HTSC Josephson break junctions in weak magnetic fields.

Surface resistance of large YBa2Cu3O7−x crystals at 3 GHz with residual losses caused by flux compounds

Abstract The surface resistance (R s ) of YBa 2 Cu 3 O 7−x (YBCO)- crystal platelets, up to 5×10 mm 2 in size, and the microwave losses of the main flux compounds CuO and BaCuO 2±x were investigated at 3GHz. R s of all crystals drops sharply at T c by about 3 orders of magnitude. The residual surface resistance (R res ) of crystals grown by the self-flux method with flux residues on the surface is dominated by the microwave losses of BaCuO 2±x . These losses are orders of magnitude higher than the losses of CuO and were found to increase below 40K. From the accordingly corrected measurement results on large crystals, we can estimate an upper limit of R res z40μΩ. With nearly flux-free but small YBCO crystals we have achieved rf field independent R res values close to the measurement limit up to 100 Oe.