Š. Beňačka

MgB2 superconducting thin films on Si and Al2O3 substrates

Thin films of MgB2 superconductor were prepared by three different procedures on sapphire and silicon substrates. Boron thin films, ex-situ annealed in magnesium vapour, resulted in textured polycrystalline films with crystal dimensions below about 1000 nm, onset critical temperature Tcon near 39 K and width of phase transition about 1 K. Both, ex-situ and in-situ annealed co-deposited boron and magnesium thin films on sapphire and silicon substrates give smooth nanocrystalline films. DC properties of nanocrystalline MgB2 films co-deposited on silicon substrate reached Tcon = 33 K and zero resistance Tco = 27 K, the highest values received until now on Si substrates. In addition, microwave analyses prove the existence of superconducting parts of the film below 39 K. This result confirms the possibility to synthesise nanocrystalline superconducting MgB2 thin films on silicon substrate with critical temperature near 39 K, prepared by vacuum co-deposition of boron and magnesium films.Abstract Thin films of the superconductor MgB 2 were prepared by three different procedures on sapphire and silicon substrates. Boron thin films, ex situ annealed in magnesium vapour, resulted in textured polycrystalline films with crystal dimensions below about 1 μm, onset critical temperature T c on near 39 K and width of phase transition ΔT ⩽1 K. Both, ex situ and in situ annealed co-deposited boron and magnesium thin films on sapphire and silicon substrates give smooth nanocrystalline films. DC properties of nanocrystalline MgB 2 films co-deposited on silicon substrate reached T c on =33 K and zero resistance T c0 =27 K, the highest values received until now on Si substrates. In addition, microwave analyses prove the existence of unconnected superconducting parts of the film below 39 K. This result confirms the possibility to synthesize nanocrystalline superconducting MgB 2 thin films on silicon substrate with critical temperature near 39 K, prepared by vacuum co-deposition of boron and magnesium films.

High Tc Y-Ba-Cu-O thin films on Si substrates

A low temperature process of vacuum codeposition at 550 °C was used to prepare superconducting Y‐Ba‐Cu‐O thin films on Si substrates without a buffer layer. A zero resistance critical temperature Tce as high as 81.5 K was reached without further high‐temperature post‐annealing treatment. The surface morphology of films has a granular or porous character dependent on substrate temperature. The atomic emission spectroscopy shows the diffusion of Si from the substrate into the films at the level of several atomic percent.

DC and microwave properties of YBa2Cu3Ox thin films in a weak magnetic field

Abstract Measurements of the transport critical current and the modulated microwave absorption in low magnetic fields up to 15 mT are presented for granular crystallographically disordered as well as for c -axis oriented YBa 2 Cu 3 O x thin films at 4.2 and 77 K. Both the transport critical current and the microwave losses are believed to be determined by the weak links existing in both types of samples. The intergranular critical state and the influence of the intragranular magnetization are introduced in order to account for the magnetic hysteresis effects. The experimental results are fitted by a theoretical model, and the differences between disordered and oriented thin films are discussed.

Surface degradation of YBa2Cu3O7-δ observed by means of contact resistance measurement

Abstract The time dependence of the YBa2Cu3O7−δ (YBaCuO) - metal (Pb, In, Sn, Au) point contact resistance was studied. The measurements were carried out in the temperature range 340 K to 77 K. The point contacts prepared at room temperature exhibit the change of resistance about 100–500 % after the time ≅ 102 sec, however at temperature 77 K no change of contact resistance was observed. The change of the resistance can be explained in terms of oxygen depleted surface layer of YBaCuO material caused by lower activation energy of oxygen on the surface. This time dependence of contact resistance may be well explained by diffusion effects. From the fitting comparison of experimental curves with theory we determined the diffusion coefficient which characterizes the diffusion process of the oxygen ions in the bulk of YBaCuO material.

Superconducting gap parameters of MgB2 obtained on MgB2/Ag and MgB2/In junctions

Abstract MgB2 superconducting wires with the critical temperature Tc approaching 40 K were used for the preparation of MgB2/Ag and MgB2/In junctions. The differential conductance vs. voltage characteristics of N–S junctions exhibit a clear contribution of the Andreev reflection. Using a modified BTK theory for s-wave superconductors two order parameters Δdirty≈4 meV and Δ3D≈2.6 meV were determined from the temperature dependencies. Surprisingly, the larger order parameter Δdirty vanishes at a lower temperature T c dirty ≈20 K compared with the smaller one Δ3D with Tc≈38 K. Both the magnitudes of the order parameters and their critical temperatures are in good agreement with theoretical calculations of electron–phonon coupling in MgB2 carried out by Liu et al. [cond-mat/0103570 (2001)].

High Tc Y-Ba-Cu-O thin films prepared by in situ low-temperature codeposition of Y, BaF2, and Cu on α-Al2O3 substrates

Thin Y‐Ba‐CU‐O films were prepared in‐situ by low‐temperature codeposition on Al2O3 substrates. The temperatures during preparation did not exceed 600 °C. As a source of Ba the BaF2 was used. The zero resistance Tc values were higher than 84 K, Jc =4×104 A/cm2 at 4.2 K/OT. The Auger electron spectroscopy analysis has shown almost a homogeneous distribution of the film components throughout the film thickness. X‐ray diffraction revealed the presence of unoriented 1‐2‐3 phase besides BaF2 and CuO and, one undefined phase, which might be oxyfluoride compound.

Influence of bias voltage history on conductance properties of YBaCuO/normal metal junctions

Abstract We have investigated I – V characteristics and their derivatives in YBa 2 Cu 3 O 7− x /normal metal planar and point contact junctions in the voltage range up to ±2 V. Both increases and decreases of the junction resistance above +0.5 V and below −0.5 V (related to metal electrode), respectively, were measured. These changes were reproducibly repeated several times on the same contact, and we ascribe them to oxygen replacement in the tunneling barrier arising from degraded YBa 2 Cu 3 O 7− x unit cells in the vicinity of the contact. The differential conductance of these junctions showed both a linear and a quadratic dependence with respect to applied voltage polarity. These features are described in terms of inelastic processes due to strong spin interactions in the barrier caused by oxygen replacement in unit cells of YBa 2 Cu 3 O 7− x . We model the linear and the quadratic background as a superposition of elastic and inelastic transport of quasiparticles through the barrier. An abrupt change of the slope of the linear part of the differential conductance was found above temperature 150 K.

Time evolution of point contact resistances of high-Tc superconductors

Abstract The time increase of the differential resistance of YBa 2 Cu 3 O x /In, Au and Bi 2 Sr 2 Ca 2 Cu 3 O y /In, Ag point contacts was experimentally observed. The results are discussed in the frame of the oxygen diffusion from the surface layer of high- T c superconductors. Our computer simulation of the oxygen diffusion in YBa 2 Cu 3 O x (YBCO) and Bi 2 Sr 2 Ca 2 Cu 3 O y (BSCCO) surface layers gives good agreement with experimental data and yields diffusion coefficients of oxygen motion at 300 K of D B ≈ 3.2×10 -19 cm 2 / s for BSCCO and D y ≈ 3.7×10 -18 cm 2 / s for YBCO. From the comparison of the two studied materials it clearly follows that the surface of BSCCO is more stable than YBCO; nevertheless, the surface degradation processes due to oxygen diffusion are significant for both materials within room temperature range. The sharp drop in the time increase of the point contact resistance at temperature T r ≈ 220 K for YBCO and ≈ 185 K for BSCCO is interpreted by oxygen reordering in the unit cells of high- T c superconductors.

Dynamic properties of a superconducting quantum interference device containing arrays of Josephson junctions

The dynamic properties of a superconducting quantum interference device (SQUID) containing arrays of Josephson junctions in a superconducting loop, and in particular the case of a four‐junction SQUID, is analyzed theoretically via computer simulations. It is shown that phase locking of Josephson junctions determines the dynamic behavior of the SQUID. In the case of a stable phase‐lock state hysteretic I‐V curves as well as unusual voltage‐flux dependencies appear. The influence of a small spread in the Josephson junction parameters upon the stability of the phase‐lock state is investigated in parameter space.

Tunneling spectroscopy in thin film superconducting junctions YBCO/Pb

Abstract We have prepared thin film superconducting tunnel junctions YBa2Cu3O7−δ/Pb of Giaevers type and performed a study of their tunneling characteristics. Dynamic conductivity of the junctions exhibits parabolic dependence on voltage with the height of the tunnel barrier above 500 mV. The shape of the second derivative of U-I curves exhibits typical features of Pb and YBCO gaps as well as some other structure above the gap. The maximum gap observed in our YBCO thin films is 2Δ= 31.6 meV which for zero-resistance critical temperature Tce = 68 K gives 2Δ kT ce = 5.4 and for onset temperatur Tco = 93 K is 2Δ kT co = 3.9 . Various possible mechanisms are suggested to explain the results.