Peter Berghuis

Properties of YBa2Cu3O7−δ thin films grown on vicinal SrTiO3 (001) substrates

Abstract We report on the structural and electrical properties of YBa 2 Cu 3 O 7− δ films grown on 2°, 4° and 6° vicinal SrTiO 3 (001) substrates. X-ray diffraction analysis revealed the alignment of the YBa 2 Cu 3 O 7− δ c -axis with the substrate [001] tilted axis and good crystalline quality of the films. A largely regular surface morphology, characteristic of step-flow growth, was observed by means of atomic force microscopy and scanning tunnelling microscopy. The step height h s in the regular film area was of the order of the YBa 2 Cu 3 O 7− δ unit cell height in the c -direction. The terrace width w T decreased with increasing vicinal angle according to the relation w T = h s /tan( θ vic ). In addition we observed holes and surface regions with sub-unit cell steps and merged individual steps (step bunching). The surface topology suggests that defects such as planar antiphase boundaries and impurities result in a localised reduction of the step velocity and in the pinning of the step growth front. Transport measurements showed an anisotropy of the in-plane resistivity but not of the critical temperature (90 K along and across the steps). The angular dependence of the critical current density is briefly reported.

The capacitance of bicrystal Josephson junctions deposited on SrTiO/sub 3/ substrates

We have investigated the capacitance of grain boundary Josephson junctions deposited on symmetric 24/spl deg/ misoriented SrTiO/sub 3/ bicrystal substrates using hysteresis and Fiske resonances in their current voltage characteristics. Our measurements show that the total junction capacitance is proportional to the junction area. The capacitance per unit area is (140/spl plusmn/30) fF /spl mu/m/sup -2/ and this implies that the main effect of the large relative permittivity of the substrate is to scale-up the grain boundary capacitance by a factor of around 10. We also find that in the planar geometry of bicrystal grain boundary junctions the velocity of light in the junction depends upon film thickness.

Preparation and properties of c-axis-oriented NdBa2Cu3O7−δ films by dc sputtering

Abstract NdBa 2 Cu 3 O 7− δ (NdBCO) is known to have the highest T c , among the rare earth REBa 2 Cu 3 O 7− δ (RBCO) family, and to show higher irreversibility fields than YBa 2 Cu 3 O 7− δ (YBCO). Recently, excellent superconducting NdBCO films have been synthesized by a laser ablation technique. In this work, the transition temperature, phase and growth morphology of NdBCO films deposited on (100) SrTiO 3 (STO) and (100) MgO substrates by dc sputtering have been investigated. The transition temperature T c is closely related with the c -axis lattice constant and shows a maximum around 1.173 nm. The c -axis lattice constant of the films strongly depends on oxygen pressure P o . High- T c NdBCO films are obtained around P o =350 Pa. The spiral growth morphology of the films is affected by deposition rate and takes place at low deposition rate. The best film has shown a zero resistance transition temperature of 92.6 K.

Current-voltage measurements as a probe of the activation barriers for flux creep in thin films of YBa2Cu3O7−δ

Abstract In order to investigate the nature of the energy barrier U ( j ) for flux creep below the irreversibility line, accurate current-voltage ( j - E ) curves have been measured on YBa 2 Cu 3 O 7−δ thin films at temperatures close to 77.4 K and fields up to 8 T. Using Taylors expansion the logarithmic derivative ( j / E ) d E /d j and the second derivative d 2 E /d j 2 can be obtained reliably from the j - E data. Below the irreversibility line the logarithmic derivative increases with decreasing j , displaying an upward curvature in a low-field regime. An evaluation of the results in terms of various models proposed for U ( j ) is presented. The Anderson-Kim picture U ( j ) ∝ (1− j / j c ), a logarithmic U ( j ) ∝ ln( j ) and various distribution functions p ( U ) for the activation energy could not be reconciled with the j dependence of the logarithmic derivative. The experimental data are, however, consistent with U ( j ) ∝ ( j c / j ) μ , as expected for a glassy vortex phase. Although the exponent μ is field dependent, our results suggest a Bose glass phase at low fields.

Determination of pinning forces on vortex lines in YBa2Cu3O7-delta

The structure adopted by the flux line lattice (FLL) in YBa2Cu3O7- (YBCO) depends strongly on the angle () between the applied field and the superconducting cuprate planes. If this angle is held constant and the field rotated about an axis normal to the cuprate planes, the structure of the FLL remains constant, while the direction of the Lorentz force on the vortex lines changes. Measurements of the critical current density (jc) with respect to this rotation angle () thus probe pinning behaviour while excluding effects due to changes in the structure of the FLL. If the value of is small, such that the FLL adopts the kinked (string- pancake) state, the variation in jc seen with must be due to the behaviour of the string vortex segments. High-resolution measurements of the variation of jc with this second angle have been made on thin (120 nm) YBCO films. A simple model is proposed to explain this variation in terms of dragging of vortex strings by the vortex pancake elements. From this model and experimental data an estimate has been made for the value of the pinning force density on vortex strings for motion in the a-b planes. This estimate is in agreement with values found for the string pinning force density found in earlier measurements on YBCO films grown on vicinal-cut substrates.

Growth of c-Axis Oriented NdBa2Cu3O7-y Films with Transition Temperature Exceeding 92 K by dc Sputtering

NdBa2Cu3O7-y (NdBCO) among the REBa2Cu3O7-y family is known to have the highest transition temperature and to show higher irreversibility fields than YBa2Cu3O7-y(YBCO) in the bulk sample. In this work, NdBCO films deposited on (100) SrTiO3 substrates by an on-axis dc sputtering technique have been investigated on transition temperature Tc, phase and growth morphology. The deposition conditions for c-axis oriented films with Tc exceeding 90 K are given. The relationships between Tc, c-axis lattice constant and oxygen pressure are discussed.