Showing papers by "Paul Seidel published in 1998"

Planar gradiometers with high- DC SQUIDs for non-destructive testing

Abstract: We investigated planar gradiometers on the basis of galvanically coupled high- DC SQUIDs for application in non-destructive testing (NDT). The stability of the sensor of better than 1% permits NDT investigations in unshielded environments. The layout of the gradiometer sensors and a corresponding determination of their effective areas and baselines will be discussed as well as the properties of the Josephson junctions in the DC SQUID based on step-edge and bicrystal grain boundaries. We show measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as well as the influence of sensor position and experimental environment such as the dewar material on the performance of the sensor and the whole testing system. As an example first investigations of the permanent magnetization of hardened valves will be presented where we look for a correlation between hardening and magnetic field distribution.

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.

Large-area YBCO films for device fabrication

Abstract: We have prepared YBCO thin films by a pulsed laser deposition (PLD) technique on substrates and on the naturally oxidized 2 in diameter silicon wafers with buffer layers. This large-area film PLD technique allows the preparation of homogeneous and reproducible YBCO films with high quality on classical and technical substrates. For 2 in silicon wafer, we obtained the critical temperature in the range from 87.5 K to 88.2 K and a critical current density at 77 K of from 0.53 to over the whole wafer.

High-Tc direct current SQUIDs on silicon bicrystal substrates operating at 77 K

Abstract: We investigated Josephson junctions and dc superconducting quantum interference devices (SQUIDs) on silicon bicrystal substrates with epitaxially grown 24° Y1Ba2Cu3O7−x, (YBCO) grain boundaries. Buffer layers and passivation/contact layers were prepared for the YBCO thin films by laser deposition techniques. The observed current–voltage characteristics of ion-beam etched, as well as direct laser patterned junctions with products of the critical current and the normal resistance up to 150 μV at 77 K, can be described within the resistively shunted junction model. The inductance of dc SQUIDs was varied by additional laser patterning. In this way, dc SQUIDs with transfer functions up to 30 μV/Φ0 and a white noise level of 30 μΦ0/Hz at 77 K were reached.

Low Noise Cold Head of a Four-Valve Pulse Tube Refrigerator

Abstract: We have designed and constructed a split type of a four-valve pulse tube refrigerator (FVPTR) in order to cool high-Tc Superconducting Quantum Interference Devices1 (SQUIDs). A SQUID, in particular, places rigorous upper limits on the tolerable levels of magnetic interference, vibrations and temperature fluctuations. The pressure wave in the system is generated by means of a commercial He-compressor in combination with a rotary valve. In order to protect the cold head from disturbances generated by the valve gear and the compressor, it is useful to separate those parts spatially. Reduction of interference signals from the compressor and the rotary valve is accomplished using flexible tubes between the rotary valve and the cold head. Most parts of the cold head, including the regenerator, were made of non-metallic, electrically insulating, and non-magnetic materials to minimize eddy currents and local magnetic fields, which interfere the sensor directly.

Effect of Photodoping on the Fiske Resonances of YBa2Cu3Ox Grain Boundary Josephson Junctions

Abstract: We have expanded our studies on illuminated YBa2Cu3O x grain boundary Josephson junctions (GBJJ) which show both dc Josephson properties (Fraunhofer pattern) and ac Josephson properties (Fiske resonance). Illuminating GBJJs with visible light changes the Josephson coupling. This change is characterized by an increase of the critical current and a large shift in the voltage position of the Fiske resonances. This effect is due to persistent photoinduced superconductivity (PPS) of the oxygen-depleted YBa2Cu3O x barrier, similar to the PPS found in illuminated oxygen-deficient YBa2Cu3O x thin films. From Fiske resonance experiments in GBJJs of different lengths, it is possible to study the velocity of the electromagnetic wave in the barrier and its change after illumination. Information on the parameters of the barrier, before and after illumination, is obtained from this study.

Tunneling results on gap anisotropy in niobium

Abstract: A recently developed theory to describe tunneling into the Meissner phase of clean superconductors is applied to a re-discussion of experimental data on (Nb single crystal)/Ag junctions taking band structure effects into account. Thereby several shortcomings are removed of the previous analysis which assumed an isotropic single gap. The analysis is based on independent band structure and non-tunneling experimental data. On this basis, quantitative account is obtained for our tunneling results concerning both broadening of characteristics in zero field and the effect of magnetic field for different crystal orientations. The results verify the theory which was based on solving the Bogoliubov–De Gennes equations for tunnel-injected quasiparticles including the Doppler shift imposed by the shielding current. Moreover, they represent realistic tunneling results on gap anisotropy in niobium.

High-quality YBa2Cu3O7-x films with CeO2/YSZ buffer layers on 2-inch Si wafers deposited by pulsed laser

Abstract: The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa2Cu3O7−x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO2 buffer layers, nonsuperconducting crystalline YBa2Cu3O7−x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T c0 from 88.4 K to 88.9 K. and critical current density J c at 77 K and zero field from 2.5 × 106 to 7× 106 A/cm2. The YSZ, CeO2 and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from φ-scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T c superconducting devices on large-area silicon wafers.

High-temperature superconducting devices on buffered silicon substrates

Abstract: The use of silicon as substrate for thin film devices based on high temperature superconducting oxides requires additional buffer layers to prevent interdiffusion, lattice mismatch, and internal stress by different thermal expansion coefficients. We tested different materials like yttrium-stabilized zirconia (YSZ), CeO 2 , and CoSi 2 . Laser deposition of a double buffer system YSZ/CeO 2 gives best results for silicon substrates up to 2 inch wafers. In this way the superconducting YBa 2 Cu 3 O 7-x (YBCO) films can reach a zero resistance temperature near 89 K and critical current densities at 77 K of up to 7(DOT)10 6 A/cm 2 . Additionally a nonsuperconducting but crystalline phase with the same stoichiometry (YBCO*) is used as passivation layer. Based on this technology we realized and investigated step- edge as well as new silicon bicrystal Josephson junctions, superconducting quantum interference devices (SQUIDs), bolometers using different compensation principles, and a new hybrid magnetometer. The hybrid magnetometer based on a simple Hall sensor was integrated with a superconducting antenna loop on the same chip.

Tunneling and Point Contact Spectroscopy of High-TcSuperconducting Thin Films

Abstract: Tunneling and point contact junctions between a normal metal and thin high temperature superconducting film were studied to obtain information on snrface properties. Surface degradation and time development of contact resistance are investigated as well as peculiarities of the conductance-voltage characteristics. The quasi-linear background is related to carrier transport through degraded surface layer described by a model of inelastic scattering from a broad flat continuum of states inside the potential barrier. An asymmetry of the characteristics appears as the result of low Fermi energy values in the high temperature superconductors in comparison to common metals. An extrinsic nature of these peculiarities is supported by effects of barrier formation by applied voltages in the 1 volt range. As an often observed anomaly a conductance peak at zero bias is observed which can be related to different mechanisms. In the case of high temperature superconductors this zero bias anomaly is related to the d-wave pairing symmetry of the pair potential. Experimental results on YBCO and BSCCO films are compared to calculations taking into account the Andreev reflections for a junction between a normal metal and a d-wave superconductor.

The threshold of phase locking in the system of two multi-junction superconducting loops

Abstract: Coherent behavior (phase locking) in the system of two superconducting loops containing arrays of Josephson junctions and influencing each other by means of mutual inductance was numerically investigated. It is shown that under certain conditions the mutual interaction between loops can result in total phase-locking of the system. The threshold of the locking state depends on the strength of the interaction (mutual inductance). It is found that the basic behavior of the system in the phase-locked state can be explained by means of simple model of single multi-junction loop with an effective inductance which depends on mutual inductance. It is shown that synchronization in this system is possible and the mutual interaction increases the stability of phase locking in arrays.

Application of silicon substrates for high-Tc Josephson junctions and SQUIDs

Abstract: We investigated the realization of YBCO Josephson junctions and SQUIDs on 10 x 10 mm 2 silicon bicrystal substrates and on 2 inch single crystalline silicon wafers. All buffer layers, YBCO films, passivation and metallization layers were deposited with the laser ablation technique. Bicrystal junctions with RSJ-like I-V characteristics, I C R N products of 150 μV at 77 K and microwave response up to the THz range were realized. DC-SQUIDs consisting of these junctions have working temperatures up to 80 K and transfer functions up to 30 μV/Φ 0 at 77 K. Further we enlarge our standard preparation technique of Josephson step-edge junctions on 5x10 mm 2 to 2 inch silicon wafers. YBCO films of this size have a critical temperature T C 87 K and critical current densities j C (77K) >3.10 6 A/cm 2 over the whole wafer area.

Submillimeter wave detection and mixing experiments using high temperature Josephson junctions

Abstract: Using high temperature grain boundary Josephson junctions (GBJJs) made of YBa2Cu3O7-(delta ) (YBCO) deposited across silicon bicrystal boundary, we successfully demonstrated direct detection at wavelength as short as 118.8 micrometer (frequency of 2.525THz) and the operation temperature up to 70 K. Radiation from a far infrared (FIR) laser was coupled to the junction, via a TPX plano convex lens and a high resistivity Si hyperhemispherical lens. The response at wavelength of 183.4 micrometer was obtained for the YBCO GBJJs on MgO bicrystal substrates. Also, investigated are the effects of response on external DC magnetic fields and polarization of electromagnetic waves as well as the harmonic mixing properties.

Influence of degraded surface layer of HTS on differential conductance of HTS/metal junctions

Abstract: We have performed a detailed study of the effect of the HTS degraded surface layer on conductance spectra of HTS/metal junctions. It has been found that the corresponding resistance changes by applying an external bias voltage can be explained as an alternation of the oxygen content near the interface. To take into account the spatially inhomogeneous nature of a few top HTS unit cells, we have examined theoretically the influence of a nonuniform normal covering on the subgap tunnelling characteristics of a superconductor. A simple approach to coherent charge transport through a planar double-barrier structure has been generalized to the cause of an anisotropic order parameter, non-uniformity of the middle non-superconducting layer, and magnetic field effect. In particular, we show that, under suitable circumstances, the conductance peak at zero bias voltage and its splitting at low temperatures often observed for HTS/metal junctions can be interpreted within the s-wave scenario, too, if above mentioned inhomogeneity is considered.