Showing papers by "Paul Seidel published in 2000"

Inductance matching of high-TC dc-SQUIDs in planar gradiometers using a laser patterning technique

Abstract: The inductance of the dc-SQUID in a planar galvanically coupled gradiometer of YBa 2 Cu 3 O 7− x was changed by lengthening the SQUID loop with a focused laser beam. The dc-SQUID inductance was increased from 50 pH to values close to 300 pH by this technique. This increased SQUID inductance changed the effective area of the gradiometer device as well as the SQUID parameter β L . We focused our investigations on the change of the peak-to-peak voltage of the SQUID modulation, on the change of the white noise properties as well as on the change in low frequency noise depending on an effective area and SQUID parameter β L . Different fixed critical currents of the dc-SQUID were considered in order to determine their influence on SQUID operation. For critical currents below 200 μA, a maximum of the white field gradient resolution could be achieved for corresponding SQUID parameters β L between 1.5 and 6 with 340 fT/(cm Hz 1/2 ) as the lowest noise level. Here, the necessity of a high I C R N product, an optimum critical current and an optimum dc-SQUID inductance was shown in order to realize a high device sensitivity. The influence of the grain boundary angle to achieve a high field gradient resolution even in unshielded environment as well as the combination of the increase of SQUID inductance with an additional ion beam etching technique for critical current reduction is considered.

Superconducting, surface and anomalous electron transport properties of BaNbO3-x films

Abstract: Review of superconducting ( T c , H c 2 (T), J c ), electron transport Rs(T), surface (XPS, UPS) and structural (XRD, RBS) properties of thin films of a novel superconductor BaNbO3−x on different substrates is presented. Superconducting films have been obtained when grown on the Al2O3 with T c =14 K , J c ≈1×10 4 A cm −2 and large H c 2 (0)=28 T . At the same time, films on NdGaO3 exhibit behavior typical to granular superconductors.

An alternative explanation of the long-range proximity effect in HTS junctions

Abstract: We investigated coplanar junctions in the variable thickness bridge (VTB) geometry made of YBa 2 Cu 3 O 7− y (YBCO)/Y 1− x Pr x Ba 2 Cu 3 O 7− y bilayers. Depending on their properties we can distinguish at least three types of junctions. We think that the “long-range proximity effect” in coplanar junctions is an artifact of the preparation resulting in residual channels and suggest the joint use of magnetic and microwave field investigations to clarify the nature of these and other junctions with unusual behavior.

Two Components in Femtosecond Optical Response of Y0.7Pr0.3Ba2Cu3O7-δ Thin Films in Superconducting State

Abstract: We studied the optical response of Y0.7Pr0.3Ba2Cu3O7-δ (YPBCO) thin films by means of a femtosecond time-resolved pump-probe method, and found that its response quite differs significantly from that of YBa2Cu3O7-δ (YBCO) thin films. It is revealed that there are two components in the transient reflectivity change ΔR(t)/R of the YPBCO thin film in the superconducting state. The temperature dependence of ΔR(t)/R suggests that the one component originates from the photoexcitation of superconducting layers and the other originates from that of normal layers. The origin of these two components is discussed in relation to the electronic structure of YPBCO.

Comparison of the electromagnetic pulse emission from YBa2Cu3O7−δ and Y0.7Pro0.3Ba2Cu3O7 excited by femtosecond laser pulses

Abstract: We observe terahertz radiation from Y 07 Pr 03 Ba 2 Cu 3 O 7 (YPBCO) thin films excited with femtosecond optical pulses, and compare its waveform and efficiency with those from YBa 2 Cu 3 O 7−gd (YBCO) It is found that YPBCO can produce much stronger radiation than YBCO We discuss the origin of the enhancement as well as the temperature dependence of the emission properties

Influence of target–substrate angle on properties of laser deposited YBa2Cu3O7−δ thin films and step-edge Josephson junctions

Abstract: YBa 2 Cu 3 O 7− δ (YBCO) thin films have been made by pulsed laser deposition (PLD) under variable angles α between target and substrate orientation. For several angles α the laser pulse energy E and the oxygen pressure p O 2 were varied to study the dependence of optimal film growth conditions on α . Film properties which were analyzed include the film thickness, the droplet density, the critical temperature T c0 , the surface morphology as seen by scanning electron microscopy (SEM) as well as crystal quality measured by Rutherford backscattering spectroscopy. With increasing angle α we find a shift of the optimal growth conditions towards higher laser pulse energy. Step-edge Josephson junctions prepared from YBCO films which were deposited under different angles α show a maximum product of critical current density and normal resistivity at α =20–45°.

Superconducting sensors for weak magnetic signals in combination with BiCMOS electronics at 77 K for different applications

Abstract: Different sensors for weak magnetic signals were realized using thin high temperature superconducting films on different substrates including buffered silicon. Superconducting quantum interference devices (SQUIDs), magnetometers and planar gradiometers based on them as well as a new type of Hall-effect sensor with a superconducting antenna were tested with respect to signal resolution, band width and spatial resolution. To realize adapted systems for biomagnetic research or non-destructive evaluation common room temperature electronics have some disadvantages. Thus we tested discrete elements as well as special adapted integrated BiCMOS circuits placed near the sensors at 77 K. We demonstrate the status of the development of such electronics.

Femtosecond optical response of Y0.7Pr0.3Ba2Cu3O7−δ thin films

Abstract: We study Y0.7Pr0.3Ba2Cu3O7-δ (YPBCO) thin films by a reflection pump-probe method with femtosecond optical pulses, and find that its response quite differs from that of a YBa2Cu3O7-δ (YBCO) thin film. There are two components in the optical response ΔR(t)/R in the superconducting state, which suggests that one component originates from the photoexcitation of the supercarriers while the other does from that of the normal carriers. We will discuss the temperature dependence of the carrier lifetime in the optically excited YPBCO in the normal and superconducting states.

Development and application of magnetic field sensors based on high-temperature superconductors

Abstract: Thin films of YBa2Cu3O7-x (YBCO) were prepared on different kinds of substrates (SrTiO3, LaAlO3, sapphire or silicon) with critical current densities jc (77K) > 2 (DOT) 106 A/cm2 and critical temperatures (zero resistance) of up to 90 K. We used laser deposition techniques for film preparation on SrTiO3-substrates with a maximum substrate size of 10 X 10 mm2. Applying bicrystallin substrates made from this material a pronounced RSJ-behavior of the grain boundary Josephson junctions was observed with ICRN-products of 410 (mu) V and critical current density of 1.2(DOT)105 A/cm2 in maximum on 24 degree grain boundaries. On that basis planar galvanically coupled dc-SQUID-gradiometers were patterned on this limited substrate size with a field gradient resolution of 308 fT/(cm(root)Hz) in the white noise level and 2 pT/(cm(root)Hz) at 1 Hz in electrically and magnetically unshielded environment. This extraordinary field gradient resolution even in unshielded environment enables this kind of dc-SQUID sensor for measurements of the magneto-cardiogram (MCG) of the human heart as well as for investigations in non-destructive testing (NDT). The layout of the dc-SQUID as well as the antenna layout of the gradiometer antennas were varied in order to determine their influence on the whole sensor performance. For the dc-SQUID layout itself we present a gradiometric scheme with decreased parasitic area of the dc-SQUID in the gradiometer. Furthermore the additional use of buffer layers to prevent interdiffusion, lattice mismatch and internal stress by different thermal expansion coefficients enables the use of silicon substrates for YBCO thin film deposition. Gradiometric flip-chip- antennas were patterned on 2'-silicon-substrates and combined with dc-SQUID-gradiometers explained above. With this sensor concept the resolution was increased by a factor of 6 in shielded environment thus also the human MCG was measured. By the integration of superconducting antennas with Hall-effect sensors on the same substrate a hybrid sensor concept is introduced having less sensitivity compared to SQUID-based sensors but improved dynamic range enabling their application in a NDT-measurement system in highly disturbed environment.

Accurate location and marking of grain boundaries using focused ion and electron beams

Abstract: We have investigated the 2×12° symmetric grain boundary of ( 0 0 1 ) oriented bicrystalline silicon substrates for high-Tc superconductor applications using scanning focused ion beam (FIB) and electron beam (SEM). We successfully used the focused ion beam to detect and to mark the grain boundary and hence, to increase the accuracy of positioning the Josephson device with respect to the grain boundary of the silicon substrate. Both imaging methods have been compared using channeling effects of focused ion and electron beams in a dual beam system.

NdBaCuO thin films for Josephson devices

Abstract: We have prepared NdBa 2 Cu 3 O 7-δ (NBCO) Josephson junctions on SrTiO 3 (STO) bicrystal substrates with a misorientation angle Ω = 24° . The superconducting films were fabricated by pulsed laser deposition (PLD). To increase the superconducting properties of the NBCO films, we used PrBa 2 Cu 3 O 7-δ (PBCO) as a template layer. The highest T c of these bilayers was about 94 K, about 8 K higher than for our single NBCO films. The Josephson junctions showed a clear resistively and capacitively shunted junction (RCSJ) type behaviour.

Charge Transport Across a Mesoscopic Superconductor-Notmal Metal Junction: Coherence and Decoherence Effects

Abstract: We present a simple scattering approach to the charge transport across a realistic superconductor–normal injector interface of a finite transmittance that is modeled by a double-barrier mesoscopic junction. For a d-wave pairing symmetry, our calculations combine a fully quantum-mechanical scattering formalism with a self-consistent estimation of Andreev reflection coefficients within the quasi-classical Eilenberger equation scheme for a free specular superconducting surface. Numerical simulations confirm experimental criteria of Cucolo for the unconventional superconducting origin of conductance anomalies in high-temperature oxides. A discussion of dephasing effects caused by inelastic scattering processes in the interlayer and their impact on the conductance spectra is given.

Photodoping of 60K and 90 K YBaCuO grain bounadry Josephson junctions

Abstract: We have illuminated with UV or visible light YBaCuO grain boundary Josephson junctions (GBJJ) of different critical temperatures of the banks (90K and 60K). These GBJJ show both dc Josephson properties (Fraunhofer pattern) and ac Josephson properties (Fiske resonance). From the decrease of the normal state resistance we can estimate the oxygen content of the barrier which is lower in the 60 K junctions. The relative increase of the critical current by illumination is higher in the 60 K than in the 90K junctions.

Flip-chip-type high-T c gradiometer for biomagnetic measurements in unshielded environment

Abstract: A flip-chip-type gradiometer has been constructed with a 10 mm x 5 mm planar DCSQUID gradiometer fabricated on a SrTiO 3 bicrystal substrate and a flux transformer made from a YBCO * /YBCO/Ce0 2 /YSZ multilayer on a Φ50.8 mm Si wafer. The coupling coefficient between the flux transformer and the planar gradiometer is 0.18. The transformer increases effectively the resolution of the gradiometer. A magnetic field gradient resolution of 73 ff. cm -1 Hz -1/2 in the white region and 596 fT- cm -1 Hz -1/2 at 1 Hz has been obtained. High quality magnetocardiogram signals have been successfully measured by using this flip-chip-type gradiometer in an unshielded environment.

Terahertz Radiation Properties from YBCO and YPBCO Thin Films

Abstract: We report the emission of terahertz (THz) radiation from Y 0.7Pr0.3Ba2Cu3O7 (YPBCO) thin film antennas excited by femtosecond laser. Under the same system configuration and excitation conditions, the maximum peak amplitude of the electromagnetic (EM) pulse in the time domain appears to be around 10 times larger in the 0.7Pr0.3Ba2Cu3O7 as compared to the YBa2Cu3O7-δ(YBCO) device. This may be due to the higher transmission of YPBCO in the THz frequency range. We employ time domain spectroscopy (TDS) to determine the electromagnetic field transmission of the film/substrate interface of films made of the above mentioned materials in the frequency range from 0.2 to.2 THz. We compare the difference in the transmission measured by TDS with the difference in the signal efficiency of the emitted electromagnetic subpicosecond pulse.