R. Gross

OBSERVATION OF BOUND SURFACE STATES IN GRAIN-BOUNDARY JUNCTIONS OF HIGH-TEMPERATURE SUPERCONDUCTORS

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.

Possible pseudogap behavior of electron-doped high-temperature superconductors

We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y) and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature Tc. In contrast, below Tc for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below Tc. Possible interpretations of this observation are the existence of a normal state pseudogap in the electron doped HTS or the existence of a spatially non-uniform superconducting state.

Large two-level magnetoresistance effect in doped manganite grain-boundary junctions

We performed a systematic analysis of the tunneling magnetoresistance (TMR) effect in single grain boundary junctions formed in epitaxial La(2/3)Ca(1/3)MnO(3) films deposited on SrTiO(3) bicrystals. For magnetic fields H applied parallel to the grain boundary barrier, an ideal two-level resistance switching behavior with sharp transitions is observed with a TMR effect of up to 300% at 4.2 K and still above 100% at 77 K. Varying the angle between H and the grain boundary results in differently shaped resistance vs H curves. The observed behavior is explained within a model of magnetic domain pinning at the grain boundary interface.

Voltage-flux-characteristics of asymmetric dc SQUIDs

We present a detailed analysis of voltage-flux V(/spl Phi/)-characteristics for asymmetric dc SQUIDs with various kinds of asymmetries. For finite asymmetry /spl alpha//sub I/ in the critical currents of the two Josephson junctions, the minima in the V(/spl Phi/)-characteristics for bias currents of opposite polarity are shifted along the flux axis by /spl Delta//spl Phi/=/spl alpha//sub I//spl beta//sub L/ relative to each other; /spl beta//sub L/ is the screening parameter. This simple relation allows the determination of /spl alpha//sub I/ in our experiments on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// dc SQUIDs and comparison with theory. Extensive numerical simulations within a wide range of /spl beta//sub L/ and noise parameter /spl Gamma/ reveal a systematic dependence of the transfer function V/sub /spl Phi// on /spl alpha//sub I/ and /spl alpha//sub R/ (junction resistance asymmetry). As for the symmetric dc SQUID, V/sub /spl Phi// factorizes into g(/spl Gamma//spl beta//sub L/)/spl middot/f(/spl alpha//sub I/,/spl beta//sub L/), where now f also depends on /spl alpha//sub I/. For /spl beta//sub L//spl lsim/5 we find mostly a decrease of V/sub /spl Phi// with increasing /spl alpha//sub I/, which however can only partially account for the frequently observed discrepancy in V/sub /spl Phi// between theory and experiment for high-T/sub c/ dc SQUIDs.

High-temperature superconducting Josephson Vortex Flow Transistors: numerical simulations and experimental results

Josephson Vortex Flow Transistors (JVFTs) based on high transition temperature superconductors (HTS) are promising candidates for three-terminal devices, which may be used e.g. at the interface between superconducting and semiconducting electronics. We have investigated the performance of JVFTs based on parallel arrays and on long HTS Josephson junctions, both theoretically and experimentally. Our numerical simulation results reveal the dependence of the current gain on various device parameters, such as number of junctions, loop size, and screening parameter /spl beta/=L/L/sub j/, where L is the loop inductance and L/sub j/ the Josephson inductance of a single junction. We have fabricated various devices using symmetrically and asymmetrically injected bias currents. The experimental results are in good agreement with our simulation results and it is shown that for asymmetric devices high values of the current gain above 20 can be obtained for temperatures below 60 K.

Fabrication and characterization of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///SrTiO/sub 3//Ag trilayer films on SrTiO/sub 3/ bicrystal substrates

We have fabricated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///SrTiO/sub 3//Ag trilayer thin films on bicrystal SrTiO/sub 3/ substrates for electric field effect three terminal devices. For the electrical transport measurements the samples were patterned using optical lithography and wet or ion beam etching. The dielectric properties of the SrTiO/sub 3/ barrier layer were analyzed by measuring the temperature and voltage dependence of the relative dielectric constant /spl epsiv//sub r/ and the resistivity /spl rho//sub iso/. By optimization of the deposition process, /spl rho//sub iso/=2/spl times/10/sup 10/ /spl Omega/m and a breakdown voltage E/sub bd/=1/spl times/10/sup 7/ V/m were obtained at 4.2 K. For /spl epsiv//sub r/ a maximum value of 460 was observed around 75 K. By the deposition of the trilayer structure on bicrystal substrates, gated grain boundary Josephson junctions (GBJs) were fabricated. High quality GBJs have been obtained covered by a SrTiO/sub 3/ gate insulator with a maximum change in the polarization of 0.08 C/m/sup 2/.

Intuition and Technology as Bases of Medical Decision-Making

At least since the nineteenth century, medicine has held a precarious place between natural science and “helping and healing”. Now, in addition to this widely discussed dualism, there are additional problems:

Role of ion beam etching in the fabrication of ramp-type junctions

The use of ramp-type junctions is not only limited to the fabrication of Josephson junctions for ultrafast high-temperature superconducting electronics but is also well suited for the controlled coupling of crystallographically compatible oxide materials such as ferromagnetic manganites and superconducting cuprates. Transport processes of ramp-type junctions strongly depend on the ramp interface generated in the fabrication process. With regard to the high potential of ramp-type junctions in the study and application of oxide materials, a detailed investigation of the process of ramp formation by modern surface analytical methods is highly desired. The enormous challenge in the fabrication of ramp-type junctions consists in the necessity of the engineering of the involved interfaces on a unit-cell length scale. We present a detailed study of the ramp formation by ion beam techniques. Special focus is put on the prevention of recrystallization of ion milled material which poses a major problem to the formation of atomically smooth ramp interfaces with desired stoichiometry.

Symmetry of the order parameter in hole and electron doped cuprate superconductors

Abstract It is well established that the order parameter symmetry in the hole doped high-temperature superconductors (HTS) is of d-wave type. In contrast, there is growing experimental evidence for a s-wave type symmetry in the electron doped cuprate superconductors. We present tunneling spectra and measurements of the London penetration depth, both supporting strongly the s-wave scenario for the electron doped HTS [1]. The absence of Andreev bound states in the tunneling spectra indicates a state without sign change of the pairk potential, and the absence of low energy excitations as revealed by the London penetration depth measurements suggests a gapped state. Based on these experiments, for the materials (Nd/Pr)1.85Ce0.15CuO4−y an about isotropic s-wave symmetry seems more likely than a strongly anisotropicl s-wave symmetry. For the ND compound we observe anomalous low temperature behavior due to the Nd3+ magnetic moments.