A. Schmehl

Tunable Quasi-Two-Dimensional Electron Gases in Oxide Heterostructures

We report on a large electric-field response of quasi–two-dimensional electron gases generated at interfaces in epitaxial heterostructures grown from insulating oxides. These device structures are characterized by doping layers that are spatially separated from high-mobility quasi–two-dimensional electron gases and therefore present an oxide analog to semiconducting high–electron mobility transistors. By applying a gate voltage, the conductivity of the electron gases can be modulated through a quantum phase transition from an insulating to a metallic state.

Enhanced supercurrent density in polycrystalline YBa2Cu3O7-δ at 77 K from calcium doping of grain boundaries

With the discovery of high-temperature superconductivity, it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization. But it was soon found that the critical current density Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small. In many materials, Jc is suppressed at grain boundaries, by phenomena such as interface charging and bending of the electronic band structure. Partial replacement (‘doping’) of the yttrium in YBa2Cu3O7-δ with calcium has been used to increase grain-boundary Jc values substantially, but only at temperatures much lower than 77 K (ref. 9). Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of J c at 77 K that far exceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures.

Design and realization of an all d-wave dc π-superconducting quantum interference device

The predominantly dx2−y2-pairing symmetry in most high-Tc superconductors provides the opportunity to fabricate Josephson junction circuits in which part of the junctions are biased by a phase difference of the superconducting order parameter of π. We present fabrication and measurements of an all high-Tc dc superconducting quantum interference device (dc SQUID) realized with thin-film technology, of which the Josephson junctions consist of one standard junction and one junction with a π-phase shift. The characteristics of the π-SQUID are compared with the properties of a standard high-Tc SQUID.

Evidence of Doping-Dependent Pairing Symmetry in Cuprate Superconductors

Scanning tunneling spectroscopy studies reveal long-range spatial homogeneity and predominantly d(x(2)-y(2))-pairing spectral characteristics in under- and optimally doped YBa2Cu 3O (7-delta) superconductors, whereas STS on YBa2(Cu 0.9934Zn 0.0026Mg (0.004))3O (6.9) exhibits microscopic spatial modulations and strong scattering near the Zn or Mg impurity sites, together with global suppression of the pairing potential. In contrast, in overdoped (Y 0.7Ca (0.3))Ba 2Cu 3O (7-delta), (d(x(2)-y(2))+s)-pairing symmetry is found, suggesting significant changes in the superconducting ground state at a critical doping value.

Adsorption-controlled growth of EuO by molecular-beam epitaxy

Using molecular-beam epitaxy, we demonstrate the adsorption-controlled growth of epitaxial EuO films on single crystalline (110) YAlO3 substrates. Four-circle x-ray diffraction (XRD) reveals phase-pure, epitaxial, (001)-oriented films with rocking curve full width at half maxima as narrow as 34 arc sec (0.0097°). The critical thickness for the onset of relaxation of (001) EuO on (110) YAlO3 (∼2% lattice mismatch) was determined from XRD measurements to be 382±25 A. A saturation magnetization of 6.96±0.07μB/Eu, a value close to the theoretical limit of 7μB/Eu, is observed.

Superconducting memory based on ferromagnetism

Answering to the need for dense superconducting memories, the authors propose a memory concept that combines ferromagnetic dots for the storage of the data and Josephson junctions for their readout. Good scalability is expected for large scale integration. Exploratory memory cells have been implemented using 3μm Nb technology and Permalloy dots. Nonvolatile data storage at 300K was demonstrated.

Possible solution of the grain-boundary problem for applications of high-Tc superconductors

It is shown that the critical current density of high-Tc wires can be greatly enhanced by using a three-fold approach, which consists of grain alignment, doping, and optimization of the grain architecture. According to model calculations, current densities of 4×106 A/cm2 can be achieved for an average grain alignment of 10° at 77 K. Based on this approach, a road to competitive high-Tc cables is proposed.

Doping induced enhancement of the critical currents of grain boundaries in high-Tc superconductors

Abstract Appropriate doping of the grains provides a means to optimize the transport properties of grain boundaries in high- T c superconductors. This is demonstrated for the exemplary case of grain boundaries in bicrystalline Ca- and Co-doped YBa 2 Cu 3 O 7−δ films. By Ca-doping the critical current density is strongly increased and the normal state resistivity significantly reduced as compared to the values obtained for equivalent junctions in undoped films.

Tailoring of high-Tc Josephson junctions by doping their electrodes

Appropriate doping of the electrodes of high-Tc Josephson junctions provides a means to systematically adjust the junctions’ electronic properties. This is demonstrated for the exemplary case of grain boundary junctions in bicrystalline Ca-doped YBa2Cu3O7−δ films. It is found that the critical current density is strongly increased and the normal state resistivity significantly reduced in comparison with the values obtained for equivalent junctions in undoped films.

Ultrafast optical tuning of ferromagnetism via the carrier density

1Department of Materials, ETH Zurich, Wolfgang-Pauli-Stra sse 10, 8093 Zurich, Switzerland 2Physikalisches Institut, Universität Bonn, Nussallee 12 , 53115 Bonn, Germany 3Institut für Physik, Universität Augsburg, Augsburg 861 35, Germany 4Department of Materials Science and Engineering, Cornell U niversity, Ithaca, New York 14853-1501, USA 5Kavli Institute at Cornell for Nanoscale Science, Ithaca, N ew York 14853-1501, USA and 6Max Planck Institute for Solid State Research, Heisenbergs traße 1, 70569 Stuttgart, Germany (Dated: May 11, 2014)Interest in manipulating the magnetic order by ultrashort laser pulses has thrived since it was observed that such pulses can be used to alter the magnetization on a sub-picosecond timescale. Usually this involves demagnetization by laser heating or, in rare cases, a transient increase of magnetization. Here we demonstrate a mechanism that allows the magnetic order of a material to be enhanced or attenuated at will. This is possible in systems simultaneously possessing a low, tunable density of conduction band carriers and a high density of magnetic moments. In such systems, the thermalization time can be set such that adiabatic processes dominate the photoinduced change of the magnetic order--the three-temperature model for interacting thermalized electron, spin and lattice reservoirs is bypassed. In ferromagnetic Eu(1-x)Gd(x)O, we thereby demonstrate the strengthening as well as the weakening of the magnetic order by ~10% and within ≤3 ps by optically controlling the magnetic exchange interaction.