P. England

As‐deposited high Tc and Jc superconducting thin films made at low temperatures

2000 A as‐deposited high Tc superconducting Y1Ba2Cu3O7−x films with zero‐resistance temperatures of ∼89 K and critical current densities about 0.7×106 A/cm2 at 77 K have been reproducibly fabricated at a substrate holder temperature of 650 °C, using pulsed laser deposition, with no post‐annealing. One key to these results is the injection of gaseous oxygen into the laser‐produced plume just in front of the target. In this way, the correct amount of oxygen is incorporated into the as‐grown film so that post‐deposition treatment becomes unnecessary. The in situ growth of such high Tc and Jc films is an important step in the use of the laser deposition technique to fabricate multilayer structures such as Josephson junctions.

Optical switching in a resonant tunneling structure

We describe the interaction of light pulses with a GaAs/AlAs resonant tunneling structure. We demonstrate that light with an average power of less than 10 μW can induce switching, and show that switching is accompanied by a change in the optical absorption. These results suggest a number of new applications for the resonant tunneling structure, including light‐by‐light switching.

Quantum interference effects in GaAs/GaAlAs bulk potential barriers

We have observed a large number of oscillations (10) in the current/voltage characteristic of GaAs/GaAlAs triangular potential barriers due to quantum interference effects. When an exact number of periods of a standing wave is present in the barrier there is a minimum in the transmission coefficient for electrons. As the bias is changed the electron wavelength is changed and there will once again be a minimum when the next complete period of the standing wave is incorporated. The observation of this quantum interference effect enables us to conclude that the scattering rate for hot electrons, high in the energy band of GaAs, is much less than previously assumed.

Ferrimagnetic rare‐earth orthoferrites: A new, magnetic substrate for the growth of epitaxial Y‐Ba‐Cu‐O thin films

We report, for the first time, growth of thin films of Y‐Ba‐Cu‐O by pulsed laser deposition onto [001] YbFeO3 single‐crystal substrates. A Tc,0 of 88 K was obtained along with a critical current greater than 105 A/cm2 at 77 K. X‐ray diffraction shows epitaxial growth of the 1‐2‐3 phase with the c axis oriented normal to the substrate plane. Rocking curve measurements show a 0.5° full width at half maximum, indicating good alignment of the layered structure. The composition was verified by Rutherford backscattering, while channeling results reveal a 17% yield, consistent with the rocking curve measurements. Growth of thin films of 1‐2‐3 on a magnetic substrate may open the way for potentially new applications.

Tunnel injection into a Wannier–Stark ladder

We describe tunneling measurements of the electronic states of superlattices in which an electric field has destroyed the miniband structure. The tunneling characteristics reveal that the minibands collapse into localized Wannier–Stark states, which can be resolved in the first two quantum wells of the superlattice. The positions can be accounted for quantitatively by a combination of electrostatic and quantum mechanical effects.

Resonant tunneling in crossed electric and magnetic fields

A wide (80 nm) double‐barrier resonant tunneling structure shows 25 features in the current‐voltage characteristic associated with resonances in the quantum well. When a transverse magnetic field is applied to the structure, the features weaken and shift to higher energies. We give a quantum mechanical description of this phenomenon which is able to account quantitatively for the magnitude of the shifts.

Tunneling measurement of the density of states of a superlattice

We have studied the current‐voltage characteristics of GaAs/AlGaAs samples consisting of an electron injector, a wide AlGaAs tunnel barrier, and a strongly coupled superlattice. The bias determines the energy of electrons injected into the superlattice, and the resulting tunnel current shows pronounced structure associated with minibands and band gaps. The position of the bands is in quantitative agreement with theory, and we resolve minibands with energies both below and above the AlGaAs conduction‐band offset.

Fabrication of submicrometer features in Y-Ba-Cu-O superconducting thin films

The authors explored some of the processes required to fabricate superconducting devices in Y-Ba-Cu-O thin films. Photolithography was used to define superconducting wires with dimensions from several micrometers to less than one micrometer in films grown both by laser and electron-beam deposition. Patterns were transferred with a variety of techniques including lift-off, chemical etching, and argon ion milling. Low-resistance contacts were reliably made by in situ Ar ion beam presputter and Au or Ag deposition. Submicrometer wires produced with these processes behaved as scaled-down bulk superconducting films; wire properties were determined largely by initial film morphology and wire geometry. No naturally occurring weak link behavior was observed in wires down to 0.5 mu m in width. The fabrication process and the electrical behavior of the resulting wires are discussed. >

Hot electron transport in high-lying minibands in semiconductor superlattices

Abstract We describe the first measurements of hot-electron transport in a semiconductor superlattice. The structures we have studied are three-terminal devices consisting of a superlattice sandwiched between a tunnel barrier injector on one side and an energy spectrometer on the other. These structures offer the unique flexibility of being able to inject carriers of arbitrary energy into the superlattice bandstructure and perform hot-electron spectroscopy on the carriers that have traversed the superlattice. We demonstrate that hot electron transport is possible in high-lying minibands (the second, third, and fourth) through the appearance of a well defined peak in the analyzed energy distribution. In addition, from the transfer function characteristic, we prove that the electrons are transporting coherently: that is, they obey the miniband dispersion relation. Finally, we describe a model for the electron transport.

Laser deposition of quality high T/sub c/ superconductor films

The laser deposition technique has been developed into a versatile method for depositing films of YBa/sub 2/Cu/sub 3/O/sub x/ material. A brief description is given of the method and important materials characteristics, such as composition, surface smoothness, crystalline structure, and interactions with substrates. T/sub c/ over 93 K, J/sub c/ in the 10/sup 6/ A/cm/sup 2/ range at 77 K, tolerance to high magnetic fields, low temperature deposition (near 650 degrees C), as-deposited superconducting films, epitaxial growth, and very smooth surfaces have been demonstrated. Relevance to practical application is briefly discussed. >