R. E. Howard

Propagation of terahertz bandwidth electrical pulses on YBa2Cu3O7−δ transmission lines on lanthanum aluminate

We study the propagation of terahertz bandwidth electrical pulses on high critical current density c‐axis oriented YBa2Cu3O7−δ (YBCO) coplanar transmission lines deposited epitaxially on the low dielectric loss substrate lanthanum aluminate (LaAlO3). The losses on the YBCO transmission line are lower than on an equivalent gold line on the same substrate at temperatures below 50 K. At higher temperatures, absorption and dispersion of the ultrashort electrical pulses are observed in reasonable agreement with Mattis–Bardeen theory.

Deviation of the quantum hall effect from exact quantization in narrow GaAs-AlxGa1−xAs heterostructure devices

Abstract We report the first observation of deviaton of the i=4 quantum Hall plateau from its quantized value, in narrow GaAs-AlxGa1−xAs quasi-1-d wires of width 2000A. The deviation arises in the form of aperiodic fluctuations as the magnetic field is varied, even though a deep minimum developes in longitudinal resistance. The fluctuation size grows with decreasing temperature and can be as large as 250Ω at 50mK. We suggest the observations arise from a combination of localization and Aharonov-Bohm quantum interference effects.

Time-domain measurement of the surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films up to 500 GHz

The surface resistance and surface reactance of high-quality YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films were determined from 10 GHz to 500 GHz using time-domain propagation measurements of picosecond electrical pulses on coplanar transmission lines. For the surface resistance, the authors find a square law dependence with frequency almost up to 500 GHz and a crossover of the losses of gold and YBa/sub 2/Cu/sub 3/O/sub 7/ films at 77 K at roughly 100 GHz. The pulse propagation is successfully modeled using the Mattis-Bardeen theory for the frequency-dependent conductivity of YBa/sub 2/Cu/sub 3/O/sub 7/. Good quantitative agreement of the surface resistance and surface reactance with this theory is obtained. The potential of YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films and equivalent gold and superconducting films for high-speed applications is studied theoretically.

YBa/sub 2/Cu/sub 3/O/sub 7/ superconductors for high-speed interconnects

The surface resistance of high-quality YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films measured over a frequency range from 10 to 500 GHz using high-speed optoelectronic techniques is discussed. A direct comparison is made with the surface resistance of gold and superconducting niobium conductors. Using the measured surface resistance, the propagation characteristic of interconnects based on YBa/sub 2/Cu/sub 3/O/sub 7/ superconductors at 77 K is simulated and compared to that of gold transmission lines at 77 K and superconducting niobium lines at 7.7 K.<<ETX>>

Ion beam induced damage and superlattice formation in epitaxial YBa2Cu3O7−δ thin films

We have studied the effect of ion beam irradiation on the microstructure of epitaxial YBa2Cu3O7−δ thin films. The ion beam induced defects are found to cluster in small (<100 A) disordered areas. The size and density of the disordered areas are found to increase with the ion fluence. The presence of these small disordered areas can lead to the reduction of phase coherence of the electron pair wave function. Ion beam irradiation is also found to reduce the orthorhombicity of the lattice structure. A new incommensurate superlattice phase due to ion beam induced defect ordering has also been observed.

Quantum transport in narrow MOSFET channels

Abstract Narrow MOSFET channels are investigated as quasi-one-dimensional electron systems containing a small number of electrons. New multiterminal devices allow resistance measurements of adjacent channel segments as small as 30 nm wide and 100 nm long. Even at high electron densities, the quantum transmission (conductance) through small segments depends strongly on electron energy (gate voltage) and magnetic field, and the behavior of adjacent segments is uncorrelated. This conductance structure saturates at low temperatures as inelastic scattering becomes negligible, and the strongest transmission “resonances” remain temperature independent as high as 20 K. The “universal conductance fluctuations” recently calculated by Lee and Stone may provide a framework for understanding these results. In larger devices, the change of scattering due to the trapping of a single electron at a particular interface trap can be isolated, and the surrounding perturbation can be spatially resolved.

Fabrication And Characterization Of An YBa2Cu3O7/Au/YBa2Cu3O7 S-N-S Microbridge

We have demonstrated proximity effect coupling between a high transition temperature superconductor and a normal metal. A device with a 1 Am long gold microbridge coupling two evaporated films of YBa2Cu3O7 has a dc supercurrent and exhibits the ac Josephson effect when irradiated with microwaves. These high quality S-N interfaces should have application to fundamental studies of the superconductor as well as to electronic devices.

Submicron tunnel junctions

We have fabricated a variety of small-area, superconducting tunnel junctions and simple superconducting interferometer circuits using novel e-beam and optical lithographic techniques. These Pb-oxide-Pb(In) tunnel junctions are made using self-aligning processes which involve multiple oblique evaporations through suspended liftoff stencils formed in two-layer resist systems. Oxide barriers are grown in situ immediately after evaporation of base electrode films. Junction areas range from 10-9cm2(optically patterned) to about 10-10cm2(e-beam patterned). Current densities as high as 2×106A/cm2have been attained. The high current-density junctions show only small hysteresis at 4K and are completely non-hysteretic at higher temperatures. These junctions have been used to make low-hysteresis interferometers with a current gain of 3. A positive feedback scheme is described which provides sharper switching characteristics in non-latching interferometers.

Switching noise in YBa/sub 2/Cu/sub 3/O/sub x/ 'macrobridges'

Intermittent switching was observed in the voltage current characteristics of thin-film bridges of YBa/sub 2/Cu/sub 3/O/sub x/. At a fixed bias point there are multiple metastable voltage states with lifetimes which depend on the bias current and applied magnetic field. The microbridges are made of thin ( >

Magneto-quantum transport in a quasi-one-dimensional GaAs/AlxGa1−xAs ring

Abstract We study the magneto-transport properties of a quasi-ID GaAs/Al x Ga 1− x As ring of mobility 40 000 cm 2 /V·a at 4 K. The inelastic scattering length is about 4000 A while the sample length is of the order of 3 μm, so that we are in the diffusive regime. A variety of quantum interference phenomena are observed, including negative magneto-resistance, hole Aharonov-Bohm effect, and aperiodic/quasiperiodic resistance fluctuations. This rich variety enables us to estimate the sample conducting width and electron phase coherence length, l θ , so that we may attempt to obtain a consistent picture of these related phenomena. In addition, we obtain a magnetic field scale on which the hole oscillations disappear.