R. S. Newrock

Two‐dimensional electron gas properties of AlGaN/GaN heterostructures grown on 6H–SiC and sapphire substrates

High quality Al0.15Ga0.85N/GaN heterostructures have been fabricated on 6H–SiC and sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). A temperature independent mobility, indicative of the presence of a two‐dimensional electron gas (2DEG), was observed in all samples below 80 K. The highest low temperature 2DEG mobility, 7500 cm2/V s, was measured in AlGaN/GaN grown on 6H–SiC; the sheet carrier density was 6×1012 cm−2. Strong, well resolved, Shubnikov–de Haas oscillations were observed in fields as low as 3 T and persisted to temperatures as high as 15 K. Hall effect measurements also revealed the presence of well‐defined plateaus in the Hall resistance. The high quality 2DEG properties of the AlGaN/GaN heterostructures grown on 6H–SiC are attributed to the absence of significant parallel conduction paths in the material.

All-electric quantum point contact spin-polarizer

The controlled creation, manipulation and detection of spin-polarized currents by purely electrical means remains a central challenge of spintronics. Efforts to meet this challenge by exploiting the coupling of the electron orbital motion to its spin, in particular Rashba spin-orbit coupling, have so far been unsuccessful. Recently, it has been shown theoretically that the confining potential of a small current-carrying wire with high intrinsic spin-orbit coupling leads to the accumulation of opposite spins at opposite edges of the wire, though not to a spin-polarized current. Here, we present experimental evidence that a quantum point contact -- a short wire -- made from a semiconductor with high intrinsic spin-orbit coupling can generate a completely spin-polarized current when its lateral confinement is made highly asymmetric. By avoiding the use of ferromagnetic contacts or external magnetic fields, such quantum point contacts may make feasible the development of a variety of semiconductor spintronic devices.

The Two-Dimensional Physics of Josephson Junction Arrays

This chapter discusses the two-dimensional physics of Josephson junction arrays. Josephson junction arrays consist of islands of superconductor, usually arranged on an ordered lattice, coupled by Josephson junctions. They may be divided into classical and quantum arrays depending on the ratio of the Josephson coupling energy to the charging energy. Josephson junction arrays may also be divided into overdamped and underdamped arrays, referring to the fact that the equation of motion for a single Josephson junction is identical to a damped pendulum. The presence of vortices is one of the natural consequences of arranging such junctions in a two-dimensional lattice. Large arrays have proven to be very useful model systems for studying a wide variety of other physical problems, for example, phase transitions in frustrated and random systems, the dynamics of coupled non-linear systems and macroscopic quantum effects. Classical two-dimensional arrays may be shown to be isomorphic to a two-dimensional XY spin system—they are physical representations of the XY model, which is a two-dimensional lattice of spins free to rotate in the XY plane. Classical two-dimensional arrays are used in zero magnetic fields for studying phase transitions such as the Kosterlitz–Thouless–Berezinskii transition, for studying the effects of disorder on phase transitions, and for studying dimensional crossover effects in phase transitions.

Paramagnetic Meissner effect in multiply-connected superconductors

We have measured a paramagnetic Meissner effect in

Possible origin of the 0.5 plateau in the ballistic conductance of quantum point contacts

\mathrm{Nb}\char21{}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\char21{}\mathrm{Nb}

Experimental studies of Coulomb drag between ballistic quantum wires

Josephson-junction arrays using a scanning superconducting quantum interference device microscope. The arrays exhibit diamagnetism for some cooling fields and paramagnetism for other cooling fields. The measured mean magnetization is always small,

Coulomb drag between ballistic one-dimensional electron systems

〈{\ensuremath{\Phi}}_{\mathrm{tot}}\ensuremath{-}{\ensuremath{\Phi}}_{\mathrm{ext}}〉l0.3{\ensuremath{\Phi}}_{0}

Observation of a 0.5 conductance plateau in asymmetrically biased GaAs quantum point contact

(in terms of flux per unit cell of the array, where

Finite-size effects and dynamical scaling in two-dimensional Josephson junction arrays

{\ensuremath{\Phi}}_{0}

Interface roughness scattering in thin, undoped GaInP/GaAs quantum wells

is the flux quantum), for the range of cooling fields investigated