A. C. Gossard

Gate-controlled spin-orbit quantum interference effects in lateral transport.

In situ control of spin-orbit coupling in coherent transport using a clean GaAs/AlGaAs two-dimensional electron gas is realized, leading to a gate-tunable crossover from weak localization to antilocalization. The necessary theory of 2D magnetotransport in the presence of spin-orbit coupling beyond the diffusive approximation is developed and used to analyze experimental data. With this theory the Rashba contribution and linear and cubic Dresselhaus contributions to spin-orbit coupling are separately estimated, allowing the angular dependence of spin-orbit precession to be extracted at various gate voltages.

Using soft lithography to fabricate GaAs/AlGaAs heterostructure field effect transistors

This letter describes the fabrication of functional GaAs/AlGaAs field effect transistors using micromolding in capillaries—a representative soft lithographic technique. The fabrication process involved three soft lithographic steps and two registration steps. Room temperature characteristics of these transistors resemble those of field effect transistors fabricated by photolithography. The fabrication of functional microelectronic devices using multilayer soft lithography establishes the compatibility of these techniques with the processing methods used in device fabrication, and opens the door for their development as a technique in this area.

Superconductivity of Nb3Al

Abstract Nb 3 Al has been found to undergo a transition to the superconducting state starting at 18.8°K. This is the highest known transition temperature for any single phase binary alloy with the β-tungsten structure. A number of its electronic properties including specific heat, magnetic susceptibility and Knight Shift have been studied. It is concluded that the one-dimensional model postulated by Labbe and Friedel to explain the properties of other high temperature superconductors is inconsistent with these measurements.

Absence of Antiferromagnetism in NbSe2 and TaSe2

NbSe2 and TaSe2 are layered metallic compounds in which antiferromagnetic ordering has previously been deduced from anomalies in magnetic and electrical behavior. The anomalies consist of magnetic susceptibility maxima, Hall coefficient sign reversals and for TaSe2, an electrical resistivity kink, at T∼30°K and T∼110°K for the two compounds respectively. We have now studied the nuclear magnetic resonances of 93Nb in NbSe2 and of 77Se in TaSe2 above and below these temperatures. Observation of resonances from T=4.2°K to T=300°K shows that the materials are definitely not magnetically ordered. In NbSe2, an axially symmetric 93Nb quadrupole powder pattern is observed at 77° and 300°K but at 4.2°K is found to be further split. This implies that the transition is instead a low‐temperature structural distortion. This distortion may be responsible for the previously observed anomalies and for the absence of magnetic order. On the basis of the observed change of the electric field gradient, the corresponding chan...

A study of intersubband scattering in GaAs-AlxGa1-xAs heterostructures by means of a parallel magnetic field

Abstract We demonstrate the influence of intersubband scattering in GaAs/AlGaAs heterostructures by means of a parallel magnetic field. For samples with a carrier density such that the second subband is lightly populated, the mobility can be increased by about 50% due to the suppression of intersubband scattering, when the subband separation is increased by a strong magnetic field parallel to the interface. This gives rise to a strong negative magnetoresistance. The onset of this negative magnetoresistance can be shifted by the application of a back-side gate voltage which varies the Fermi energy.

Mode spectroscopy and level coupling in ballistic electron waveguides

A tunable quantum point contact with modes occupied in both transverse directions is studied by magnetotransport experiments. We find conductance quantization that can be suppressed by degeneracies of one-dimensional modes. The mode spectrum is determined as a function of the magnetic field of different orientations with respect to the quantum wire. A magnetic field applied parallel to the direction of the current flow couples the modes. This can be described by an extension of the Darwin-Fock model. Anticrossings are observed as a function of the parallel magnetic field, but not for zero field or perpendicular field directions, indicating coupling of the subbands due to nonparabolicity in the electrical confinement.

LOW-DENSITY HIGH-MOBILITY ELECTRON GAS IN WIDE PARABOLIC GAAS/ALXGA1-XAS WELLS

Remotely doped wide parabolic GaAs/AlxGa1−xAs wells are used to create thick (≳ 1000 A) layers of high‐mobility (≳ 2×105 cm2/Vu2009s) electron gas with three‐dimensional densities below (by a factor ∼3) the metal‐insulator transition for doped GaAs. The temperature dependences of the Hall mobility and sheet density show no qualitative changes in a series of three samples spanning the metal‐insulator transition. Shubnikov–de Haas oscillation measurements are used to determine the width of the electron gas layers.

Light scattering in GaAs parabolic quantum wells

Abstract We report the observation of electronic light scattering in photoexcited parabolic GaAsue5f8Al x Ga 1-x As quantum wells. The spectra show sharp peaks corresponding to transitions between sublevels in the conduction band. The precision of the measurements allows a test of the simple harmonic oscillator model for the energy level structure. The energy gap and alloy composition of the Al x Ga 1-x As layers are determined from resonance Raman scattering by optical phonons. We use these results to calculate the conduction band offset Q e . We obtain Q e ∼ 0.7, in agreement with recent experiments in square quantum wells.

Magnetic ordering of a d1 compound: VF4

Abstract Magnetic susceptibility and nuclear magnetic resonance above and below the ordering temperature show that VF4 is an S = 1 2 , 3d1 canted antiferromagnet with a transition temperature near 28°K and a canting angle of ∼ .02 rad. The compound is not an example of a proposed nonmagnetic singlet ground state of resonating covalent bonds.

Covalent distribution of spin in V2O3:O17 nuclear resonance

Abstract O 17 nuclear magnetic resonance has been observed in metallic V 2 O 3 with frequency shifts from (−0.10 ± 0.02)-(−0.05 ± 0.02) per cent between 170 and 460°K respectively, a linewidth of 37 ± 5 oe and spin-lattice relaxation rate 1/ T 1 ≈ 60 sec −1 at 296°K. From these quantities, covalency parameters f s /2 S = − 0.35 × 10 −3 and ⨍ π /2S ≈ − 0.07 are calculated. One of the two vanadium 3 d electrons in the antiferromagnetic state below the 170°K metal-insulator transition is inferred to lie in a non-magnetic state, while covalent charge transfer augments the spin moment of the other 3 d electron to the observed 1.2 μ B .