D. Anderson

Probing Spin-Charge Separation in a Tomonaga-Luttinger Liquid

Electron Breakdown An electron possesses charge and spin. In general, these properties are confined to the electron. However, in strongly interacting many-body electronic systems, such as one-dimensional wires, it has long been theorized that the charge and spin should separate. There have been tantalizing glimpses of this separation experimentally, but questions remain. Jompol et al. (p. 597) looked at the tunneling current between an array of one-dimensional wires and a two-dimensional electron gas and argue that the results reveal a clear signature of spin-charge separation. Electronic spin and charge respond differently during tunneling between low-dimensional electron systems. In a one-dimensional (1D) system of interacting electrons, excitations of spin and charge travel at different speeds, according to the theory of a Tomonaga-Luttinger liquid (TLL) at low energies. However, the clear observation of this spin-charge separation is an ongoing challenge experimentally. We have fabricated an electrostatically gated 1D system in which we observe spin-charge separation and also the predicted power-law suppression of tunneling into the 1D system. The spin-charge separation persists even beyond the low-energy regime where the TLL approximation should hold. TLL effects should therefore also be important in similar, but shorter, electrostatically gated wires, where interaction effects are being studied extensively worldwide.

Spin-up of a stratified ocean, with applications to upwelling

Abstract The method by which an ocean, initially stratified but motionless, adjusts to a suddenly applied wind stress is examined. For wind stress in the east-west direction, the zonal velocity in the ocean interior builds up linearly with time until a long planetary wave arrives from the eastern boundary. Then the interior flow stops increasing and oscillates about a steady value. At the west, a boundary layer forms and gets progressively thinner. The rate of thinning of this boundary layer is reduced by the arrival of long planetary waves from the east but is not stopped. A comparison is made between the barotropic and baroclinic responses. A comparison is also made between the upwelling at the eastern boundary induced by a longshore wind stress when there are no planetary waves (f-plane) and when there are planetary waves (s-plane). In the former, upwelling is restricted to within a distance of ∼ 3 km of the coast, whereas when planetary waves are included the width of the upwelling zone increases with time. On both f- and s-plane, Kelvin waves carry energy poleward along the eastern boundary. For a wind stress which varies sinusoidally with y this results in the mean upwelling being nearly 90° out of phase with the wind stress. The amplitude of the Kelvin wave is damped on the s-plane but suffers no attenuation on the f-plane. The difference between the results for coastal upwelling on an f-plane and that on a s-plane increases as the north-south scale of the forcing is increased. In the case where the scale is infinite, the f-plane upwelling increases indefinitely with time whereas the s-plane solution attains a steady value.

On-demand single-electron transfer between distant quantum dots

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times—a cumulative distance of 0.25 mm—without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

Spin-up of a stratified ocean, with topography

Abstract This paper examines the linear response of a motionless stratified ocean in the presence of large-scale topography aligned north-south to a wind stress turned on at time zero. An unstratified ocean is first examined. Its response differs from the barotropic response discussed by Anderson and Gill (1975, Spin-up of a stratified ocean, with application to upwelling. Deep-Sea Research , 22 , 583–596) in two major respects: a higher frequency ‘basin mode’ dominates much of the solution, and thinning boundary layers can occur to the east of the topography. The position of the topography strongly affects the properties of the solution. When stratification is included, both barotropic and baroclinic responses are possible. The baroclinic response is affected very little by topography, and essentially the baroclinic spin-up resembles the flat-bottom case. The barotropic response, however, is very different from the unstratified case. Initially the response resembles the unstratified case—in particular the high frequency basin modes are unaffected by stratification—but the slow westward propagation of the baroclinic planetary wave leaves in its wake what is essentially a steady, flat-bottom Sverdrup solution for both barotropic and baroclinic modes. This can be interpreted as there being no motion at great depth, which removes the topographic effect.

An overview of coupled ocean‐atmosphere models of El Niño and the Southern Oscillation

This review summarizes, organizes, and compares coupled ocean-atmosphere models of El Nino and the Southern Oscillation (ENSO). The models are arranged in a hierarchy of increasing dynamical complexity, and useful categories for this purpose are (1) conceptual and simple models, (2) intermediate models, and (3) coupled general circulation models (GCMs). Conceptual and simple models illustrate three potentially important mechanisms of ocean-atmospheric interaction in the tropics: the slow propagation of oceanic Rossby waves across the Pacific Ocean; the generation of two equilibrium states (a normal state and an ENSO state) with a trigger that switches the system from one state to the other; and the development of coupled instabilities. Intermediate models develop oscillations in two different ways. In one type, coupled instabilities appear in the western and central ocean, intensify and propagate into the eastern ocean, and eventually dissipate there. In the other, the instabilities are confined to the central and eastern oceans, and oceanic Rossby waves that reflect from the western boundary are involved in the onset and decay of ENSO events. Coupled GCMs have so far had limited success in simulating realistic ENSO events, one reason for this being that even moderate errors in model physics can cause solutions to drift away from realistic climatologies. Nevertheless, ENSO-like oscillations also occur in coupled GCMs, apparently arising from several types of processes. None of the more complex models have yet been shown to have the ability to generate two equilibrium states.

Beta-dispersion of low-frequency Rossby waves

Abstract An investigation is made into the dispersion of oceanic internal Rossby waves at annual and semi-annual frequencies. Turning of the group velocity vector due to latitudinal variations in the radius of deformation cannot be neglected, particularly in basins as large as the Pacific. This turning allows disturbances to propagate from high lattitudes into the equatorial zone and distorts the solutions in the western part of the basin. For no mean flow, and a coastline aligned north-south, an almost exact focus of wave energy is found very close to the equator at a distance of just under πc/4ω from the eastern boundary, where c is the eigenspeed of a high-frequency internal wave mode, and ω is the angular frequency of the low-frequency wave being studied. The focus depends on a long meridional wavelength excited at the coast, and a frequency small compared with c/a , where a is the radius of the Earth. For the lowest baroclinic mode and waves of annual period, this distance is about 12 000 km. Equivalence of the ray theory and the theory of equatorial meridional modes is demonstrated for the simple cases where the latter applies. The effects of mean currents and irregular coastlines are examined. Barotropic mean currents may change the turning latitude and ray shapes, inducing critical layers and enhancing reflection. Baroclinic mean currents are seen to affect the rays by simply changing the speed in proportion to the depth of the thermocline. As long as the mean currents are geostrophically balanced, no “effective beta” term from variations in the thermocline depth appears, in contrast to the topographic Rossby wave problem.

Narrow emission linewidths of positioned InAs quantum dots grown on pre-patterned GaAs(100) substrates

We report photoluminescence measurements on a single layer of site-controlled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) on pre-patterned GaAs(100) substrates with a 15 nm re-growth buffer separating the dots from the re-growth interface. A process for cleaning the re-growth interface allows us to measure single dot emission linewidths of 80 µeV under non-resonant optical excitation, similar to that observed for self-assembled QDs. The dots reveal excitonic transitions confirmed by power dependence and fine structure splitting measurements. The emission wavelengths are stable, which indicates the absence of a fluctuating charge background in the sample and confirms the cleanliness of the re-growth interface.

Site-Control of InAs Quantum Dots using Ex-Situ Electron-Beam Lithographic Patterning of GaAs Substrates

Conventional e-beam lithography followed by either dry or wet etching of small holes in GaAs substrates has been used to control the position of InAs self-assembled quantum dots. The dependence of hole occupancy on both hole area and hole depth has been investigated. We show a range of hole sizes where greater than 30% of sites contain a single dot with up to 60% single dot occupancy seen for dry-etched holes ~60 nm wide, ~35 nm deep and for wet-etched holes ~90 nm wide, ~20 nm deep. Single dot luminescence from these placed dots is demonstrated despite only a 10 nm GaAs buffer between dots and regrowth interface.

Modulation of single quantum dot energy levels by a surface-acoustic-wave

This letter presents an experimental investigation into the effect of a surface-acoustic-wave (SAW) on the emission of a single InAs quantum dot. The SAW causes the energy of the transitions within the dot to oscillate at the frequency of the SAW, producing a characteristic broadening of the emission lines in their time-averaged spectra. This periodic tuning of the transition energy is used as a method to regulate the output of a device containing a single quantum dot and we study the system as a high-frequency periodic source of single photons.

Enhancement and suppression of spontaneous emission by temperature tuning InAs quantum dots to photonic crystal cavities

We report on the control of the spontaneous emission rates in InAs self-assembled quantum dots weakly coupled to the mode of a modified H1 defect cavity in a two-dimensional photonic crystal slab. Changes in sample temperature are used to spectrally tune the exciton emission from a single quantum dot to the monopole mode of the microcavity. A Purcell enhancement of the spontaneous emission rate of up to a factor of 11.4 is seen on-resonance, while suppression by up to a factor of 4.4 is seen off-resonance. Also, a two orders of magnitude increase in the intensity of light detected from the exciton is measured when compared to a quantum dot in bulk GaAs.