arXiv subjects
D. A. Ritchie
Publications and source records attributed to D. A. Ritchie.
Measurements of Composite Fermion Conductivity Dependence on Carrier Density
We present the first experimental study of the carrier density dependence of the composite fermion conductivity $σ^{CF}_{xx}$ at Landau level filling factors $ν= {1/2}$ and $ν= {3/2}$ in high-quality front-gated GaAs/Al$_{0.33}$Ga$_{0.67}$As heterostructures. Extracting $α$ from the power law ln$(σ^{CF}_{xx}) \propto$ ln$(n_e)^α$ shows that $α\approx 1$. The measured $α \approx 1$ is placed between the predicted value 3/4 in the strong random magnetic field regime, and 3/2 in the weak random magnetic field regime. Comparisons between our results and theory are discussed.
Can the conductance of an adiabatic ballistic constriction be lower than $2e^2/h$?
We have performed four-terminal conductance measurements of a one-dimensional (1D) channel in which it is possible to modulate the potential profile using three overlaying finger gates. In such a 1D ballistic structure we have observed, {\em for the first time,} that the conductance steps show a gradual decrease from $2e^2/h$ to $0.97 \times 2e^2/h$ with increasing negative finger gate voltage in a short, clean 1D constriction. We suggest this phenomenon is due to differing shifts of 1D subbands with changing spilt-gate voltage. Both a simple analytical estimate for an adiabatic constriction and, realistic modeling of the device, give the same magnitude of the conductance decrease as observed in our experiments.
On the low-field insulator-quantum Hall conductor transitions
We studied the insulator-quantum Hall conductor transition which separates the low-field insulator from the quantum Hall state of the filling factor $ν=4$ on a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. To enter the $ν=4$ quantum Hall state directly from the low-field insulator, the two-dimensional system undergoes a crossover from the low-field localization to Landau quantization. The crossover, in fact, covers a wide range with respect to the magnetic field rather than only a small region near the critical point of the insulator-quantum Hall conductor transition.
Surface acoustic wave-induced electroluminescence intensity oscillation in planar light-emitting devices
Electroluminescence emission from surface acoustic wave-driven light-emitting diodes (SAWLEDs) is studied by means of time-resolved techniques. We show that the intensity of the SAW-induced electroluminescence is modulated at the SAW frequency (~1 GHz), demonstrating electron injection into the p-type region synchronous with the SAW wavefronts.
Coulomb-Blockade directional coupler
A tunable directional coupler based on Coulomb Blockade effect is presented. Two electron waveguides are coupled by a quantum dot to an injector waveguide. Electron confinement is obtained by surface Schottky gates on single GaAs/AlGaAs heterojunction. Magneto-electrical measurements down to 350 mK are presented and large transconductance oscillations are reported on both outputs up to 4.2 K. Experimental results are interpreted in terms of Coulomb Blockade effect and the relevance of the present design strategy for the implementation of an electronic multiplexer is underlined.
Spontaneous current generation in gated nanostructures
We have observed an unusual dc current spontaneously generated in the conducting channel of a short-gated GaAs transistor. The magnitude and direction of this current critically depend upon the voltage applied to the gate. We propose that it is initiated by the injection of hot electrons from the gate that relax via phonon emission. The phonons then excite secondary electrons from asymmetrically distributed impurities in the channel, which leads to the observed current.
Fano factor reduction on the 0.7 structure in a ballistic one-dimensional wire
We have measured the non-equilibrium current noise in a ballisticone-dimensional wire which exhibits an additional conductanceplateau at $0.7\times2e^2/h$. The Fano factor shows a clearreduction on the 0.7 structure, and eventually vanishes uponapplying a strong parallel magnetic field. These results provideexperimental evidence that the 0.7 structure is associated withtwo conduction channels which have different transmissionprobabilities.
Three Key Questions on Fractal Conductance Fluctuations: Dynamics, Quantization and Coherence
Recent investigations of fractal conductance fluctuations (FCF) in electron billiards reveal crucial discrepancies between experimental behavior and the semiclassical Landauer-Buttiker (SLB) theory that predicted their existence. In particular, the roles played by the billiard's geometry, potential profile and the resulting electron trajectory distribution are not well understood. We present measurements on two custom-made devices - a 'disrupted' billiard device and a 'bilayer' billiard device - designed to probe directly these three characteristics. Our results demonstrate that intricate processes beyond those proposed in the SLB theory are required to explain FCF.
A dynamic localization of 2D electrons at mesoscopic length scales
We have investigated the local magneto-transport in high-quality 2D electron systems at low carrier densities. The positive magneto-resistance in perpendicular magnetic field in the strongly insulating regime has been measured to evaluate the spatial concentration of localized states within a mesoscopic region of the samples. An independent measurement of the electron density within the same region shows an unexpected correspondence between the density of electrons in the metallic regime and that of the localized states in the insulating phase. We have argued that this correspondence manifests a rigid distribution of electrons at low densities.
Evolution of the bilayer nu = 1 quantum Hall state under charge imbalance
We use high-mobility bilayer hole systems with negligible tunneling to examine how the bilayer nu = 1 quantum Hall state evolves as charge is transferred from one layer to the other at constant total density. We map bilayer nu = 1 state stability versus imbalance for five total densities spanning the range from strongly interlayer coherent to incoherent. We observe competition between single-layer correlations and interlayer coherence. Most significantly, we find that bilayer systems that are incoherent at balance can develop spontaneous interlayer coherence with imbalance, in agreement with recent theoretical predictions.
Interaction Correction to the Longitudinal Conductivity and Hall Resistivity in High Quality Two-Dimensional GaAs Electron and Hole Systems
We present a systematic study of the corrections to both the longitudinal conductivity and Hall resistivity due to electron-electron interactions in high quality GaAs systems using the recent theory of Zala et al. [Phys. Rev. B 64, 214204 (2001)]. We demonstrate that the interaction corrections to the longitudinal conductivity and Hall resistivity predicted by the theory are consistent with each other. This suggests that the anomalous metallic drop in resistivity at B=0 is due to interaction effects and supports the theory of Zala et al.
Surface-acoustic-wave driven planar light-emitting device
Electroluminescence emission controlled by means of surface acoustic waves (SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital transducers for SAW generation were integrated onto pLEDs fabricated following the scheme which we have recently developed. Current-voltage, light-voltage and photoluminescence characteristics are presented at cryogenic temperatures. We argue that this scheme represents a valuable building block for advanced optoelectronic architectures.
Shot Noise in Mesoscopic Transport Through Localised States
We show that shot noise can be used for studies of hopping and resonant tunnelling between localised electron states. In hopping via several states, shot noise is seen to be suppressed compared with its classical Poisson value $S_I=2eI$ ($I$ is the average current) and the suppression depends on the distribution of the barriers between the localised states. In resonant tunnelling through a single impurity an enhancement of shot noise is observed. It has been established, both theoretically and experimentally, that a considerable increase of noise occurs due to Coulomb interaction between two resonant tunnelling channels.
Interactions in high-mobility 2D electron and hole systems
Electron-electron interactions mediated by impurities are studied in several high-mobility two-dimensional (electron and hole) systems where the parameter $k_BTτ/\hbar $ changes from 0.1 to 10 ($τ$ is the momentum relaxation time). This range corresponds to the \textit{intermediate} and \textit {ballistic} regimes where only a few impurities are involved in electron-electron interactions. The interaction correction to the Drude conductivity is detected in the temperature dependence of the resistance and in the magnetoresistance in parallel and perpendicular magnetic fields. The effects are analysed in terms of the recent theories of electron interactions developed for the ballistic regime. It is shown that the character of the fluctuation potential (short-range or long-range) is an important factor in the manifestation of electron-electron interactions in high-mobility 2D systems.
Non-invasive detection of the evolution of the charge states of a double dot system
Coupled quantum dots are potential candidates for qubit systems in quantum computing. We use a non-invasive voltage probe to study the evolution of a coupled dot system from a situation where the dots are coupled to the leads to a situation where they are isolated from the leads. Our measurements allow us to identify the movement of electrons between the dots and we can also identify the presence of a charge trap in our system by detecting the movement of electrons between the dots and the charge trap. The data also reveals evidence of electrons moving between the dots via excited states of either the single dots or the double dot molecule.
Quantized charge pumping through a quantum dot by surface acoustic waves
We present a realization of quantized charge pumping. A lateral quantum dot is defined by metallic split gates in a GaAs/AlGaAs heterostructure. A surface acoustic wave whose wavelength is twice the dot length is used to pump single electrons through the dot at a frequency f=3GHz. The pumped current shows a regular pattern of quantization at values I=nef over a range of gate voltage and wave amplitude settings. The observed values of n, the number of electrons transported per wave cycle, are determined by the number of electronic states in the quantum dot brought into resonance with the fermi level of the electron reservoirs during the pumping cycle.
Imaging Fractal Conductance Fluctuations and Scarred Wave Functions in a Quantum Billiard
We present scanning-probe images and magnetic-field plots which reveal fractal conductance fluctuations in a quantum billiard. The quantum billiard is drawn and tuned using erasable electrostatic lithography, where the scanning probe draws patterns of surface charge in the same environment used for measurements. A periodicity in magnetic field, which is observed in both the images and plots, suggests the presence of classical orbits. Subsequent high-pass filtered high-resolution images resemble the predicted probability density of scarred wave functions, which describe the classical orbits.