Double-layer-gate architecture for few-hole GaAs quantum dots

We report the fabrication of single and double hole quantum dots using a double-layer-gate design on an undoped accumulation mode AlxGa1-xAs/GaAs heterostructure. Electrical transport measurements of a single quantum dot show varying addition energies and clear excited states. In addition, the two-level-gate architecture can also be configured into a double quantum dot with tunable inter-dot coupling

Metal-insulator transition at B=0 in an ultra-low density (rs=23r_{s}=23) two dimensional GaAs/AlGaAs hole gas

We have observed a metal-insulator transition in an ultra-low density two dimensional hole gas formed in a high quality GaAs-AlGaAs heterostructure at B=0. At the highest carrier density studied ($p_{s}=2.2x10^{10} cm^{-2}, r_{s}=16$) the hole gas is strongly metallic, with an exceptional mobility of $425,000 cm^{2}V^{-1}s^{-1}$. The low disorder and strength of the many-body interactions in this sample are highlighted by the observation of re-entrant metal insulator transitions in both the fractional ($\nu < 1/3$) and integer ($2 > \nu > 1$) quantum Hall regimes. On reducing the carrier density the temperature and electric field dependence of the resistivity show that the sample is still metallic at $p_{s}=1.3x10^{10} cm^{-2}$ ($r_{s}=21$), becoming insulating at $p_{s}{\simeq}1x10^{10} cm^{-2}$. Our results indicate that electron-electron interactions are dominant at these low densities, pointing to the many body origins of this metal-insulator transition. We note that the value of $r_{s}$ at the transition ($r_{s}=23 +/- 2$) is large enough to allow the formation of a weakly pinned Wigner crystal, and is approaching the value calculated for the condensation of a pure Wigner crystal.Comment: 4 pages, latex, 4 postscript figures, submitted to EP2DS-12 on 21st August 1997, to appear in Physica

Anisotropic Pauli Spin Blockade of Holes in a GaAs Double Quantum Dot

Electrically defined semiconductor quantum dots are attractive systems for spin manipulation and quantum information processing. Heavy-holes in both Si and GaAs are promising candidates for all-electrical spin manipulation, owing to the weak hyper- fine interaction and strong spin-orbit interaction. However, it has only recently become possible to make stable quantum dots in these systems, mainly due to difficulties in device fabrication and stability. Here we present electrical transport measurements on holes in a gate-defined double quantum dot in a GaAs/AlxGa1−xAs heterostructure. We observe clear Pauli spin blockade and demonstrate that the lifting of this spin blockade by an external magnetic field is highly anisotropic. Numerical calculations of heavy-hole transport through a double quantum dot in the presence of strong spin-orbit coupling show quantitative agreement with experimental results and suggest that the observed anisotropy can be explained by both the anisotropic effective hole g-factor and the surface Dresselhaus spin-orbit interaction

Deconstruction of the Trap Model for the New Conducting State in 2D

A key prediction of the trap model for the new conducting state in 2D is that the resistivity turns upwards below some characteristic temperature, $T_{\rm min}$. Altshuler, Maslov, and Pudalov have argued that the reason why no upturn has been observed for the low density conducting samples is that the temperature was not low enough in the experiments. We show here that $T_{\rm min}$ within the Altshuler, Maslov, and Pudalov trap model actually increases with decreasing density, contrary to their claim. Consequently, the trap model is not consistent with the experimental trends.Comment: Published version of Deconstructio

Improving reproducibility of quantum devices with completely undoped architectures

The reproducible operation of quantum electronic devices is a key requirement for future quantum information processing and spintronics applications. Traditionally, quantum devices have been fabricated from modulation-doped heterostructures, where there is an intrinsic lack of reproducibility due to the random potential from ionized donors. Here, we show that we can greatly improve reproducibility over modulation-doped devices by using a completely undoped architecture, with superior uniformity in the confinement potential and more consistent operating voltages for both electron and hole devices. Our results demonstrate that undoped heterostructures have significant advantages over modulation doping for reproducible manufacturing of quantum devices

The relative importance of electron-electron interactions compared to disorder in the two-dimensional "metallic" state

The effect of substrate bias and surface gate voltage on the low temperature resistivity of a Si-MOSFET is studied for electron concentrations where the resistivity increases with increasing temperature. This technique offers two degrees of freedom for controlling the electron concentration and the device mobility, thereby providing a means to evaluate the relative importance of electron-electron interactions and disorder in this so-called ``metallic'' regime. For temperatures well below the Fermi temperature, the data obey a scaling law where the disorder parameter ($k_{\rm{F}}l$), and not the concentration, appears explicitly. This suggests that interactions, although present, do not alter the Fermi-liquid properties of the system fundamentally. Furthermore, this experimental observation is reproduced in results of calculations based on temperature-dependent screening, in the context of Drude-Boltzmann theory.Comment: 5 pages, 6 figure

Temperature dependent resistivity of spin-split subbands in GaAs 2D hole system

We calculate the temperature dependent resistivity in spin-split subbands induced by the inversion asymmetry of the confining potential in GaAs 2D hole systems. By considering both temperature dependent multisubband screening of impurity disorder and hole-hole scattering we find that the strength of the metallic behavior depends on the symmetry of the confining potential (i.e., spin-splitting) over a large range of hole density. At low density above the metal-insulator transition we find that effective disorder reduces the enhancement of the metallic behavior induced by spin-splitting. Our theory is in good qualitative agreement with existing experiments

Tomonaga-Luttinger features in the resonant Raman spectra of quantum wires

The differential cross section for resonant Raman scattering from the collective modes in a one dimensional system of interacting electrons is calculated non-perturbatively using the bosonization method. The results indicate that resonant Raman spectroscopy is a powerful tool for studying Tomonaga-Luttinger liquid behaviour in quasi-one dimensional electron systems.Comment: 4 pages, no figur

Bias-voltage induced phase-transition in bilayer quantum Hall ferromagnets

We consider bilayer quantum Hall systems at total filling factor $\nu=1$ in presence of a bias voltage $\Delta_v$ which leads to different filling factors in each layer. We use auxiliary field functional integral approach to study mean-field solutions and collective excitations around them. We find that at large layer separation, the collective excitations soften at a finite wave vector leading to the collapse of quasiparticle gap. Our calculations predict that as the bias voltage is increased, bilayer systems undergo a phase transition from a compressible state to a $\nu=1$ phase-coherent state {\it with charge imbalance}. We present simple analytical expressions for bias-dependent renormalized charge imbalance and pseudospin stiffness which are sensitive to the softening of collective modes.Comment: 12 pages, 5 figures. Minor changes, one reference adde

Spin polarization and spin-dependent scattering of holes observed in transverse magnetic focusing

In two-dimensional systems with a spin-orbit interaction, magnetic focusing can be used to create a spatial separation of particles with different spin. Here we measure hole magnetic focusing for two different magnitudes of the Rashba spin-orbit interaction. We find that when the Rashba spin-orbit magnitude is large there is significant attenuation of one of the focusing peaks, which is conventionally associated with a change in the spin polarization. We instead show that in hole systems with a k3 spin-orbit interaction, this peak suppression is due to a change in the scattering of one spin state, not a change in spin polarization. We also show that the change in scattering length extracted from magnetic focusing is consistent with results obtained from measurements of Shubnikov-de Haas oscillations. This result suggests that scattering must be considered when relating focusing peak amplitude to spin polarization in hole systems