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We have measured the magnetoresistance measurements of a 2D electron gas subjected to a periodic magnetic field created by the presence of type II superconducting stripes on the surface of the heterostructure. We observe oscillatory behaviour due to a commensurability effect between the cyclotron radius and the period of the magnetic field. The results can be understood in terms of a simple semi-classical theory.

A donor-related resonance is observed in double-barrier resonant tunneling devices with Si donors incorporated in the quantum well. In high magnetic fields the resonance becomes dominant over the Is resonance associated with the ground state of a single donor. The bias position of the donor resonance, its magnetic field dependence, and large amplitude indicate unambiguously that the resonance is due to tunneling through the ground state of a shallow donor with two bound electrons (D- level).

We report on the motion of two-dimensional electrons in a magnetic field which is periodic on the scale of a few hundred nanometres. Such a system exhibits novel magnetoresistance oscillations that we unambiguously identify as due to a commensurability effect between the period of the magnetic field and the diameter of the cyclotron orbit. We find that a semiclassical picture of electron motion can quantitatively describe all of the observed experimental features. We also propose methods of optimizing the effects of the magnetic modulation in hybrid semiconductor-superconductor devices.

We report on tunnelling transport through a novel quantum device consisting of a double barrier structure fabricated as a submicron square loop in the plane of growth. We find two kinds of quantum confinement coexist in such a device: near the current onset, tunelling starts through zero-dimensional states located in the four corners of the loop, whereas at higher bias the resonant current exhibits kinks due to one-dimensional confinement in the arms of the loop. The use of a high magnetic field permits us to measure the binding energy of the dot ground state to the first one-dimensional subband in the connecting wires. This value is then compared to existing theory. Moreover, we show that the different kinds of lateral confinement are not only evidenced in the resonant structure, but also in the scattering assisted replica peaks.

We have studied fluctuations in the differential conductance of resonant tunnelling diodes which incorporate a δ-layer of donor impurities in the centre plane of the quantum well. The system is well-suited to studying this type of conductance fluctuation, as we can control the number and position of the impurity states and also vary their effective linewidth by the application of a magnetic field perpendicular to the current direction.

Tunnelling through highly localised impurity states in the quantum well of a resonant tunnelling diode allows us to study the properties of the two-dimensional electron gas formed at the emitter barrier of the device. In high magnetic fields, the electrons form quantum dots in the disordered potential due to unscreened donors in the depleted collector contact.

Electron tunnelling through donor-related states is discussed. This tunnelling process, which occurs well below the threshold voltage for conventional resonant tunnelling into the two-dimensional continuum states of the quantum well, reveals a new type of Fermi edge singularity effect which arises from the Coulomb interaction between the tunnelling electron on the localized site and the Fermi sea of electrons in the emitter layer. A new means of forming laterally confined resonant tunnelling devices is also described. By studying the effect of an applied magnetic field, the additional structure that appears in the current-voltage characteristics of these devices can be unambiguously associated with a lateral quantum mechanical confinement effect.

We have measured the low-temperature magnetoresistance of long (>50 mu m), narrow (similar to 1 mu m) quantum wires fabricated from a high-mobility two-dimensional electron gas. In the quantum Hall regime at high magnetic fields we observe that the Shubnikov-de Haas oscillations split into a series of aperiodic sharp peaks. Both the amplitude and sharpness of the peaks are strongly dependent on temperature below 1 K. We attribute the peaks to resonant tunneling between edge states through localized states in the bulk. The results are in quantitative agreement with the theoretical model of Jain and Kivelson.

We have investigated mesoscopic conductance fluctuations in a GaAs quantum well with five electrically quantized two-dimensional (2D) subbands occupied. In contrast to the case of a single occupied subband, where the fluctuations depend only on the component of magnetic field perpendicular to the plane of the electrons, in our multi-subband structures the conductance fluctuations occur even in a magnetic field parallel to the plane. We attribute the fluctuations to rapid electron-impurity scattering between different 2D subbands which leads to quasi-3D motion of electrons within the quantum well. We are able to confirm the 3D character of the fluctuations by correlations between the magnetoresistances observed when the magnetic field is applied at various angles relative to the plane of the quantum well.

We have investigated resonant tunnelling in GaAs/(AlGa)As heterostructures which have been fabricated into square mesas 6 x 6 mum. A delta-layer of donors (n approximately 2 x 10(9) cm-2) has been incorporated at the centre of the quantum well which is 9 nm wide. The I(V) characteristics show a feature at approximately 70 mV, which is below the threshold for the main resonance and is due to resonant tunnelling through single donor states in the well. This feature is also present in large area mesas. At lower biases and at low temperatures we see a new set of resonances which, although they occur in all small area mesas, differ in detail between devices with regard to their strength and bias position. The form of the low-bias structure is strongly dependent on temperature, T, below 4 K where several very sharp steps appear, becoming sharper as T is decreased. We have also investigated the dependence of the new structure on magnetic field, B, parallel to the current direction. We attribute the new features to tunnelling through potential fluctuations on the mesoscopic scale due to donor clustering.

We have investigated the I ( V ) characteristics of a sub-micron gated GaAs/ (AlGa)As resonant tunnelling diode. We find that, relative to a large area device, there is additional structure at both high and low source-drain bias. The low bias features are shown to be due to the presence of impurities in the quantum well. This is confirmed by experiments on δ-doped large area devices. Using the gate on the sub-micron devices we are able to control the current through a single donor state. The high voltage structure occurs within the main resonance and only in forward bias where the peak-to-valley ratio (PVR) is poor. We ascribe the poor PVR to a lateral variation of the resonance condition and discuss the high voltage structure in these devices, and in similar two-terminal devices, in the light of this hypothesis.

We have studied universal conductance fluctuations in the magnetoresistance of n+-type GaAs submicrometer wires, which represent an orthodox mesoscopic system but also allow us to reach the high-magnetic-field regime, omega(c)tau > 1, where omega(c) is the cyclotron frequency and tau the electron scattering time. The Lee-Stone correlation field B(c) increases by more than an order of magnitude as the magnetic field increases from 0 to 18 T, but the amplitude of the fluctuations remains unchanged. This implies that the universal scaling of conductance fluctuations is not valid in high magnetic fields, in strong disagreement with theoretical predictions. We show that this behavior is not specific to the nonlocal geometry of measurements, where the breakdown has been reported earlier, but that it also occurs in the local magnetoresistance and rectification fluctuations. The violation of universal scaling is attributed to the appearance of a second phase-breaking length, in the regime omega(c)tau > 1, due to extended electron diffusion near the sample boundaries.

We report the observation of the fractional quantum Hall effect in single GaAs-(Ga,Al)As quantum well structures with well widths of 102 A and 68 A. In both samples we have observed a strong v= 4 3 fraction in both the longitudinal resistivity ϱ xx and the Hall resistivity ϱ xy . The surprizing result in our data (in comparison with the FQHE in conventional single heterojunctions) is that the 5 3 is heavily suppressed or possibly absent. In a tilted magnetic field the energy gap of 4 3 depends only on the perpendicular field. We associate this difference between our samples and the conventional heterojunctions with the energy (well width) dependence of the Lande g ∗ factor and the influence of the Zeeman energy on the (partially) spin-polarized energy levels and hence the activation energies.

We have observed remarkable new structure in the source-drain I(V) characteristics of a symmetric double barrier resonant tunneling device in which the cross sectional area may be varied from ≅ 1μm2 to ≅0.1 μm2 by applying a voltage to a gate. In the source-drain voltage range close to, but slightly below, the threshold for current flow, peaks are observed with a peak value of ≅ 20 pA, and peak/valley ratio up to 10 1 . The structure in I(V) is independent of temperature between 35 mK and 10 K, and of gate voltage between 0 and −2 V. The first peak in I(V) in each polarity is unaffected by a magnetic field applied parallel to the current flow for fields up to 6 T. This structure cannot be explained by lateral quantisation or Coulomb blockade arising from the confinement. We propose that it is due to resonant tunneling between zero-dimensional states formed in the active region by the Coulombic potential of a single ionised donor, combined with the barrier potential.

The classical phase space for hot electrons resonantly injected into a wide quantum well (QW) exhibits a transition from stable regular motion to strong classical chaos when an applied magnetic field B is tilted away from the normal to the well walls1. The corresponding quantised energy level spectra and eigenfunctions of this experimentally accessible system have been investigated in the regime of strong classical chaos using resonant tunneling spectroscopy of very wide QWs (~ 100 nm)1,2. In this domain, distinct series of periodic resonant peaks can be identified and related to periodic fluctuations3 in the density of levels in the QW and periodic patterning of the wavefunctions4.

We have investigated reflection of edge states from the boundary between a disordered region and a high-mobility region of a two-dimensional electron gas. \ensuremath{\alpha}-particle bombardment has been employed to reduce selectively the mobility. We observe an exponential increase in reflection of the innermost edge state from the disordered region as the temperature decreases. The results are explained in terms of the Milne reflection of electrons which are elastically scattered back before thermalization can occur.

Hydrostatic pressure and high magnetic fields are used to study space charge buildup under resonant tunneling conditions in an asymmetric double barrier structure based on n-type GaAs/(AlGa)As. The space charge buildup gives rise to an intrinsic bistability effect in the current-voltage characteristics. The pressure-induced increase in Γ-X tunneling and scattering reduces the resonant space charge buildup and allows us to study the transition from intrinsic bistablity to ordinary negative differential conductivity in I(V). Over a certain pressure range a sharp resonance is observed in which electrons tunnel into the well via Γ-states and scatter out mainly via X-states, a clear example of sequential tunneling. The final part of the paper describes a remarkable enhancement of the intrinsic bistability when a quantizing magnetic field is applied perpendicular to the barriers. This is associated with the strong degeneracy of Landau levels at high magnetic fields.

Intense, pulsed magnetic field studies of 2D hole gas systems at low temperatures examine competition of fractional QHE ground states with an insulating phase in the region ν

We have used resonant tunnelling spectroscopy to investigate the energy level spectrum of a wide 60 nm potential well with a strong magnetic field applied at an angle θ to the normal to the barriers. In this geometry, the current-voltage characteristic I(V) reveal distinct series of resonances which change dramatically with both V and θ. In a classical picture, the electron orbits in the well are strongly chaotic for 15° ≤ θ ≤ 75° and voltages V ≤ 1.2 V. However, we have identified a hierarchy of unstable but periodic orbits whose calculated properties explain the origin and θ-dependence of the resonant structure. We show that the classical motion becomes non-chaotic at high bias voltages and that the changeover to stable orbits is characteriaed by the appearance of widely-spaced resonances in I(V) .

The electrical properties of a series of double barrier tunnelling devices with well widths between 5 and 60 nm are investigated. It is shown that the bistability effect in the current-voltage characteristics of a typical resonant tunnelling device can be removed by connecting a suitable capacitance or resistance to the device. These measurements cast serious doubt on the recent interpretation of the bistability as an intrinsic space-charge effect. In the stabilized section of the I(V) curve, at voltages above the main resonant peak, the magnetoquantum oscillations observed with B‖J are used to investigate tunnelling assisted by LO phonon emission and by elastic scattering processes. The resonant tunnelling device with well width 60 nm exhibits sixteen regions of negative differential conductivity. The effect of a transverse magnetic field J⊥B on the resonances in the I(V) characteristics is investigated. At sufficiently high magnetic field, a transition from tunnelling into hybrid magneto-electric quantised states to tunnelling into magnetically quantised cycloidal skipping states is observed.

Resonant tunneling spectroscopy is used to study the electronic energy levels of a wide potential well when a strong magnetic field is applied at an angle θ to the normal to the plane of the well. Even though the classical motion of electrons in the potential well is chaotic in the tilted field geometry, the current-voltage characteristics reveal distinct series of quasi-periodic resonances. The voltage range and period of each series of resonances change dramatically with θ. We explain this behavior by considering the unstable periodic orbits within the chaotic sea of the potential well.

A novel fabrication process, based on selective wet etching and photolithography, has been developed to produce quantum wire resonant tunnelling diodes contacted via a free-standing GaAs bridge. Current-voltage characteristics of quantum wire resonant tunnelling diodes have been studied at 300 mK in the presence of a magnetic field oriented either parallel to or perpendicular to the current. For the smallest device, in the presence of a magnetic field oriented perpendicular to the current and parallel to the wire, we are able to deduce the probability density of the lowest three bound states from the magnetic field dependence of the current and show that the one-dimensional confining potential is close to parabolic. In the presence of a magnetic field oriented parallel to the current a continuous transition from electrostatic (at low field) to magnetic (at high field) confinement is observed.

Resonant tunnelling through a GaAs-AlAs triple-barrier structure with coupled quantum wells of unequal widths has been studied experimentally. Peaks in the current-voltage characteristic due to tunnelling via the lowest subband of each well were observed. Application of a magnetic field parallel to the interfaces broadens the peaks and at a certain critical field an additional resonance peak appears at a bias voltage between the original resonances. The field-induced peak is interpreted as a double-resonance effect, even though the subbands are not energetically aligned. This interpretation is supported by detailed calculations of current-voltage characteristics using a transfer-matrix approach.

Resonant tunneling spectroscopy is used to study the energy-level spectrum of a new chaotic dynamical system, an electron in a trapezoidal potential well in the presence of a high magnetic field tilted relative to the confining barriers. Distinct series of quasi-periodic resonances are observed in the current-voltage chracteristics which change dramatically with tilt angle. These resonances are related to unstable closed orbits within the chaotic domain. The experimental results are explained by identifying and studying the properties of periodic orbits accessible to the tunneling electrons.

We investigate the current-voltage characteristics of a {ital p}-doped resonant tunneling diode. In the voltage range slightly above the bias corresponding to resonant tunneling of holes into the first light-hole subband of the quantum well, we observe two satellite peaks which we attribute to plasmon-assisted tunneling transitions. A theoretical model is presented to account for these peaks. The model is based on the excitation of intrasubband and intersubband heavy-hole plasmons in the quantum well by hot holes injected close to the energy of the first light-hole subband. We also study the behavior of the satellites when a magnetic field is applied either parallel to or perpendicular to the current. {copyright} {ital 1996 The American Physical Society.}

Resonant tunnelling spectroscopy is used to investigate the energy level spectrum of a wide potential well in the presence of a large magnetic field oriented at angles theta between 0 degrees and 90 degrees to the normal to the plane of the well. In the tilted field geometry, the current-voltage characteristics exhibit a large number of quasiperiodic resonant peaks, even though the classical motion of electrons in the potential well is chaotic. The voltage range and spacing of the resonances both change dramatically with theta . We give a quantitative explanation for this behaviour by considering the classical period of unstable periodic orbits within the chaotic sea of the potential well.

We have measured the low temperature current-voltage characteristics ( I ( V )) of GaAs/A1As resonant tunnelling diodes with sub-micron lateral dimensions. Additional peaks in I ( V ) are observed due to resonant tunnelling via one-dimensional quantum wire states. In the presence of a magnetic field oriented perpendicular to the current and parallel to the wire the peaks show a complex splitting evolving into a regular series at high field with upt to 20 resonances. For the smallest device we are able to deduce the probability density of the lowest three bound states from the magnetic field dependence of the current and show that the confining potential is close to parabolic. For a magnetic field which is perpendicular to both the current and the wire a much weaker dependence on magnetic field is observed confirming the one-dimensional nature of our device. Finally, in the presence of a field oriented parallel to the current a continuous transition from electrostatic (at low field) to magnetic confinement (at high field) is observed.

A single-barrier GaAs/AlAs/GaAs heterostructure, with self-assembled In-based quantum dots incorporated in the AlAs tunnel barrier, exhibits a series of resonant peaks in the low temperature current‐voltage characteristics. We argue that each peak arises from singleelectron tunneling through the discrete zero-dimensionalstate of an individual InAs dot. We use the tunneling for fine probing of the local density of states in the emitter-accumulation layer. Landau-quantized states are resolved at magnetic field Bk I as low as 0.2 T. Spinsplitting of the dot electron states has been observed for B? I . c 1997 Academic Press Limited

Resonant tunneling diodes with wide quantum wells are investigated in the presence of a large magnetic field oriented at various angles to the plane of the well. The current-voltage characteristics exhibit a large number of resonant peaks, even when the electron motion in the quantum well is classically chaotic. We show that the spacing of the peaks in the chaotic region is determined by the period of unstable periodic orbits in the potential well and provides direct evidence of an effect analogous to period doubling.