F.W. Sheard

Space‐charge buildup and bistability in resonant‐tunneling double‐barrier structures

Using the sequential theory of resonant tunneling, the dc current‐voltage characteristic of a double‐barrier structure is calculated, taking into account the effect of space charge in the quantum well. A region of current bistability is found over a voltage range which is determined by the maximum space charge and the capacitance of the structure. These parameters are directly related to the periodicity of magnetoquantum oscillations in the current.

Chaotic electron diffusion through stochastic webs enhances current flow in superlattices.

Understanding how complex systems respond to change is of fundamental importance in the natural sciences. There is particular interest in systems whose classical newtonian motion becomes chaotic as an applied perturbation grows. The transition to chaos usually occurs by the gradual destruction of stable orbits in parameter space, in accordance with the Kolmogorov–Arnold–Moser (KAM) theorem—a cornerstone of nonlinear dynamics that explains, for example, gaps in the asteroid belt. By contrast, ‘non-KAM’ chaos switches on and off abruptly at critical values of the perturbation frequency. This type of dynamics has wide-ranging implications in the theory of plasma physics, tokamak fusion, turbulence, ion traps, and quasicrystals. Here we realize non-KAM chaos experimentally by exploiting the quantum properties of electrons in the periodic potential of a semiconductor superlattice with an applied voltage and magnetic field. The onset of chaos at discrete voltages is observed as a large increase in the current flow due to the creation of unbound electron orbits, which propagate through intricate web patterns in phase space. Non-KAM chaos therefore provides a mechanism for controlling the electrical conductivity of a condensed matter device: its extreme sensitivity could find applications in quantum electronics and photonics.

Sequential tunneling due to intersubband scattering in double‐barrier resonant tunneling devices

Magnetoquantum oscillations in the tunnel current of double‐barrier n‐GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi‐bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.

Electrical and spectroscopic studies of space-charged buildup, energy relaxation and magnetically enhanced bistability in resonant-tunneling structures

Abstract Photoluminescence measurements and magnetoquantum oscillations in the differential capacitance are used to measure space-charged buildup and study electron thermalization in a double-barrier resonant tunneling structure based on n-type (AlGa)As. The intrinsic bistability observed in the I(V) characteristics is also seen in the linewidth and photon energy of the photoluminescence. The spectroscopic data reveal clearly the importance of intersubband transitions in the voltage range at which electrons tunnel resonantly into the second bound state of the quantum well. A novel field-induced enhancement of the intrinsic bistability effect is reported for B ‖ J .

Observation of space-charge bulk-up and thermalisation in an asymmetric double-barrier resonant tunnelling structure

By means of a study of magnetoquantum oscillations in the differential capacitance, the authors have observed the thermalisation of the space charge stored dynamically in the quantum well of an asymmetric double-barrier resonant tunnelling heterostructure based on n-GaAs/(AlGa)As. Fourier analysis of the oscillations was used to monitor the charge build-up in both the emitter accumulation layer and in the well. The storage time of an electron in the well was found to be approximately=0.5 mu s. The resonant tunnelling is truly sequential rather than coherent.

Magnetic field studies of negative differential conductivity in double barrier resonant tunnelling structures based on n-InP/(InGa)As

Abstract Negative differential conductivity (NDC) with a peak/valley ratio of 4.5:1 (4 K) and 2:1 (150 K) is observed in double barrier resonant tunnelling devices based on n-InP/(InGa)As. A transverse magnetic field applied in the plane of the tunnelling barriers ( J ¦ B ) significantly changes the current-voltage characteristics and eliminates the NDC for fields above −10 T. This behaviour is explained qualitatively in terms of the effect of the magnetic vector potential on the tunnelling electrons. The magneto-oscillations in the tunnelling current for J ‖ B are discussed in terms of a simple model of resonant tunnelling.

Charge build-up and intrinsic bistability in an asymmetric resonant-tunnelling structure

Intrinsic bistability is observed in an asymmetric resonant-tunnelling structure based on n-GaAs/(AlGa)As, incorporating a thin emitter barrier and a thick collector barrier. The resonant charge build-up in the quantum well which gives rise to the bistability is monitored by the Landau level structure in the magneto-capacitance.

Inter-Landau-level transitions of resonantly tunnelling electrons in tilted magnetic fields

The effect of a tilted magnetic field on the current-voltage characteristics of an n-type double barrier resonant tunnelling device based on GaAs/(AlGa)As is investigated. The component of magnetic field perpendicular to the tunnelling barriers quantizes the electronic states of the quantum well and emitter accumulation layer into discrete Landau levels. The effect of the component parallel to the barriers is to allow tunnelling transitions in which the Landau level index is not conserved. This gives rise to a spitting of the resonant peak in the current-voltage curve into a series of equally spaced sub-peaks. These features are explained using the sequential theory of resonant tunnelling and treating the effect of the transverse component of magnetic field in a perturbation approximation.

Space-charge effects and ac response of resonant tunneling double-barrier diodes

Abstract We analyse theoretically the small-signal ac response of a resonant tunneling double-barrier semiconductor diode using the sequential tunneling approach. Electrostatic feedback effects due to space charge buildup in the quantum well are included in the analysis. The results are expressed in terms of an equivalent circuit which is used to interpret measurements of the capacitance of an asymmetric double-barrier structure at low frequencies (

Anomalous quantum reflection of Bose-Einstein condensates from a silicon surface : The role of dynamical excitations

We investigate the effect of interatomic interactions on the quantum-mechanical reflection of Bose-Einstein condensates from regions of rapid potential variation. The reflection process depends critically on the density and incident velocity of the condensate. For low densities and high velocities, the atom cloud has almost the same form before and after reflection. Conversely, at high densities and low velocities, the reflection process generates solitons and vortex rings that fragment the condensate. We show that this fragmentation can explain the anomalously low reflection probabilities recently measured for low-velocity condensates incident on a silicon surface.