V. J. Goldman

Resonant tunneling and intrinsic bistability in asymmetric double‐barrier heterostructures

We report measurements of the current‐voltage characteristics of an asymmetric GaAs/AlGaAs double‐barrier resonant tunneling device. The structure was designed to increase the space charge in the well under forward bias and consequently enhance the electrostatic feedback that leads to intrinsic bistability. The magnetotunneling data demonstrate unambiguously that the observed bistability is the property of the device, rather than the biasing circuit.

Observation of Single-Electron Charging in Double-Barrier Heterostructures

Incremental single-electron charging of size-quantized states has been observed in the well in submicrometer double-barrier resonant tunneling devices. In order to distinguish between the effects of size quantization and the single-electron charging, the heterostructure material was grown asymmetrical so that one barrier is substantially less transparent than the other. In the voltage polarity such that the emitter barrier is more transparent than the collector barrier, electrons accumulate in the well; incremental electron occupation of the well is accompanied by Coulomb blockade, which leads to sharp steps of the tunneling current. In the opposite voltage polarity the emitter barrier is less transparent than the collector barrier and the tunneling current reflects resonant tunneling through size-quantized well states.

Possibility of infrared laser in a resonant tunneling structure

We analyze the possibility of creating population inversion and optical gain in a specially designed resonant tunneling superstructure when the energy between active quantum levels is greater than the optical phonon energy. Each of twenty periods of the structure represents a complete resonant tunneling, double quantum well diode designed to provide escape time from the lower level of the active well much shorter than the electron‐optical phonon relaxation time. Three heterostructure materials are considered: InAs/AlSb, InGaAs/AlAs, and GaAs/AlAs. In all three cases optical gains of 50–90 cm−1 were calculated to be present for the photon energy of ≊0.1 eV.

Realization of a Laughlin quasiparticle interferometer: Observation of fractional statistics

We report experimental realization of a novel Laughlin quasiparticle interferometer, where quasiparticles of the 1/3 fractional quantum Hall fluid encircle an island of the 2/5 fluid. Interference fringes are observed as conductance oscillations, as in the Aharonov‐Bohm effect. For the first time, we observe an Aharonov‐Bohm superperiod of five magnetic flux quanta (5h/e) through the island. The corresponding 2e charge period is confirmed in calibrated gate experiments. These results are interpreted in terms of the fractional statistical phase acquired by a quasiparticle of the 1/3 fluid encircling the quasiparticles of the 2/5 fluid.

Resonant tunneling in submicron double‐barrier heterostructures

We have fabricated submicron resonant tunneling devices from double‐barrier AlGaAs/GaAs heterostructures using electron beam lithography and wet chemical etching. These devices exhibit step‐like features in the current‐voltage curves. We interpret these steps as arising from additional size quantization of the electronic states in the well due to in‐plane lithographic confinement. Magnetotunneling experiments on these devices are reported for the first time. A simple model calculation describes well the experimental data.

Aharonov-Bohm superperiod in a laughlin quasiparticle interferometer

We report an Aharonov-Bohm superperiod of five magnetic flux quanta (5h/e) observed in a Laughlin quasiparticle interferometer, where an edge channel of the 1/3 fractional quantum Hall fluid encircles an island of the 2/5 fluid. This result does not violate the gauge invariance argument of the Byers-Yang theorem because the magnetic flux, in addition to affecting the Aharonov-Bohm phase of the encircling 1/3 quasiparticles, creates the 2/5 quasiparticles in the island. The superperiod is accordingly understood as imposed by the anyonic statistical interaction of Laughlin quasiparticles.

Growth of low‐density two‐dimensional electron system with very high mobility by molecular beam epitaxy

We report on the growth of modulation‐doped GaAs/Alx Ga1−xAs heterostructures with mobilities (μ) on the order of 1×106 cm2 /V s (at 4.2 K) and areal densities (ns ) below 8×1010 cm−2 . In growing these structures we employ the atomic plane doping technique and ultrathick (>1000 A) spacer layers. The mobilities of these structures are the highest ever reported for low densities. Measurements of μ vs ns as a function of illumination or gate voltage indicate μ∼nαs behavior with α≂0.6 and, even for ns ≂1.4×1010 cm−2 , μ has a value in excess of 0.3×106 cm2 /V s.

Transport in double‐barrier resonant tunneling structures

We report measurements of current‐voltage characteristics of AlGaAs/GaAs double‐barrier resonant tunneling structure (DBRTS) with current peak‐to‐valley ratio exceeding 13:1 at 77 K. We also show that space‐charge layers formed in a biased device influence significantly the charge transport in DBRTS.

Aharonov-Bohm electron interferometer in the integer quantum Hall regime

We report experiments on a quantum electron interferometer fabricated from high mobility, low density AlGaAs/GaAs heterostructure material. In this device, a nearly circular electron island is defined by four front gates deposited in etched trenches. The island is separated from the two-dimensional 2D electron bulk by two nearly open constrictions. In the quantum Hall regime, two counterpropagating edge channels are coupled by tunneling in the constrictions, thus forming a closed electron interference path. For several fixed front gate voltages, we observe periodic Aharonov-Bohm interference oscillations in four-terminal resistance as a function of the enclosed flux. The oscillation period B gives the area of the interference path S via the quantization condition S=h/eB. We experimentally determine the dependence of S on the front gate voltage, and find that the Aharonov-Bohm quantization condition does not require significant corrections due to the confining potential. These results can be interpreted as a constant integrated compressibility of the island with respect to the front gates. We also analyze experimental results using two classical electrostatics models: one modeling the 2D electron density due to depletion from an etch trench, and another modeling the gate voltage dependence of the electron density profile in the island.

Dependence of the Fractional Quantum Hall Edge Critical Exponent on the Range of Interaction

Comment on A.M.Chang et al, PRL 86, 143 (2001).