High-mobility Two-dimensional Electron System Research Articles

Structured back gates for high-mobility two-dimensional electron systems using oxygen ion implantation

We present a reliable method to obtain patterned back gates compatible with high mobility molecular beam epitaxy via local oxygen ion implantation that suppresses the conductivity of an 80 nm thick silicon doped GaAs epilayer. Our technique was optimized to circumvent several constraints of other gating and implantation methods. The ion-implanted surface remains atomically flat which allows unperturbed epitaxial overgrowth. We demonstrate the practical application of this gating technique by using magneto-transport spectroscopy on a two-dimensional electron system (2DES) with a mobility exceeding 20 × 106 cm2/V s. The back gate was spatially separated from the Ohmic contacts of the 2DES, thus minimizing the probability for electrical shorts or leakage and permitting simple contacting schemes.

Evidence of radiation-driven Landau states in 2D electron systems: Magnetoresistance oscillations phase shift

We provide the ultimate explanation of one of the core features of microwave-induced magnetoresistance oscillations in high-mobility two-dimensional electron systems: the 1/4-cycle phase shift of minima. We start with the radiation-driven electron orbits model with the novel concept of scattering flight-time between Landau states. We calculate the extrema and nodes positions obtaining an exact coincidence with the experimental ones. The main finding is that the physical origin of the phase shift is a delay of of the radiation-driven Landau guiding center with respect to radiation, demonstrating the oscillating nature of the irradiated Landau states. We analyze the dependence of this minima on radiation frequency and power and its possible shift with the quality of the sample.

Spin-Selective Electron Quantum Transport in Nonmagnetic MgZnO/ZnO Heterostructures.

We report magnetotransport measurements on a high-mobility two-dimensional electron system at the nonmagnetic MgZnO/ZnO heterointerface showing distinct behavior for electrons with spin-up and spin-down orientations. The low-field Shubnikov-de Haas oscillations manifest alternating resistance peak heights which can be attributed to distinct scattering rates for different spin orientations. The tilt-field measurements at a half-integer filling factor reveal that the majority spins show usual diffusive behavior, i.e., peaks with the magnitude proportional to the index of the Landau level at the Fermi energy. By contrast, the minority spins develop "plateaus" with the magnitude of dissipative resistivity that is fairly independent of the Landau level index and is of the order of the zero-field resistivity.

Excitation, detection, and electrostatic manipulation of terahertz-frequency range plasmons in a two-dimensional electron system.

Terahertz frequency time-domain spectroscopy employing free-space radiation has frequently been used to probe the elementary excitations of low-dimensional systems. The diffraction limit, however, prevents its use for the in-plane study of individual laterally-defined nanostructures. Here, we demonstrate a planar terahertz frequency plasmonic circuit in which photoconductive material is monolithically integrated with a two-dimensional electron system. Plasmons with a broad spectral range (up to ~ 400 GHz) are excited by injecting picosecond-duration pulses, generated and detected by a photoconductive semiconductor, into a high mobility two-dimensional electron system. Using voltage modulation of a Schottky gate overlying the two-dimensional electron system, we form a tuneable plasmonic cavity, and observe electrostatic manipulation of the plasmon resonances. Our technique offers a direct route to access the picosecond dynamics of confined electron transport in a broad range of lateral nanostructures.

Metallic state of low-mobility silicon at high carrier density induced by an ionic liquid

High mobility and dilute two-dimensional electron systems exhibit metallic behavior down to the lowest experimental temperatures. In studies of ionic liquid gated insulating silicon, we have observed transitions to a metallic state in low mobility samples at much higher areal carrier densities than found for samples of high mobility. We have also observed a mobility peak in metallic samples as the carrier density was increased beyond $10^{13} \text{cm}^{-2}$.

Geometric Resonance of Composite Fermions Near the ν=1/2 Fractional Quantum Hall State.

We observe geometric resonance features of composite fermions on the flanks of the even-denominator ν=1/2 fractional quantum Hall state in high-mobility two-dimensional electron and hole systems confined to wide GaAs quantum wells and subjected to a weak, strain-induced, unidirectional periodic potential modulation. The features provide a measure of how close to ν=1/2 the system stays single-component and supports a composite fermion Fermi sea before transitioning into a ν=1/2 fractional quantum Hall state, presumably, the two-component Ψ331 state.

Theory of magnetothermoelectric phenomena in high-mobility two-dimensional electron systems under microwave irradiation

The response of two-dimensional electron gas to a temperature gradient in perpendicular magnetic field under steady-state microwave irradiation is studied theoretically. The electric currents induced by the temperature gradient and the thermopower coefficients are calculated taking into account both diffusive and phonon-drag mechanisms. The modification of thermopower by microwaves takes place because of Landau quantization of the electron energy spectrum and is governed by the microscopic mechanisms which are similar to those responsible for microwave-induced oscillations of electrical resistivity. The magnetic-field dependence of microwave-induced corrections to phonon-drag thermopower is determined by mixing of phonon resonance frequencies with radiation frequency, which leads to interference oscillations. The transverse thermopower is modified by microwave irradiation much stronger than the longitudinal one. Apart from showing prominent microwave-induced oscillations as a function of magnetic field, the transverse thermopower appears to be highly sensitive to the direction of linear polarization of microwave radiation.

Polarization-dependent Landau level crossing in a two-dimensional electron system in a MgZnO/ZnO heterostructure

We report electrical transport measurements in a tilted magnetic field on a high-mobility two-dimensional electron system confined at the MgZnO/ZnO heterointerface. The observation of multiple crossing events of spin-resolved Landau levels (LLs) enables the mapping of the sequence of electronic states. We further measure the renormalization of electron spin susceptibility at zero field and the susceptibility dependence on the electron spin polarization. The latter manifests the deviation from the Pauli spin susceptibility. As the result, the crossing of spin-resolved LLs shifts to smaller tilt angles and the first Landau level coincidence event is absent even when the magnetic field has only a perpendicular component to the 2DES plane.

Local imaging of high mobility two-dimensional electron systems with virtual scanning tunneling microscopy

Correlated electron states in high mobility two-dimensional electron systems (2DESs), including charge density waves and microemulsion phases intermediate between a Fermi liquid and Wigner crystal, are predicted to exhibit complex local charge order. Existing experimental studies, however, have mainly probed these systems at micron to millimeter scales rather than directly mapping spatial organization. Scanning probes should be well-suited to study the spatial structure of these states, but high mobility 2DESs are found at buried semiconductor interfaces, beyond the reach of conventional scanning tunneling microscopy. Scanning techniques based on electrostatic coupling to the 2DES deliver important insights, but generally with resolution limited by the depth of the 2DES. In this letter, we present our progress in developing a technique called “virtual scanning tunneling microscopy” that allows local tunneling into a high mobility 2DES. Using a specially designed bilayer GaAs/AlGaAs heterostructure where the tunnel coupling between two separate 2DESs is tunable via electrostatic gating, combined with a scanning gate, we show that the local tunneling can be controlled with sub-250 nm resolution.

Interaction of microwave radiation with the high mobility two-dimensional electron system in GaAs/AlGaAs heterostructures

The influence of microwave excitation on the magnetotransport properties of the high mobility two-dimensional electron system (2DES) in the GaAs/AlGaAs heterostructure system is investigated by exploring (a) the dependence of the amplitude of the microwave-induced magnetoresistance-oscillations on the polarization direction of the linearly polarized microwaves and (b) the microwave reflection from the 2DES. The polarization study indicates that the amplitude of the magnetoresistance oscillations is remarkably responsive to the relative orientation between the linearly polarized microwaves and the current-axis in the specimen. At low microwave power, P, experiments indicate a strong sinusoidal variation in the diagonal resistance Rxx vs. θ at the oscillatory extrema of the microwave-induced magnetoresistance oscillations. The reflection study indicates strong correlations between the microwave induced magnetoresistance oscillations and oscillatory features in the microwave reflection in a concurrent measurement of the magnetoresistance and the microwave magnetoreflection from the 2DES. The correlations are followed as a function of the microwave frequency and the microwave power, and the results are reported.

Spin polarization, dephasing, and photoinduced spin diffusion in (110)-grown two-dimensional electron systems

We study the optically induced spin polarization, spin dephasing, and diffusion in several high-mobility two-dimensional electron systems, which are embedded in GaAs quantum wells grown on (110)-oriented substrates. The experimental techniques comprise a two-beam magneto-optical spectroscopy system and polarization-resolved photoluminescence. Under weak excitation conditions at liquid-helium temperatures, we observe spin lifetimes above 100 ns in one of our samples, which are reduced with increasing excitation density due to additional, hole-mediated, spin dephasing. The spin dynamic is strongly influenced by the carrier density and the ionization of remote donors, which can be controlled by temperature and above-barrier illumination. The absolute value of the average electron spin polarization in the samples is directly observable in the circular polarization of photoluminescence collected under circularly polarized excitation and reaches values of about 5%. Spin diffusion is studied by varying the distance between pump and probe beams in microspectroscopy experiments. We observe diffusion lengths above 100 $\ensuremath{\mu}$m and, at high excitation intensity, a nonmonotonic dependence of the spin polarization on the pump-probe distance.

Enhanced quantum oscillatory magnetization and nonequilibrium currents in an interacting two-dimensional electron system in MgZnO/ZnO with repulsive scatterers

Torque magnetometry at low temperature and in high magnetic fields B is performed on a MgZnO/ZnO heterostructure incorporating a high-mobility two-dimensional electron system. We find a sawtooth-like quantum oscillatory magnetization M(B), i.e., the de Haas-van Alphen (dHvA) effect. At the same time, unexpected spike-like overshoots in M and non-equilibrium currents are observed which allow us to identify the microscopic nature and density of the residual disorder. The acceptor-like scatterers give rise to a magnetic thaw down effect which enhances the dHvA amplitude beyond the electron-electron interaction effects being present in the MgZnO/ZnO heterostructure

Valley-degenerate two-dimensional electrons in the lowest Landau level

We report low temperature magnetotransport measurements on a high mobility ($\ensuremath{\mu}=325\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$) two-dimensional electron system on a H-terminated Si(111) surface. We observe the integral quantum Hall effect at all filling factors $\ensuremath{\nu}\ensuremath{\le}6$ and find that $\ensuremath{\nu}=2$ develops in an unusually narrow temperature range. An extended, exclusively even numerator, fractional quantum Hall hierarchy occurs surrounding $\ensuremath{\nu}=3/2$, consistent with twofold valley-degenerate composite fermions (CFs). We determine activation energies and estimate the CF mass.

Observation of the intrinsic spin Hall effect in a two-dimensional electron gas

The experimental demonstration of the spin Hall effect in a high mobility two-dimensional electron system is reported. The spatial dependence is studied by Kerr rotation as a function of the external magnetic field using an applied electric field amplitude and direction as control parameters. We observe that the effect is robust in a bilayer structure with a nonzero Rashba coefficient displayed by an electrically controllable internal magnetic field, a large spin Hall conductivity in the range of the universal intrinsic value, and a mobility-enhanced spin diffusion constant. With the application of an unidirectional electric field, the role of the spin drift was also studied. The data was analyzed following both phenomenological and microscopic approaches and compared with experimental references in a single-layer configuration.

Magnetoresistance in a High Mobility Two-Dimensional Electron System as a Function of Sample Geometry

In a high mobility two-dimensional electron gas (2DEG) realized in a GaAs / Al0.3Ga0.7As quantum well we observe changes in the Shubnikov-de Haas oscillations (SdHO) and in the Hall resistance for different sample geometries. We observe for each sample geometry a strong negative magnetoresistance around zero magnetic field which consists of a peak around zero magnetic field and of a huge magnetoresistance at larger fields. The peak around zero magnetic field is left unchanged for different geometries.

Semi-analytical model of Hall resistance anomalies (overshooting) in the fractional quantized Hall effect

We predict resistance anomalies to be observed at high-mobility two-dimensional electron systems (2DESs) in the fractional quantized Hall regime, where the narrow (L < 10 μm) Hall bar is defined by top gates. A semi-analytic calculation scheme is used to describe the formation of integral and fractional incompressible strips. We incorporate the screening properties of the 2DES, together with the effects of perpendicular magnetic field, to calculate the effective widths of the current carrying channels. The many-body effects are included to our calculation scheme via the energy gap obtained from the well-accepted formulation of the composite fermions. We show that the fractional incompressible strips at the edges, assuming different filling factors, become evanescent and can co-exist in certain magnetic field intervals yielding an overshoot at the Hall resistance, similar to that of the integral quantized Hall effect. We also provide a mechanism to explain the absence of 1/3 state in Fabry-Perot type interference experiments. A gate defined narrow sample design is proposed to enhance the visibility of fragile effects like interference and overshooting based on our semi-analytical model.

Tunable charge detectors for semiconductor quantum circuits

Nanostructures defined in high-mobility two-dimensional electron systems offer a unique way of controlling the microscopic details of the investigated device. Quantum point contacts play a key role in these investigations, since they are not only a research topic themselves, but also turn out to serve as convenient and powerful detectors for their electrostatic environment. We investigate how the sensitivity of charge detectors can be further improved by reducing screening, increasing the capacitive coupling between charge and detector and by tuning the quantum point contacts' confinement potential into the shape of a localized state. We demonstrate the benefits of utilizing a localized state by performing fast and well-resolved charge detection of a large quantum dot in the quantum Hall regime.

Frequency anomaly in the Rashba-effect induced magnetization oscillations of a high-mobility two-dimensional electron system

With the direct measurement of the quantum oscillatory magnetization M of a two-dimensional electron system (2DES) in an InGaAs/InP asymmetric quantum well we discover a frequency anomaly of the de Haas–van Alphen effect which is not consistent with existing theories on spin-orbit interaction (SOI). Strikingly, the oscillatory magnetoresistance of the same heterostructure, that is, the Shubnikov–de Haas effect conventionally used to explore SOI, does not show the frequency anomaly. This explains why our finding has not been reported for almost three decades. The understanding of the ground state energy of a 2DES is evidenced to be incomplete when SOI is present.

Microwave-induced resistance oscillations in tilted magnetic fields

We have studied the effect of an in-plane magnetic field on microwave-induced resistance oscillations in a high mobility two-dimensional electron system. We have found that the oscillation amplitude decays exponentially with an in-plane component of the magnetic field $B_\parallel$. While these findings cannot be accounted for by existing theories, our analysis suggests that the decay can be explained by a $B_\parallel$-induced correction to the quantum scattering rate, which is quadratic in $B_\parallel$.

Nonlinear response in overlapping and separated Landau levels of GaAs quantum wells

We have studied magnetotransport properties of a high-mobility two-dimensional electron system subject to weak electric fields. At low magnetic field $B$, the differential resistivity acquires a correction $\delta r \propto -\lambda^2 j^2/B^2$, where $\lambda$ is the Dingle factor and $j$ is the current density, in agreement with theoretical predictions. At higher magnetic fields, however, $\delta r$ becomes $B$-independent, $\delta r \propto -j^2$. While the observed change in behavior can be attributed to a crossover from overlapping to separated Landau levels, full understanding of this behavior remains a subject of future theories.