Search

Using inelastic light scattering, we measure the zone-center electronic excitation modes in a set of multiple quantum wells. The width of the wavefunction barriers was chosen such that it prevents significant coupling of the electron ground states between wells yet is transparent to electron tunneling under an electric field. Under these conditions, we find charge-density-like and spin-density-like plasmons whose energies do not correspond to the excitations calculated for either a single well or a set of Coulomb-coupled wells. The observed energies are proportional to the electric field strength and the lower energy modes agree with predictions for plasmons tunneling between the Wannier-Stark ladder states. [ABSTRACT FROM AUTHOR]

We introduce a design of electrically isolated "floating" bilayer GaAs quantum wells (QW), in which application of a large gating voltage controllably and highly reproducibly induces charges that remain trapped in the bilayer after removal of the gating voltage. At smaller gate voltages, the bilayer is fully electrically isolated from external electrodes by thick insulating barriers. This design permits full control of the total and differential densities of two coupled 2D electron systems. The floating bilayer design provides a unique approach for studying systems inaccessible by simple transport measurements. It also provides the ability to measure the charge transfer between the layers, even when the in-plane resistivities of the 2D systems diverge. We measure the capacitance and inter-layer tunneling spectra of the QW bilayer with independent control of the top and bottom layer electron densities. Our measurements display strongly enhanced inter-layer tunneling current at v T = 1 , a signature of exciton condensation of a strongly interlayer-correlated bilayer system. With fully tunable densities of individual layers, the floating bilayer QW system provides a versatile platform to access previously unavailable information on the quantum phases in electron bilayer systems. [ABSTRACT FROM AUTHOR]

A fast, voltage-tunable terahertz mixer based on the intersubband transition of a high-mobility 2-dimensional electron gas has been fabricated from a single 40 nm GaAs-AlGaAs square quantum well heterostructure. The device is called a Tunable Antenna-Coupled Intersubband Terahertz mixer and shows tunability of the detection frequency from 2.52 to 3.11 THz with small (<1 V) top gate and bottom gate voltage biases. Mixing at 2.52 THz has been observed at 60 K with a −3dB intermediate frequency bandwidth exceeding 6 GHz. [ABSTRACT FROM AUTHOR]

Indirect excitons (IXs) are bound pairs of electrons and holes confined in spatially separated layers. We present wide single quantum well (WSQW) heterostructures with high IX mobility, spectrally narrow IX emission, voltage-controllable IX energy, and long and voltage-controllable IX lifetime. This set of properties shows that WSQW heterostructures provide an advanced platform both for studying basic properties of IXs in low-disorder environments and for the development of high-mobility excitonic devices. [ABSTRACT FROM AUTHOR]

Presents a study on the picosecond dynamics of free carriers and excitons in high-quality gallium arsenide quantum wells using frequency upconversion and nonlinear picosecond excitation-correlation luminescence spectroscopy under nonresonant excitation conditions. Theoretical considerations; Experimental procedure; Conclusions.

We present a concept and experimental proof of principle for split-gate devices for indirect excitons (IXs). The split-gate forms a narrow channel, a point contact, for IX current. Control of IX flow through the split-gate with both gate voltage and excitation power is demonstrated. [ABSTRACT FROM AUTHOR]

We compare micro-photoluminescence (µPL) as a measure of the electron density in a clean, two-dimensional (2D) system confined in a GaAs quantum well (QW) to the standard magneto-transport technique. Our study explores the PL shape evolution across a number of molecular beam epitaxy-grown samples with different QW widths and 2D electron densities and notes its correspondence with the density obtained in magneto-transport measurements on these samples. We also measure the 2D density in a top-gated quantum well sample using both PL and transport and find that the two techniques agree to within a few percent over a wide range of gate voltages. We find that the PL measurements are sensitive to gate-induced 2D density changes on the order of 109 electrons/cm². The spatial resolution of the PL density measurement in our experiments is 40 µm, which is already substantially better than the millimeter-scale resolution now possible in spatial density mapping using magneto-transport. Our results establish that µPL can be used as a reliable high spatial resolution technique for future contactless measurements of density variations in a 2D electron system. [ABSTRACT FROM AUTHOR]

We review our work on high-frequency conductance in two-dimensional high-mobility electronic systems in wide n-AlGaAs/GaAs/AlGaAs quantum wells. Using simultaneous measurements of the attenuation and velocity of a surface acoustic wave we obtained both real and imaginary components of the complex high-frequency conductance. Based on the experimental results and their analysis we conclude that close to the filling factor v =1/5, as well as in the interval 0.18>v>0.125, a Wigner crystal pinned by disorder is formed. Both the melting temperature and the correlation length of the pinning-induced domains in the Wigner crystal were found. In close vicinities of v =1 and 2, transitions from single-electron localization to a Wigner crystal were observed. [ABSTRACT FROM AUTHOR]

We fabricated a He-3 immersion cell for transport measurements of semiconductor nanostructures at ultra low temperatures and in strong magnetic fields. We have a new scheme of field-independent thermometry based on quartz tuning fork Helium-3 viscometry which monitors the local temperature of the sample's environment in real time. The operation and measurement circuitry of the quartz viscometer is described in detail. We provide evidence that the temperature of two-dimensional electron gas confined to a GaAs quantum well follows the temperature of the quartz viscometer down to 4 mK. [ABSTRACT FROM AUTHOR]

We report the results of tilted magnetic field experiments on three high quality wide GaAs quantum wells, with particular emphasis on the N ≥ 1 Landau levels. With an increasing component of in-plane magnetic field, B∥, we observe reentrant behavior for the odd filling factor quantum Hall states. This reentrance is not explained within a single-particle energy level crossing scheme and may be related to the collapse of interlayer tunneling. [ABSTRACT FROM AUTHOR]

The conductivity of an Al0.24Ga0.76As/GaAs quantum well was studied as a function of the surface charge generated by electron bombardment of the sample in the absence of an externally applied surface electric field. Under a suitable rate of electron irradiation, it was possible to completely shut off the conductive channel, implying a surface density $n=2.5 \times 10^{11}\,\mbox{el/cm}^2 $. Light illumination quenches the increase of the resistivity, apparently due to photoemission from the metastable surface states. Upon turning off the electron bombardment the surface charge on adsorbed layers of xenon and water at 8 K decays in room temperature darkness with a lifetime τ= 0.30 ±0.02 s. The average charging efficiency, is μ0 ≃0.001. Surface charging is shown to be an effective method for contactless gating of field effect devices. [ABSTRACT FROM AUTHOR]

We study the Landau level spectrum of a two-dimensional hole gas in carbon δ-doped (100) GaAs/Al0.4Ga0.6As square quantum well (width of 15 nm) by means of microwave cyclotron resonance (CR) and tilted field magnetotransport. Only one type of hole, with an effective mass of 0.4me, was detected in CR. By analyzing the spin-splitting level coincident conditions at B∼1 T, we find that the effective g-factor is large enough to cause Landau level crossing even in zero tilt, and the product of gm* increases with total magnetic field. [ABSTRACT FROM AUTHOR]

Microwave cyclotron resonance of low-density high-mobility two-dimensional hole gases of different densities in (100) oriented GaAs/AlGaAs heterostructures has been measured at 4.2 K. The measurements show that the hole effective mass depends strongly on the confining potential. For holes in a 20 nm quantum well, the cyclotron mass decreases from 0.48me at density p=6.9×1010/cm2 to 0.29me at p=0.8×1010/cm2. The cyclotron mass for holes confined at a heterojunction is fairly insensitive to the density and has a value of approximately 0.5me. [ABSTRACT FROM AUTHOR]

The results of experimental transport studies involving a series of five narrow Al0.1Ga0.9As/GaAs quantum wells with well widths ranging from 7.9 to 33.0 nm are reported. The total transport scattering rate measured in thin AlGaAs/GaAs quantum wells is typically limited by electron scattering from the interfacial roughness of the quantum well. For a relatively low constant electron density (ne∼5.8×1010 cm-2), the authors find that interfacial roughness is the dominant scattering mechanism for L≤16.0 nm and describe the data using a finite quantum well model with adjustable interfacial roughness parameters. Temperature dependence data are also presented [ABSTRACT FROM AUTHOR]

We report on the transport properties of a high mobility two-dimensional hole system (2DHS) confined in GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy on the (100) surface of GaAs. The quantum wells are modulation doped with carbon utilizing a resistive filament source. At T=0.3 K and carrier density p=1×1011 cm-2, a mobility of 106 cm² /Vs is achieved. At fixed carrier density p=1011 cm-2, the mobility is found to be a nonmonotonic function of the quantum well width. The mobility peaks at 106 cm² /Vs for a 15-nm well and is reduced for both smaller and larger well widths for these (100) samples. The mobility anisotropy is found to be small. Mobility along [011] is approximately 20% higher than along the [011] direction. In addition, the low-temperature carrier density is found to have low sensitivity to light. The hole density increases by only ,10% after exposure to red light at T=4.2 K. In structures designed for a lower carrier density of 3.6×1010 cm-2, a mobility of 800 000 cm² /Vs is achieved at T=15 mK. [ABSTRACT FROM AUTHOR]

The surface charge generated on an Al0.24Ga0.76As/GaAs quantum well sample by electron bombardment was monitored by measuring the change in the conductivity of the channel. Upon turning off the electron bombardment the surface charge on adsorbed layers of xenon and water at 8 K decays in room temperature darkness with a lifetime τ=0.30±0.02 s. The average charging efficiency, μ0, defined as the ratio of the charge collected by the surface to the beam current times the charging time, is μ0≃0.001. Surface charging proves to be an effective method for contactless gating of field effect devices. [ABSTRACT FROM AUTHOR]

Very dilute two-dimensional hole systems were probed by resonant inelastic light scattering. This extremely high mobility system occurs in AlxGa1-xAs/GaAs quantum wells designed specifically for their light insensitivity. We find that the depolarization shift in intersubband transitions decreases linearly for hole densities below p=6.6×1010 cm-2, dropping to zero below p=1×1010 cm-2. [ABSTRACT FROM AUTHOR]

A new technique for creating independent ohmic contacts to closely spaced two-dimensional electron systems in double quantum well (DQW) structures is described. Without use of shallow diffusion or precisely controlled etching methods, the present technique results in low-resistance contacts which can be electrostatically switched between the two-conducting layers. The method is demonstrated with a DQW consisting of two 200 Å GaAs quantum wells separated by a 175 Å AlGaAs barrier. A wide variety of experiments on Coulomb and tunnel-coupled 2D electron systems is now accessible. [ABSTRACT FROM AUTHOR]

Provides a corrected reprint of an article that was previously published in the May 15, 1997 issue of the 'Journal of Applied Physics.'

The single-particle spectral function measures the density of electronic states in a material as a function of both momentum and energy, providing central insights into strongly correlated electron phenomena. Here we demonstrate a high-resolution method for measuring the full momentum- and energy-resolved electronic spectral function of a two-dimensional (2D) electronic system embedded in a semiconductor. The technique remains operational in the presence of large externally applied magnetic fields and functions even for electronic systems with zero electrical conductivity or with zero electron density. Using the technique on a prototypical 2D system, a GaAs quantum well, we uncover signatures of many-body effects involving electron-phonon interactions, plasmons, polarons, and a phonon analog of the vacuum Rabi splitting in atomic systems. [ABSTRACT FROM AUTHOR]

We describe quantum wires and point contacts fabricated in GaAs/Al[sub x]Ga[sub 1-x]As heterostructures that are free of the disorder introduced by modulation doping and in which the electron density and the confining potential are separately adjustable by lithographically defined gates. We observe conductance plateaus quantized near even multiples of e[sup 2]/h in 2 μm wires and up to 15 conductance steps in 5 μm wires at temperatures below 1 K. Near the conductance threshold the quantum point contact and the 2 μm wire both show additional structure below 2e[sup 2]/h. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on quantum magneto-oscillations in an ultrahigh mobility GaAs/AlGaAs quantum well at very high (≥ 87°) tilt angles. Unlike previous studies, we find that the spin and cyclotron splittings become equal over a continuous range of angles, but only near certain, angle-dependent filling factors. At high enough tilt angles, Shubnikov-de Haas oscillations reveal a prominent beating pattern, indicative of consecutive level crossings, all occurring at the same angle. We explain these unusual observations by an in-plane field-induced increase of the carrier mass, which leads to accelerated, filling-factor-driven crossings of spin sublevéis in tilted magnetic fields. [ABSTRACT FROM AUTHOR]

We present the transport and capacitance measurements of 10 nm wide GaAs quantum wells with hole densities around the critical point of the 2D metal-insulator transition (critical density pc down to 0.8 X 1010/cm2, rs ∼ 36). For metallic hole density pc < p < pc + 0.15 X 1010/cm2, a reentrant insu-lating phase (RIP) is observed between the v = 1 quantum Hall state and the zero-field metallic state and it is attributed to the formation of pinned Wigner crystal. Through studying the evolution of the RIP versus 2D hole density, we show that the RIP is incompressible and continuously connected to the zero-field insulator, suggesting a similar origin for these two phases. [ABSTRACT FROM AUTHOR]

The recently developed AFM local anodic oxidation (LAO) technique offers a convenient way of patterning nanodevices, but imposes even more stringent requirements on the underlying quantum well structure. We developed a new very shallow quantum well design which allows the depth and density of the 2D gas to be independently controlled during the growth. A high mobility (0.5×106 cm2 V−1 s−1 at 4.2 K) 2D hole gas just 350 A˚ below the surface is demonstrated. A quantum point contact, fabricated by AFM LAO nanopatterning from this wafer, shows nine quantum steps at 50 mK. [Copyright &y& Elsevier]

The concept of a Fermi surface, the constant-energy surface containing all the occupied electron states in momentum, or wave-vector (k), space plays a key role in determining electronic properties of conductors. In two-dimensional (2D) carrier systems, the Fermi surface becomes a contour which, in the simplest case, encircles the occupied states. In this case, the area enclosed by the contour, which we refer to as the Fermi sea (FS), is a simple disk. Here we report the observation of an FS with a new topology, namely, an FS in the shape of an annulus. Such an FS is expected in a variety of 2D systems where the energy band dispersion supports a ring of extrema at finite k, but its experimental observation has been elusive. Our study provides (1) theoretical evidence for the presence of an annular FS in 2D hole systems confined to wide GaAs quantum wells and (2) experimental signatures of the onset of its occupation as an abrupt rise in the sample resistance, accompanied by a sudden appearance of Shubnikov-de Haas oscillations at an unexpectedly high frequency whose value does not simply correspond to the (negligible) density of holes contained within the annular FS. [ABSTRACT FROM AUTHOR]

We report a new correlated phase of two-dimensional charged carriers in high magnetic fields, manifested by an anisotropic insulating behavior at low temperatures. It appears in a large range of low Landau level fillings 1/3≲ν≲2/3 in hole systems confined to wide GaAs quantum wells when the sample is tilted in magnetic field to an intermediate angle. The parallel field component (B|) leads to a crossing of the lowest two Landau levels, and an elongated hole wave function in the direction of B|. Under these conditions, the in-plane resistance exhibits an insulating behavior, with the resistance along B| about 10 times smaller than the resistance perpendicular to B|. We interpret this anisotropic insulating phase as a two-component, striped Wigner crystal. [ABSTRACT FROM AUTHOR]

Interacting two-dimensional electrons confined in a GaAs quantum well exhibit isotropic transport when the Fermi level resides in the first excited (N=1) Landau level. Adding an in-plane magnetic field (B‖) typically leads to an anisotropic, stripelike (nematic) phase of electrons with the stripes oriented perpendicular to the B‖ direction. Our experimental data reveal how a periodic density modulation, induced by a surface strain grating from strips of negative electron-beam resist, competes against the B‖-induced orientational order of the stripe phase. Even a minute (<0.25%) density modulation is sufficient to reorient the stripes along the direction of the surface grating. [ABSTRACT FROM AUTHOR]

We report on high-order magnetoplasmon resonances detected in photoresistance in high-mobility GaAs quantum wells. These resonances manifest themselves as a series of photoresistance extrema in the regime of Shubnikov-de Haas oscillations. Extending to orders as high as 25, the extrema exhibit alternating strength, being less (more) pronounced at even- (odd)-order magnetoplasmon modes. This experimental technique provides a sensitive and elegant means to detect and investigate multiple magnetoplasmon modes and could be applied to other systems. [ABSTRACT FROM AUTHOR]

In previous studies, an inhomogeneous strain field was used as a trapping mechanism for interwell excitons in coupled GaAs/Al0.45Ga0.55 AS quantum wells. Photoluminescence measurements in this system demonstrated the presence of a dark population of excitons at trap center in the low-temperature, high-stress, high-density regime. The dramatic appearance of this effect at low temperature and high density initially suggested that it may be indicative of a Bose-Einstein condensation phase transition. Further experiments revealed that this effect appears more readily in wider quantum wells and occurs at strain values near the heavy-hole/light-hole crossover point. In this paper, it will be shown that this effect occurs in a regime where the heavy-hole valence band maximum shifts away from k|| = 0, which is expected to strongly suppress the recombination rate at low temperatures. Simulations based on this assumption will be shown to match the experimentally observed behavior, without requiring the appearance of a Bose-Einstein condensate. [ABSTRACT FROM AUTHOR]

In two-dimensional electron systems confined to wide GaAs quantum wells we observe a remarkable sequence of transitions of the fractional quantum Hah states in the filling factor range 1 < v < 3 near the crossing of two N = 0 Landau levels from different subbands and with opposite spins. The transitions attest to the interplay between the spin and subband degrees of freedom and can be explained as pseudospin polarization transitions where the pseudospins are the two crossing levels. The magnetic field positions of the transitions yield a quantitative measure of the transition energies or, equivalently, the composite fermions' discrete energy level separations. Surprisingly, these energies are much larger than those reported for other systems with SU(2) symmetry; moreover, they are larger when the electron system is less spin polarized. [ABSTRACT FROM AUTHOR]

In two-dimensional hole systems confined to wide GaAs quantum wells, where the heavy- and light-hole states are close in energy, we observe a very unusual crossing of the lowest two Landau levels as the sample is tilted in a magnetic field. At a magic tilt angle θ ≃ 34°, which surprisingly is independent of the well width or hole density, in a large filling factor range near ν = 1, the lowest two levels are nearly degenerate as evidenced by the presence of two-component quantum Hall states. Remarkably, a quantum Hall state is seen at ν = 1, consistent with a correlated Ψ111 state. [ABSTRACT FROM AUTHOR]

In two-dimensional (2D) electron systems, Wigner crystals (WC) and fractional quantum Hall effect (FQHE) liquids are competing ground states under low temperatures (T ) and high magnetic fields (B). Here we report differential conductivity results demonstrating the reentrant insulating phase around v = 1/5 in a 2D hole system in AlGaAs/GaAs quantum wells and unexpected features in the solid-liquid phase transition between WC and FQHE liquids in ultrahigh magnetic fields up to 45 T. Remarkably, the electric field (E) plays an equivalent role as the temperature does in our phase diagram. From the E - T "duality" analysis, a characteristic length of 450 nm is derived, which can be understood as the phase-coherent domain size of WC. Moreover, evidence shows that with weak disorder the insulating phase and composite fermion liquid could be coexisting around v = 1/5, pointing to the possibility that the insulating phase is the four flux quantum Wigner crystal, as proposed by theories. [ABSTRACT FROM AUTHOR]

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 singlecomponent and supports a composite fermion Fermi sea before transitioning into a ν = 1/2 fractional quantum Hall state, presumably, the two-component Ψ331 state. [ABSTRACT FROM AUTHOR]

In a quasi-two-dimensional electron system with nonzero layer thickness, a parallel magnetic field can couple to the out-of-plane electron motion and lead to a severe distortion and eventual splitting of the Fermi contour. Here we directly and quantitatively probe this evolution through commensurability and Shubnikov-de Haas measurements on electrons confined to a 40-nm-wide GaAs (001) quantum well. We are able to observe the Fermi contour splitting phenomenon, in good agreement with the results of semiclassical calculations. Experimentally, we also observe intriguing features, suggesting magneticbreakdown- type behavior when the Fermi contour splits. [ABSTRACT FROM AUTHOR]

Via measurements of commensurability features near the Landau filling factor υ = 1/2, we probe the shape of the Fermi contour for hole-flux composite fermions confined to a wide GaAs quantum well. The data reveal that the composite fermions are strongly influenced by the characteristics of the Landau level in which they are formed. In particular, their Fermi contour is warped when their Landau level originates from a hole band with significant warping. [ABSTRACT FROM AUTHOR]

Magnetotransport measurements on two-dimensional electrons confined to wide GaAs quantum wells reveal a remarkable evolution of the ground state at filling factor v = 1/2 as we tilt the sample in the magnetic field. Starting with a compressible state at zero tilt angle, a strong v = 1/2 fractional quantum Hall state appears at intermediate angles. At higher angles an insulating phase surrounds this state and eventually engulfs it at the highest angles. This evolution occurs because the parallel component of the field renders the charge distribution increasingly bilayer-like. The evolution is qualitatively similar to the one seen, in the absence of parallel field, as a function of increasing the electron density in the quantum well, but there are some notable differences. [ABSTRACT FROM AUTHOR]

We report ballistic transport commensurability minima in the magnetoresistance of ν=3/2 composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to ν=3/2, which we explain quantitatively by comparing three CF density models and concluding that the ν=3/2 CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe the shape and size of the CF Fermi contour. At a fixed electron density of ≃1.8×1011 cm−2, as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field (B∥) leads to a significant distortion of the CF Fermi contour as B∥ couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the ν=1/2 CF case at comparable B∥. Moreover, the applied B∥ further spin-polarizes the ν=3/2 CFs as deduced from the positions of the commensurability minima. [ABSTRACT FROM AUTHOR]

We study the critical spin-polarization energy (αC) above which fractional quantum Hall states in two-dimensional electron systems confined to symmetric GaAs quantum wells become fully spin polarized. We find a significant decrease of a c as we increase the well width. In systems with comparable electron layer thickness, a c for fractional states near Landau level filling v = 3/2 is about twice larger than those near v = 1/2, suggesting a broken particle-hole symmetry. Theoretical calculations, which incorporate Landau level mixing through an effective three-body interaction, and finite layer thickness capture certain qualitative features of the experimental results. [ABSTRACT FROM AUTHOR]

We observe the fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor v=1/2 in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayerlike charge distributions. The v=1/2 FQHE is stable when the charge distribution is symmetric and only in a range of intermediate densities, qualitatively similar to what is seen in two-dimensional electron systems confined to approximately twice wider GaAs quantum wells. Despite the complexity of the hole Landau level structure, originating from the coexistence and mixing of the heavy- and light-hole states, we find the hole v=1/2 FQHE to be consistent with a two-component, Halperin-Laughlin (ψ331) state. [ABSTRACT FROM AUTHOR]

We report an experimental investigation of the fractional quantum Hall effect (FQHE) at the even-denominator Landau-level filling factor v = 1/2 in very-high-quality wide GaAs quantum wells and at very high magnetic fields up to 45 T. The quasi-two-dimensional electron systems we study are confined to GaAs quantum wells with widths W ranging from 41 to 96 nm and have variable densities in the range of ≃4 × 1011 to ≃4 × 1010 cm-2. We present several experimental phase diagrams for the stability of the v = 1/2 FQHE in these quantum wells. In general, for a given W, the 1/2 FQHE is stable in a limited range of intermediate densities where it has a bilayerlike charge distribution: it makes a transition to a compressible phase at low densities and to an insulating phase at high densities. The densities at which the v = 1/2 FQHE is stable are larger for narrower quantum wells. Moreover, even a slight charge distribution asymmetry destabilizes the v = 1/2 FQHE and turns the electron system into a compressible state. We also present a plot of the symmetric-to-antisymmetric subband separation (ΔSAS), which characterizes the interlayer tunneling, vs density for various W. This plot reveals that ASas at the boundary between the compressible and FQHE phases increases linearly with density for all the samples. There is no theoretical explanation for such a simple dependence. Finally, we summarize the experimental data in a diagram that takes into account the relative strengths of the interlayer and intralayer Coulomb interactions and ΔSAS. We conclude that consistent with the conclusions of some of the previous studies, the v = 1/2 FQHE observed in wide GaAs quantum wells with symmetric charge distribution is stabilized by a delicate balance between the interlayer and intralayer interactions and is very likely described by a two-component (Ψ331) state. [ABSTRACT FROM AUTHOR]

Intense terahertz (THz) pulse induces photoluminescence (PL) flash from undoped high-quality GaAs/AlGaAs quantum wells under continuous wave laser excitation. The number of excitons increases 10?000-fold from that of the steady state under only laser excitation. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes impurity states during the 1 ps period of the THz pulse and release carriers from a giant reservoir containing impurity states in the AlGaAs layers. [ABSTRACT FROM AUTHOR]