Search

Expanding edge experiments are promising to open new physics windows of quantum Hall systems. In a static edge, the edge excitation, which is described by free fields decoupled with the bulk dynamics, is gapless, and the dynamics preserve conformal symmetry. When the edge expands, such properties need not be preserved. We formulate a quantum field theory in 1+1 dimensional curved spacetimes to analyze the edge dynamics. We propose methods to address the following questions using edge waveforms from the expanding region: Does the conformal symmetry survive? Is the nonlinear interaction of the edge excitations induced by edge expansion? Do the edge excitations interact with the bulk excitations? We additionally show that the expanding edges can be regarded as expanding universe simulators of two-dimensional dilaton-gravity models, including the Jackiw-Teitelboim gravity model. As an application, we point out that our theoretical setup might simulate emission of analog Hawking radiation with the Gibbons-Hawking temperature from the future de Sitter horizon formed in the expanding edge region., Comment: A subtitle and arguments about Hawking radiation in de Sitter space are added

The drift of electron spin helices in an external in-plane electric field in GaAs quantum wells is studied by means of time-resolved magneto-optical Kerr microscopy. The evolution of the spin distribution measured for different excitation powers reveals that, for short delay times and higher excitation powers, the spin helix drift slows down while its envelope becomes anisotropic. The effect is understood as a local decrease of the electron gas mobility due to electron collisions with nonequilibrium holes within the excitation spot and is reproduced well in the kinetic theory framework. For larger delay times, when the electrons constituting the spin helix and nonequilibrium holes are separated by an electric field, the spin helix drift accelerates and the mobility reaches its unperturbed value again.

Unintentionally doped impurities formed in the microstructures of free-standing GaN grown with facets were studied using confocal magneto-photoluminescence (PL) microscopy. Donor-bound exciton related peaks in PL spectra and their magnetic behavior allowed us to distinguish typical donor impurity atoms, such as silicon and oxygen. Combining this technique with confocal microscopy also revealed the spatial distribution of the impurities. The results showed that angled facets tend to incorporate oxygen. Moreover, even facets angled at a few degrees with respect to the (0001) surface cause a noticeable change in oxygen incorporation on the order of 1 × 1016 cm−3.

We conduct photoluminescence microscopy that is sensitive to both electron and nuclear spin polarization to investigate the changes that occur in the magnetic ordering in the vicinity of the first integer quantum Hall state in a GaAs two-dimensional electron system (2DES). We observe a discontinuity in the electron spin polarization and nuclear spin longitudinal relaxation time which heralds a spontaneous transition in the magnetic ordering. We image in real space the spin phase domains that coexist at this transition, and observe hysteresis in their formation as a function of the 2DES's chemical potential. Based on measurements in a tilted magnetic field orientation, we found that the transition is protected by an energy gap containing the Zeeman energy. We explain that these observations are consistent with a phase of skyrmions forming at the transition.

Using photoluminescence microscopy enhanced by magnetic resonance, we visualize in real space both electron and nuclear polarization occurring in nonequilibrium fraction quantum Hall (FQH) liquids. We observe stripelike domain regions comprising FQH excited states which discretely form when the FQH liquid is excited by a source-drain current. These regions are deformable and give rise to bidirectionally polarized nuclear spins as spin-resolved electrons flow across their boundaries.

Electron spin transport and dynamics are investigated in a single, high-mobility, modulation-doped, GaAs quantum well using ultrafast two-color Kerr-rotation micro-spectroscopy, supported by qualitative kinetic theory simulations of spin diffusion and transport. Evolution of the spins is governed by the Dresselhaus bulk and Rashba structural inversion asymmetries, which manifest as an effective magnetic field that can be extracted directly from the experimental coherent spin precession. A spin precession length L-SOI is defined as one complete precession in the effective magnetic field. It is observed that application of (a) an out-of-plane electric field changes the spin decay time and L-SOI through the Rashba component of the spin-orbit coupling, (b) an in-plane magnetic field allows for extraction of the Dresselhaus and Rashba parameters, and (c) an in-plane electric field markedly modifies both the L-SOI and diffusion coefficient. While simulations reproduce the main features of the experiments, the latter results exceed the corresponding simulations and extend previous studies of drift-current-dependent spin-orbit interactions., Comment: 11 pages, 7 figures

The electronic band structure of van der Waals (vdW) layered crystals has properties that depend on the composition, thickness and stacking of the component layers. Here we use density functional theory and high field magneto-optics to investigate the metal chalcogenide InSe, a recent addition to the family of vdW layered crystals, which transforms from a direct to an indirect band gap semiconductor as the number of layers is reduced. We investigate this direct-to-indirect bandgap crossover, demonstrate a highly tuneable optical response from the near infrared to the visible spectrum with decreasing layer thickness down to 2 layers, and report quantum dot-like optical emissions distributed over a wide range of energy. Our analysis also indicates that electron and exciton effective masses are weakly dependent on the layer thickness and are significantly smaller than in other vdW crystals. These properties are unprecedented within the large family of vdW crystals and demonstrate the potential of InSe for electronic and photonic technologies.

We perform real-space imaging of propagating magnetic domains in the fractional quantum Hall system using spin-sensitive photoluminescence microscopy. The propagation is continuous and proceeds in the direction of the conventional current, i.e. opposite to the electron flow direction. The mechanism of motion is shown to be connected to polarized nuclear spins around the domain walls. The propagation velocity increases when nuclei are depolarized, and decreases when the source-drain current generating this nuclear polarization is increased. We discuss how these phenomena may arise from spin interactions along the domain walls., 5 figures

Exciton, trion, and electron spin dynamics in a 20-nm-wide modulation-doped GaAs single quantum well are investigated using resonant ultrafast two-color Kerr rotation spectroscopy. Excitons and trions are selectively detected by resonant probe pulses while their relative spectral weight is controlled by adjusting the gate voltage which tunes the carrier density. Tuning the carrier density markedly influences the spin decay time of the two-dimensional electron gas. The spin decay time can be enhanced by a factor of 3 at an intermediate carrier concentration in the quantum well where excitons and trions coexist in the system. In addition, we explore the capability to tune the $g$ factor of the electron gas via the carrier density.

Electrons in semiconductors usually behave like a gas--as independent particles. However, when confined to two dimensions under a perpendicular magnetic field at low temperatures, they condense into an incompressible quantum liquid. This phenomenon, known as the fractional quantum Hall (FQH) effect, is a quantum-mechanical manifestation of the macroscopic behaviour of correlated electrons that arises when the Landau-level filling factor is a rational fraction. However, the diverse microscopic interactions responsible for its emergence have been hidden by its universality and macroscopic nature. Here, we report real-space imaging of FQH liquids, achieved with polarization-sensitive scanning optical microscopy using trions (charged excitons) as a local probe for electron spin polarization. When the FQH ground state is spin-polarized, the triplet/singlet intensity map exhibits a spatial pattern that mirrors the intrinsic disorder potential, which is interpreted as a mapping of compressible and incompressible electron liquids. In contrast, when FQH ground states with different spin polarization coexist, domain structures with spontaneous quasi-long-range order emerge, which can be reproduced remarkably well from the disorder patterns using a two-dimensional random-field Ising model. Our results constitute the first reported real-space observation of quantum liquids in a class of broken symmetry state known as the quantum Hall ferromagnet.

We demonstrate initialization of nuclear spin qubits using a resistively-detected nuclear magnetic resonance (NMR) device. In our device, nuclear spins are dynamically polarized in a sub-micron scale region defined by split gates. The population of each nuclear spin state is estimated from a resistively-detected NMR spectrum combined with numerical analysis. By applying radio frequency pulses to the polarized nuclear spins, we create two-qubit effective pure states, which is a crucial step toward NMR quantum computation. We also demonstrate simple logic gate operations such as controlled-NOT and SWAP gates in this two-qubit system. The obtained spectra are consistent with numerically calculated ones. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study the nuclear quadrupolar interaction using a resistively-detected NMR device. The nuclear spins are dynamically polarized in a sub-micron scale region defined by split gates. In pulsed NMR spectra for 69Ga and 75As nuclei, three distinct peaks are observed. The peak splitting depends on the electric quadrupolar coupling constant EQ. By comparing the spectra of both nuclei, we find that the relative peak intensity is opposite, which is independent of the condition for the dynamic nuclear spin polarization. This result indicates that the electric-field gradient in EQ has opposite sign at 69Ga and 75As nuclear site. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Manipulations of electron spin and nuclear spin have been studied in AlGaAs/GaAs semiconductor nanosystems. Non-local manipulation of electron spins has been realized by using the correlation effect between localized and mobile electron spins in a quantum dot- quantum wire coupled system. Interaction between electron and nuclear spins was exploited to achieve a coherent control of nuclear spins in a semiconductor point contact device. Using this device, we have demonstrated a fully coherent manipulation of any two states among the four spin levels of Ga and As nuclei. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have developed semiconductor point contact devices in which nuclear spins in a nanoscale region are coherently controlled by all-electrical methods. Different from the standard nuclear-magnetic resonance technique, the longitudinal magnetization of nuclear spins is directly detected by measuring resistance, resulting in ultra-sensitive detection of the microscopic quantity of nuclear spins. All possible coherent oscillations have been successfully demonstrated between two levels from four nuclear spin states of I = 3/2 nuclei. Quantum information processing is discussed based on two fictitious qubits of an I = 3/2 system and methods are described for performing arbitrary logical gates both on one and two qubits. A scheme for quantum state tomography based on Mz-detection is also proposed. As the starting point of quantum manipulations, we have experimentally prepared the effective pure states for the I = 3/2 nuclear spin system., 16 pages, 5 figures

Nuclear magnetic resonance (NMR), the analytical technique familiar in magnetic resonance imaging, tracks a quantum mechanical property of nuclei called spin, where each nucleus acts like a tiny barmagnet. Large coils and quite bulky samples are usually required to detect these magnets indirectly. But a new system makes it possible to detect spin directly, allowing NMR measurements on a nanoscale sample on a semiconductor chip. It also provides tighter control over nuclei than conventional NMR, so can probe nuclei containing multiple spin levels, one of the systems that can be used in quantum computing. The analytical technique of nuclear magnetic resonance (NMR1,2) is based on coherent quantum mechanical superposition of nuclear spin states. Recently, NMR has received considerable renewed interest in the context of quantum computation and information processing3,4,5,6,7,8,9,10,11, which require controlled coherent qubit operations. However, standard NMR is not suitable for the implementation of realistic scalable devices, which would require all-electrical control and the means to detect microscopic quantities of coherent nuclear spins. Here we present a self-contained NMR semiconductor device that can control nuclear spins in a nanometre-scale region. Our approach enables the direct detection of (otherwise invisible) multiple quantum coherences between levels separated by more than one quantum of spin angular momentum. This microscopic high sensitivity NMR technique is especially suitable for probing materials whose nuclei contain multiple spin levels, and may form the basis of a versatile multiple qubit device.

We study the photoluminescence (PL) spectrum of a two-dimensional electron system at the high magnetic field limit, where all electrons reside at the lowest Landau level (ν

Quantum energy teleportation (QET) is a proposed protocol related to the quantum vacuum. The edge channels in a quantum Hall system is well suited for the experimental verification of QET. For this purpose, we examine a charge density wave excited and detected by capacitively coupled front gate electrodes. We observe the waveform of the charge density wave, which is proportional to the time derivative of the applied square voltage wave. Further, we study the transmission and reflection behaviors of the charge density wave by applying a voltage to another front gate electrode to control the path of the edge state. We show that the threshold voltages where the dominant direction is switched in either transmission or reflection for dense and sparse waves are different from the threshold voltage where the current stops flowing in an equilibrium state., 4 pages, 4 figures

We study the photoluminescence spectrum of a low density (ν

We discuss methods of quantum state tomography for solid-state systems with a large nuclear spin $I=3/2$ in nanometer-scale semiconductors devices based on a quantum well. Due to quadrupolar interactions, the Zeeman levels of these nuclear-spin devices become nonequidistant, forming a controllable four-level quantum system (known as quartit or ququart). The occupation of these levels can be selectively and coherently manipulated by multiphoton transitions using the techniques of nuclear magnetic resonance (NMR) [Yusa et al., Nature (London) 434, 101 (2005)]. These methods are based on an unconventional approach to NMR, where the longitudinal magnetization $M_z$ is directly measured. This is in contrast to the standard NMR experiments and tomographic methods, where the transverse magnetization $M_{xy}$ is detected. The robustness against errors in the measured data is analyzed by using the condition number based on the spectral norm. We propose several methods with optimized sets of rotations yielding the highest robustness against errors, as described by the condition number equal to 1, assuming an ideal experimental detection. This robustness is only slightly deteriorated, as given by the condition number equal to 1.05, for a more realistic "noisy" $M_z$ detection based on the standard cyclically-ordered phase sequence (CYCLOPS) method., 19 pages, 9 figures

The scattering of carriers in a two-dimensional electron gas (2DEG) in a series of novel GaAs/ n-AlGaAs heterojunctions is studied for the case where InAlAs anti-dots are embedded near the channel. Mobilities μ are measured as functions of the electron concentrations N e and are compared with theoretical models which take into account the shape and distribution of anti-dots. In samples with low dot densities, mobilities are well explained in terms of the scattering by anti-dots with a random distribution. In contrast, in samples with dense and overlapping dots and a thick wetting layer, a roughness-like potential with Gaussian-type correlation is shown to account for the data. Influences of growth conditions and the alloy composition on the diameter and the density of InAlAs dots are also discussed.

By a computer simulation we study the real space and energy distributions of 0D electrons bound in a planar array of quantum dots, including both intra-dot charging and inter-dot Coulomb interaction effects, size fluctuations, as well as the screening by a parallel gas of 2D electrons. It is demonstrated that the mutual Coulomb shifts between different dots cause pronounced many-body correlation effects and in-plane potential fluctuations, which can be significant for experiments such as capacitance and tunneling spectroscopy. In addition we investigate the influence of charged dot scattering on the mobility of a conducting channel parallel to the dot layer.

We study the coherence time of nuclear spins of 75As nuclei in a GaAs quantum well using a submicron-scale all-electrical nuclear magnetic resonance (NMR) device. We estimate the extrinsic transverse relaxation time T2* and intrinsic transverse relaxation time T2 of nuclear spins using different pulse sequences to measure the decay of spin–echo signals. We significantly improve the coherence time from 0.05 to 1.4 ms by eliminating the spatial field inhomogeneity and temporal fluctuation of surrounding electrons and nuclei with the aid of heteronuclear and electron–nuclear spin decoupling techniques. By analyzing the data, we extract individual sources of decoherence in GaAs. We discuss the relationship between the obtained T2* and the peak width of the NMR spectra.

We have studied single electron and hole storage in self-assembled InAs quantum dots (QDs) embedded in GaAs/n-AlGaAs field effect transistors (QD-FETs). We prepared two types of QD-FETs. A single electron and a photo-generated single hole can be stored in each QD in Type 1. In the new Type II, single-electron discharge processes can be controlled by a surface gate voltage ( V g ) as well as single-electron storage processes. We demonstrate possible application to novel photo devices and quantum dot memory devices.

The trapping of electrons and holes in self-assembled InAs quantum dots (QDs) has been studied at low temperatures in GaAs/n-AlGaAs heterostructures. It has been found that the concentration Ns of two-dimensional electrons at a given gate voltage Vg is persistently enhanced by laser illumination, because of the trapping of holes by QDs. We also studied how the number of trapped holes depends on the energy of excitation photons from a Ti-sapphire laser. Applications of this device for a memory and near-infrared detector are discussed.

Quantum energy teleportation (QET) is, from the operational viewpoint of distant protocol users, energy transportation via local operations and classical communication. QET has various links to fundamental research fields including black-hole physics, the quantum theory of Maxwell's demon, and quantum entanglement in condensed-matter physics. However, the energy that has been extracted using a previous QET protocol is limited by the distance between two protocol users; the upper bound of the energy being inversely proportional to the distance. In this paper, we prove that introducing squeezed vacuum states with local vacuum regions between the two protocol users overcomes this limitation, allowing energy teleportation over practical distances.

We propose an experimental method for a quantum protocol termed quantum energy teleportation (QET), which allows energy transportation to a remote location without physical carriers. Using a quantum Hall system as a realistic model, we discuss the physical significance of QET and estimate the order of energy gain using reasonable experimental parameters.

We report a novel GaAs-based device in which I = 3/2 nuclear spins of 69Ga, 71Ga and 75As in a nanometer scale region can be manipulated by all-electrical means. The device comprises a quantum point contact (QPC), a narrow conduction channel in a GaAs quantum well defined by split gates, integrated with an additional metal strip on top for applying a radio-frequency (RF) pulse. With the device set in a special condition characterized by the Landau-level filling factor v = 2/3, nuclear spins in the narrow region near the QPC can be selectively polarized by driving a current through the QPC. By applying a resonant RF pulse, the polarized nuclei can be coherently manipulated, which we detect through the electrical resistance of the QPC. Different from the conventional nuclear magnetic resonance measuring the transverse component of the magnetization, our device measures the longitudinal component, which enables us to observe otherwise invisible multiple quantum coherences between states with z projection of the angular momentum differing by more than one. By appropriately tuning the length, intensity, and detuning of the RF pulse, all possible coherent superposition between two out of the four Zeeman levels can be created for each nuclide.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We study decoherence of nuclear spins in a GaAs quantum well structure using resistively detected nuclear magnetic resonance. The transverse decoherence time T2 of 75As nuclei is estimated from Rabi-type coherent oscillations as well as by using spin-echo techniques. By analyzing T2 obtained by decoupling techniques, we extract the role of dipole-dipole interactions as sources of decoherence in GaAs. Under the condition that the device is tilted in an external magnetic field, we exhibit enhanced decoherence induced by the change in strength of the direct dipole-dipole interactions between first nearest-neighbor nuclei. The results agree well with simple numerical calculations., Comment: 4 pages, 4 figures

We study the nuclear spin population in a GaAs quantum well structure and demonstrate its initialization using an all-electrical nuclear magnetic resonance (NMR) device. In our device, nuclear spins are dynamically polarized in a sub-micron scale region defined by split gates. The nuclear spin populations under various polarization conditions are estimated from resistively-detected pulsed NMR spectra. We find that nuclear spin populations are determined by electron spin configurations. By applying radio frequency pulses to the strongly polarized nuclear spins, we demonstrate the creation of two-qubit effective pure states, which is a crucial step toward NMR quantum computation., Comment: 6 pages, 3 figures, Accepted for publication in App. Phys. Lett

Yusa et al. reported an anomalous cooling of radiative quantum dot (QD) excitons by THz‐radiation in [Proc. 24th ICPS, 1083 (1998)] We have analyzed this experiment using continuum elasticity, multi‐band k⋅p and spin‐resolved Monte‐Carlo methods. We show that the unexpected discovery is related to hole relaxation via piezo‐electric potential minima, induced in the QD sample by InP stressor islands. The THz‐radiation gives rise to a drift of dark excitons from the piezo‐electric minima to radiative states in the deformation potential minimum. This increases the QD ground state luminescence at the expense of the luminescence from higher QD states. We reproduce also the delayed flash of QD ground state luminescences when a THz‐radiation pulse hits the sample even ∼ 1 s after switching off the carrier generation.

Coherent oscillations between any two levels from four nuclear spin states of I=3/2 have been demonstrated in a nanometre-scale NMR semiconductor device, where nuclear spins are all-electrically controlled. Using this device, we discuss quantum logic operations on two fictitious qubits of the I=3/2 system, and propose a quantum state tomography scheme based on the measurement of longitudinal magnetization, $M_z$., 5 pages, 4 figures

The trapping of photogenerated carriers by embedded InAs quantum dots (QDs) has been studied at 77 K in novel GaAs/n-AlGaAs structures. It is found that the concentration Ns of two dimensional electrons at a given gate voltage Vg is persistently increased by light illumination, because of the trapping of holes by QDs. By the interplay of the gate voltage and photocarrier generation, a distinct hysteresis is observed in the Ns-Vg characteristics. A drastic change of electron mobility by a factor of 19 is achieved by light illumination. The applications of this device for a novel light-controllable floating dot memory is suggested.

The resonant tunneling of electrons through a 20 nm scale InAs quantum dot bound by a pair of very thin AlAs barriers is studied. A well-resolved composite peak resulting from the ground 1s states was observed at 4.2 K in current–voltage characteristics. By investigating the effects of inplane magnetic fields, the shape of the wave function and the spatial extent of the first two electronic states are clarified.

We study electron‐nuclear spin coupling implemented in mesoscopic fractional quantum Hall (FQH) devices. We find that longitudinal resistance in such systems oscillates with a period of several hundreds of seconds driven by a constant voltage instead of a constant current. The anomalous behavior suggests that an average nuclear spin polarization self‐sustainingly oscillates between randomized and polarized states, which reveal nonlinear nature of the mesoscopic electron‐nuclear spin coupled systems.

We study electron-nuclear spin coupled systems implemented in mesoscopic fractional quantum Hall devices. We find that longitudinal resistance in such systems, oscillates with a period of several hundreds of seconds driven by a constant voltage instead of a constant current. The anomalous behavior suggests that an average nuclear spin polarization self-sustainingly oscillates between randomized and polarized states, which reveal the nonlinear nature of the mesoscopic electron-nuclear spin coupled systems.

We study the magnetoresistance R x x , and the Hall resistance R x x in GaAs/Al x Ga 1 - x As/GaAs bilayer systems, in which the electron channel of each layer is latterally confined in a narrow Hall bar of width W = 0.3-5μm. We observe that intralayer backscattering destroy some states peculiar to the bilayer system. R x y plateaus at odd filling factors largely deviate from their integer values in monolayer quantum Hall (QH) states due to backscattering. while the normal R x y plateau is observed from coupled QH stales. Examining the backscattering both in the Hall bars and in their voltage probes, we discuss the suppression of the intralayer backscattering between edge channels strongly modified by the interlayer charge transfer between the front and the back channels.

We study electron-nuclear spin coupled systems implemented in microscopic fractional quantum Hall (FQH) devices. We find that the longtitudinal resistance in such systems oscillates with a period of about ∼200 s and is driven by a constant voltage instead of a constant current. This anomalous behavior suggests that an average nuclear spin polarization self-sustainingly oscillates between randomized and polarized states which reveal the nonlinear nature of the mesoscopic electron-nuclear spin coupled systems.

We study the photoluminescence spectrum of a low density ($��, 10 pages, 5 figures

We study the evolution of the absorption spectrum of a modulation doped GaAs/AlGaAs semiconductor quantum well with decreasing the carrier density. We find that there is a critical density which marks the transition from a Fermi edge singularity to a hydrogen-like behavior. At this density both the lineshape and the transitions energies of the excitons change. We study the density dependence of the singularity exponent $\alpha $ and show that disorder plays an important role in determining the energy scale over which it grows., Comment: 10 pages, 3 figures

The electron-spin/nuclear-spin interactions in semiconductors are summarized by putting emphasis on dynamical nuclear-spin polarization and detection achieved by using electrical means. These have been demonstrated in quantum dots in the spin-blockade regime, edge channel in the integer quantum-Hall-effect regime and bulk in the fractional quantum-Hall-effect regime. The electron-spin/nuclear-spin interactions, especially at the spin transition point of ν = 2/3 fractional filling, result in an almost linear relationship between nuclear-spin magnetization and the resistance value. As the nuclear-spin magnetization can be measured for a single layer and even for nanostructures by just measuring the resistance, the powerful features of nuclear magnetic resonance can be successfully applied to semiconductor quantum wells, bilayers and point-contact structures where characteristics are well controlled by gates. In GaAs point-contact devices, full coherent control of a quantum four-level system has been demonstrated for I = 3/2 As and Ga nuclei toward nuclear-spin-based quantum information processing. Multiple quantum coherence was clearly observed reflecting the direct detection of nuclear-spin magnetization. In quantum wells and bilayer systems, novel electron-spin features, such as spin texture, a canted spin state and related low-frequency spin fluctuations arising from the breakdown of planar symmetry, have been sensitively detected by using nuclear-spin-based measurements. We also discuss electron-spin fluctuations originating from spin–orbit interactions observed via a nuclear relaxation experiment and the characterization of the nanoscale strain obtained through quadrupolar splitting. Finally, a possible extension of nuclear-spin manipulation and nuclear-spin-based measurements is briefly discussed.

Novel GaAs/n-AlGaAs FETs have been developed by placing InAs quantum dots near the channel. The authors show that the electron concentration increases linearly with gate voltage but its threshold voltage can be programmable by trapping electrons in these dots. Analysis has shown that one electron is trapped by each InAs dot.

We report on a novel method of fabricating 10-nm-scale GaAs dot structures using molecular beam epitaxy and in situ gas etching. Self-assembled nanometer-scale InAs dots are formed first and are used as masks for the subsequent etching of GaAs by Cl2 gas. In this approach, we also make use of HCl gas etching to remove selectively the InAs wetting layer as well as the InAs dots to yield clean and strain-free GaAs dot structures. The height and base diameter of resultant GaAs dots are studied by atomic force microscopy and found to be 10±2 nm and 30±5 nm, respectively.