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Fractional quantum Hall states (FQHSs) at even-denominator Landau level filling factors ($\nu$) are of prime interest as they are predicted to host exotic, topological states of matter. We report here the observation of a FQHS at $\nu=1/2$ in a two-dimensional electron system of exceptionally high quality, confined to a wide AlAs quantum well, where the electrons can occupy multiple conduction-band valleys with an anisotropic effective mass. The anisotropy and multi-valley degree of freedom offer an unprecedented tunability of the $\nu=1/2$ FQHS as we can control both the valley occupancy via the application of in-plane strain, and the ratio between the strengths of the short- and long-range Coulomb interaction by tilting the sample in the magnetic field to change the electron charge distribution. Thanks to this tunability, we observe phase transitions from a compressible Fermi liquid to an incompressible FQHS and then to an insulating phase as a function of tilt angle. We find that this evolution and the energy gap of the $\nu=1/2$ FQHS depend strongly on valley occupancy.

Electrons' multiple quantum degrees of freedom can lead to rich physics, including a competition between various exotic ground states, as well as novel applications such as spintronics and valleytronics. Here we report magneto-transport experiments demonstrating how the valley degree of freedom impacts the fractional quantum states (FQHSs), and the related magnetic-flux-electron composite fermions (CFs), at very high magnetic fields in the extreme quantum limit when only the lowest Landau level is occupied. Unlike in other multivalley two-dimensional electron systems such as Si or monolayer graphene and transition-metal dichalcogenides, in our AlAs sample we can continuously tune the valley polarization via the application of in-situ strain. We find that the FQHSs remain exceptionally strong even as they make valley polarization transitions, revealing a surprisingly robust ferromagnetism of the FQHSs and the underlying CFs. Our observation implies that the CFs are strongly interacting in our system. We are also able to obtain a phase diagram for the FQHS and CF valley polarization in the extreme quantum limit as we monitor transitions of the FHQSs with different valley polarizations.

The interplay between the Fermi sea anisotropy, electron-electron interaction, and localization phenomena can give rise to exotic many-body phases. An exciting example is an anisotropic two-dimensional (2D) Wigner solid (WS), where electrons form an ordered array with an anisotropic lattice structure. Such a state has eluded experiments up to now as its realization is extremely demanding: First, a WS entails very low densities where the Coulomb interaction dominates over the kinetic (Fermi) energy. Attaining such low densities while keeping the disorder low is very challenging. Second, the low-density requirement has to be fulfilled in a material that hosts an anisotropic Fermi sea. Here, we report transport measurements in a clean (low-disorder) 2D electron system with anisotropic effective mass and Fermi sea. The data reveal that at extremely low electron densities, when the r_{s} parameter, the ratio of the Coulomb to the Fermi energy, exceeds ≃38, the current-voltage characteristics become strongly nonlinear at small dc biases. Several key features of the nonlinear characteristics, including their anisotropic voltage thresholds, are consistent with the formation of a disordered, anisotropic WS pinned by the ubiquitous disorder potential.

Fractional quantum Hall states (FQHSs) exemplify exotic phases of low-disorder two-dimensional (2D) electron systems when electron-electron interaction dominates over the thermal and kinetic energies. Particularly intriguing among the FQHSs are those observed at even-denominator Landau level filling factors, as their quasi-particles are generally believed to obey non-Abelian statistics and be of potential use in topological quantum computing. Such states, however, are very rare and fragile, and are typically observed in the excited Landau level of 2D electron systems with the lowest amount of disorder. Here we report the observation of a new and unexpected even-denominator FQHS at filling factor {\nu} = 3/4 in a GaAs 2D hole system with an exceptionally high quality (mobility). Our magneto-transport measurements reveal a strong minimum in the longitudinal resistance at {\nu} = 3/4, accompanied by a developing Hall plateau centered at (h/e2)/(3/4). This even-denominator FQHS is very unusual as it is observed in the lowest Landau level and in a 2D hole system. While its origin is unclear, it is likely a non-Abelian state, emerging from the residual interaction between composite fermions., Comment: 6+6 pages, 3+3 figures; featured in Physics; featured as Editor's suggestion

The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors ($\nu\lesssim1/6$) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultra-high-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density $n=6.1\times10^{10}$ /cm$^2$ and mobility $\mu=25\times10^6$ cm$^2$/Vs, we find a deep minimum in the longitudinal magnetoresistance ($R_{xx}$) at $\nu=1/7$ when $T\simeq104$ mK. There is also a clear sign of a developing minimum in the $R_{xx}$ at $\nu=2/13$. While insulating phases are still predominant when $\nu\lesssim1/6$, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence $\nu=p/(2mp\pm1)$ even in the very low Landau level filling limit. The magnetic field dependent activation energies deduced from the relation $R_{xx}\propto e^{E_A/2kT}$ corroborate this view, and imply the presence of pinned Wigner solid states when $\nu\neq p/(2mp\pm1)$. Similar results are seen in another sample with a lower density, further generalizing our observations., Comment: 6 pages, 4 figures

The complex band structure, large spin-orbit induced band splitting, and heavy effective mass of two-dimensional (2D) hole systems hosted in GaAs quantum wells render them rich platforms to study many-body physics and ballistic transport phenomena. Here we report ultra-high-quality (001) GaAs 2D hole systems, fabricated using molecular beam epitaxy and modulation doping, with mobility values as high as $5.8\times10^6$ cm$^2$/Vs at a hole density of $p=1.3\times10^{11}$ /cm$^2$, implying a mean-free path of $\simeq27$ $\mu$m. In the low-temperature magnetoresistance trace of this sample, we observe high-order fractional quantum Hall states up to the Landau level filling $\nu=12/25$ near $\nu=1/2$. Furthermore, we see a deep minimum develop at $\nu=1/5$ in the magnetoresistance of a sample with a much lower hole density of $p=4.0\times10^{10}$ /cm$^2$ where we measure a mobility of $3.6\times10^6$ cm$^2$/Vs. These improvements in sample quality were achieved by reduction of residual impurities both in the GaAs channel and the AlGaAs barrier material, as well as optimization in design of the sample structure., Comment: 9 pages, 4 figures

In a low-disorder two-dimensional electron system, when two Landau levels of opposite spin or pseudospin cross at the Fermi level, the dominance of the exchange energy can lead to a ferromagnetic, quantum Hall ground state whose gap is determined by the exchange energy and has skyrmions as its excitations. This is normally achieved via applying either hydrostatic pressure or uniaxial strain. We study here a very high-quality, low-density, two-dimensional hole system, confined to a 30-nm-wide (001) GaAs quantum well, in which the two lowest-energy Landau levels can be gate tuned to cross at and near filling factor $\nu=1$. As we tune the field position of the crossing from one side of $\nu=1$ to the other by changing the hole density, the energy gap for the quantum Hall state at $\nu=1$ remains exceptionally large, and only shows a small dip near the crossing. The gap overall follows a $\sqrt{B}$ dependence, expected for the exchange energy. Our data are consistent with a robust quantum Hall ferromagnet as the ground state., Comment: 7+2 pages, 4+4 figures

We studied electron-hole plasma in separated electron and hole layers. We observed a strong enhancement of photoluminescence intensity at the Fermi energy of the ultracold plasma that evidences the emergence of excitonic Fermi edge singularity.

Degenerate conduction-band minima, or `valleys', in materials such as Si, AlAs, graphene, and MoS$_2$ allow them to host two-dimensional electron systems (2DESs) that can access a valley degree of freedom. These multivalley 2DESs present exciting opportunities for both pragmatic and fundamental research alike because not only are they a platform for valleytronic devices, but they also provide a tool to tune and investigate the properties of complex many-body ground states. Here, we report ultra-high quality, modulation doped AlAs quantum wells containing 2DESs that occupy two anisotropic valleys and have electron mobilities peaking at $2.4\times10^6$ cm$^{2}$V$^{-1}$s$^{-1}$ at a density of $2.2\times10^{11}$ cm$^{-2}$. This is more than an order of magnitude improvement in mobility over previous results. The unprecedented quality of our samples is demonstrated by magneto-transport data that show high-order fractional quantum Hall minima up to the Landau level filling $\nu=8/17$, and even the elusive $\nu=1/5$ quantum Hall state.

The enigmatic even-denominator fractional quantum Hall state at Landau level filling factor ν=5/2 is arguably the most promising candidate for harboring Majorana quasiparticles with non-Abelian statistics and, thus, of potential use for topological quantum computing. The theoretical description of the ν=5/2 state is generally believed to involve a topological p-wave pairing of fully-spin-polarized composite fermions through their condensation into a non-Abelian Moore-Read Pfaffian state. There is, however, no direct and conclusive experimental evidence for the existence of composite fermions near ν=5/2 or for an underlying fully-spin-polarized Fermi sea. Here, we report the observation of composite fermions very near ν=5/2 through geometric resonance measurements and find that the measured Fermi wave vector provides direct demonstration of a Fermi sea with full spin polarization. This lends crucial credence to the model of 5/2 fractional quantum Hall effect as a topological p-wave paired state of composite fermions.

The fractional quantum Hall effect (FQHE) states at half integer Landau fillings ($\nu$) have long been of great interest, since they have correlations that differ from those of the fundamental Laughlin states found at odd denominators. At $\nu=1/2$ the FQHE has been observed in wide or double quantum wells, and is ascribed to the two-component Halperin-Laughlin $\Psi_{331}$ state. $\Psi_{331}$ excitations carry charge $\pm e/4$, like the carriers of $\nu=5/2$ states which are of interest in quantum computation. Further, such an excitation (quasiparticle or -hole) of $\Psi_{331}$ has unequal, opposite charge in the top and bottom layers, and hence an up or down dipole moment. Here we report evidence for a Wigner solid (WS) of such dipolar quasiholes from a quantitative study of the microwave spectra of a wide quantum well (WQW) at $\nu$ close to 1/2.

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.

We report on the transport properties of a two-dimensional electron gas (2DEG) confined in an AlGaN∕GaN heterostructure grown by plasma-assisted molecular-beam epitaxy on a semi-insulating GaN template prepared by hydride vapor phase epitaxy with a threading dislocation density of ∼5×107cm−2. Using a gated Hall bar structure, the electron density (ne) is varied from 4.1to9.1×1011cm−2. At T=300mK, the 2DEG displays a maximum mobility of 167000cm2∕Vs at a sheet density of 9.1×1011cm−2, corresponding to a mean-free-path of ∼3μm. Shubnikov–de Haas oscillations, typically not observed at magnetic fields below 2T in GaN, commence at B=0.6T.

We report on the transport properties of a tunable two-dimensional electron gas (2DEG) confined at the lower interface of a GaN∕Al0.06Ga0.94N∕GaN heterostructure grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy. Using an insulated gate Hall bar structure, the electron density is continuously tuned from ∼2×1012 down to 1.5×1011cm−2. At T=300mK, the 2DEG displays a maximum mobility of 80 000cm2∕Vs at a sheet density of 1.75×1012cm−2. At low densities, the mobility exhibits a power law dependence on density −μ∼neα, with α∼1.0, over the range of 2×1011–1×1012cm−2. In this density regime, the mobility is no longer limited by alloy scattering and long-range Coulomb scattering dominates. We discuss the dominant scattering mechanisms that presently limit low temperature mobility at electron densities below 1×1012cm−2.

We have studied the temperature dependent resistivity at exactly half Landau level filling of a high-quality two-dimensional electron system in high magnetic field. The experimental data can be well described in terms of a standard degenerate system of fermions at zero magnetic field being scattered by impurities and acoustic phonons. The gauge-field-mediated phonon scattering contributes a ${T}^{\ensuremath{-}3}$ dependence to the composite fermion mobility. The effective mass of composite fermions is obtained from the temperature dependence of composite fermion-impurity scattering.

In transmission studies of two-dimensional AlGaAs-based photonic crystal waveguide structures, we demonstrate high coupling efficiencies from ridge waveguides to photonic crystal waveguides using photonic crystal waveguide tapers. Enhanced bending efficiencies and bend bandwidths are achieved by use of modified waveguide bends.

We report a severe, spin-dependent, Fermi contour anisotropy induced by parallel magnetic field in a high-mobility (001) GaAs two-dimensional hole system. Employing commensurability oscillations created by a unidirectional, surface-strain-induced, periodic potential modulation, we directly probe the anisotropy of the two spin subband Fermi contours. Their areas are obtained from the Fourier transform of the Shubnikov-de Haas oscillations. Our findings are in semi-quantitative agreement with the results of parameter-free calculations of the energy bands., Comment: 4 pages, 4 figures

Today the fractional quantum Hall effect (FQHE) is well understood as the condensation of two-dimensional electrons in a high magnetic field into a sequence of quantum liquids. Thus far the relationship between the different liquids has been viewed as a hierarchy in which higher-order fractional states develop from the condensation of the elementary excitation of the next fewer-order fractional state. Recent theory has shed a new light on these relationships and provided us with a new framework for the FQHE. The central ingredients of this new picture are bizarre new particles, often termed composite fermions. This paper reviews our recent experimental evidence for the existence of such objects.

We have recently developed a spectroscopic technique which allows direct measurement of quantum energy levels. The method is based on observation of the capacitance signal resulting from single electrons tunneling into discrete quantum levels. The electrons tunnel between a metallic layer and confined states of a microscopic capacitor fabricated in GaAs. Charge transfer occurs only for bias voltages at which a quantum level resonates with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions directly reflect the electronic spectrum of the confined structure. Using this “single-electron capacitance spectroscopy”, we map the magnetic field dependence of the ground state energies of a single quantum dot containing from 0 to 50 electrons. Along with a spectroscopic measurement of the dot's ground states, we probe tunneling rates of electrons to individual quantum states. The experimental spectra reproduce many features of a noninteracting electron model with an added fixed charging energy. However, in detailed observations deviations are apparent: exchange induces a two-electron singlet-triplet transition, self-consistency of the confinement potential causes the dot to assume a quasi two-dimensional character, and features develop which are suggestive of the fractional quantum Hall effect.

We report magnetotransport measurements in wide GaAs quantum wells with a tunable density to probe the stability of the fractional quantum Hall effect at a filling factor of ν=5/2 in the vicinity of the crossing between Landau levels (LLs) belonging to the different (symmetric and antisymmetric) electric subbands. When the Fermi energy (E(F)) lies in the excited-state LL of the symmetric subband, the 5/2 quantum Hall state is surprisingly stable and gets even stronger near this crossing, and then suddenly disappears and turns into a metallic state once E(F) moves to the ground-state LL of the antisymmetric subband. The sharpness of this disappearance suggests a first-order transition.

A new picture of fractional quantum Hall effect (FQHE) in terms of a novel particle called composite fermion has emerged recently. A composite fermion is a composite of two flux quanta which are effectively bound to an electron as a result of electron-electron interaction. A system of electrons at half-filled Landau level can be transformed to an equivalent system of composite fermions at zero effective magnetic field with a distinct Fermi surface. The FQHE is then viewed as the integral quantum Hall effect of composite fermions away ffrom half-filling. in order to test for these new particles, we have studied transport of anti-dot superlattices in a two-dimensional electron gas. At low magnetic fields electron transport exhibits well-known resonances at fields where the classical cyclotron orbit becomes commensurate with the anti-dot lattice. At half-filling we observe the same dimensional resonances. This establishes the “semi-classical” behavior of composite fermions.

Using single-electron capacitance spectroscopy, we map the magnetic field dependence of the ground state energies of a single quantum dot containing from 0 to 50 electrons. The experimental spectra reproduce many features of a noninteracting electron model with an added fixed charging energy. However, in detailed observations deviations are apparent: Exchange induces a two-electron singlet-triplet transition, self-consistency of the confinement potential causes the dot to assume a quasi-two-dimensional character, and features develop which are suggestive of the fractional quantum Hall effect.

This paper presents a technique which permits a quantitative spectroscopy of discrete quantum levels in structures containing as few as one electron. The ground state energy in a quantum dot can be measured for an arbitrary number of electrons and followed as a function of magnetic field. The method involves monitoring the capacitance signal resulting from the tunneling of single electrons. In a microscopic capacitor fabricated in GaAs we study the confined states of a single 1 μm disk to which electrons can tunnel from a nearby metallic layer. Charge transfer occurs only for bias voltages at which a quantum level is resonant with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions can ve directly converted to an energy scale to determine the electronic spectrum of the confined structure. The evolution of the spectrum in magnetic field allows deduction of the nature of the bound states.

We present a technique which permits a quantitative spectroscopy of discrete quantum levels in semiconductor microstructures. The method involves monitoring the capacitance signal resulting from single-electron tunneling. In a microscopic capacitor fabricated in GaAs we study the confined states of a single 1 μm disk to which electrons can tunnel from a nearby metallic layer. Charge transfer occurs only for bias voltages at which a quantum level is resonant with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions can be directly converted to an energy scale to determine the electronic spectrum of the confined structure. The evolution of the spectrum in magnetic fields allows deduction of the nature of the bound states.

We review a new molecular beam epitaxy (MBE) technique we call cleaved edge overgrowth (CEO), which makes possible fabrication of quantum wires or other lower dimensional quantum structures with atomic precision. CEO is accomplished by performing two separate MBE overgrowths separated by an in situ cleave of the substrate sample. We review our development of this novel method and give several examples of new structures recently fabricated using it.

Few-layer graphene films are grown using a Molecular Beam Deposition (MBD) technique in ultra-high vacuum by evaporation of atomic carbon and subsequent annealing of the samples at 800–900 °C. The graded thickness layers are grown on strip-shaped oxidized silicon substrates which are covered with 300 nm thick nickel films deposited by e-beam evaporation. The thickness of the deposited carbon layers changes continuously from 70 Å to less than 4 Å. The relatively narrow optical phonon bands in Raman spectroscopy reveal that good quality multilayer graphene films form on the Ni surface., This work is supported by ONR (N000140610138 and Graphene Muri), NSF (CHE-0117752 and CHE-0641523),NYSTAR, Spanish CAM (Q&C Light, S2009ESP-1503), Spanish MICINN (NANINPHO-QD, TEC2008-06756-C03- 01, CONSOLIDER QOIT and Salvador de Madariaga Grant no. PR2007-0036).

We report on a strikingly linear behavior of the magnetoresistivity, ϱxx versus magnetic field B in two-dimensional electron systems at low-temperatures. While linearity of the Hall resistivity ϱxy versus magnetic field, punctuated in the region of the quantum Hall effect, is a direct consequence of the classical Hall geometry, there does not exist a theoretical basis for a linearity of ϱxx. The curious behavior of ϱxx is implicitly contained in a phenomenological relationship between ϱxx and ϱxy proposed earlier by Chang and Tsui. Our new data on very high mobility samples dramatically confirm their conjecture of this still unresolved proportionality between ϱxx and B dϱ xy dB .

We have studied edge magnetoplasmons (EMP's) created by pulses of 100 ps width in a high-mobility two-dimensional electron gas, using a cryogenic transistor. We find that magnetoplasmon motion along the edge occurs only in the direction expected for negatively charged electrons; there is no hint of propagation in the opposite direction expected for positively charged quasiholes. We also measure the periods and decay rates of the edge modes continuously as a function of the magnetic field. From these characteristics we deduce the width of the EMP boundary layer as well as the magnetic-field-dependent momentum-relaxation time.

We demonstrate that ballistic electrons in ultra-high mobility two-dimensional electron systems can be controlled in ways analogous to the manipulation of photons in optical systems. Among the electron-optical structures we demonstrate are emitters, detectors, absorbers, and refractors. These advances may enable the design of new types of electronic circuits.

Understanding the flow of spins in magnetic layered structures has enabled an increase in data storage density in hard drives over the past decade of more than two orders of magnitude1. Following this remarkable success, the field of 'spintronics' or spin-based electronics is moving beyond effects based on local spin polarisation and is turning its attention to spin-orbit interaction (SOI) effects, which hold promise for the production, detection and manipulation of spin currents, allowing coherent transmission of information within a device. While SOI-induced spin transport effects have been observed in two- and three-dimensional samples, these have been subtle and elusive, often detected only indirectly in electrical transport or else with more sophisticated techniques. Here we present the first observation of a predicted 'spin-orbit gap' in a one-dimensional sample, where counter-propagating spins, constituting a spin current, are accompanied by a clear signal in the easily-measured linear conductance of the system., Comment: 10 pages, 5 figures, supplementary information

We have recently demonstrated the existence of a high mobility (6.1×10 5 cm 2 /V·s) two-dimensional electron gas (2DEG) at the (110) vicinal surface formed by cleaving [L. Pfeiffer et al., Appl. Phys. Letters 56 (1990) 1697] a (100) GaAs wafer. We have now expanded this work to modulation-doped overgrowth on the cleaved edge of a multiperiod superlattice. We report here the first observation of the quantum Hall characteristics in such a two-dimensional system containing an atomically precise 71 A GaAs by 31 A Al 0.24 Ga 0.76 As compositional superlattice. The onset of Shubnikov-De Haas oscillations occurs at only 3000 G, implying the Landau cyclotron orbits are phase coherent over diameters as large as 5000 A, corresponding to more than 200 GaAs/AlGaAs interface crossings.

We present a spectrum of experimental data on the fractional quantum Hall effect (FQHE) states in the first excited Landau level, obtained in an ultrahigh mobility two-dimensional electron system (2DES) and at very low temperatures and report the following results: For the even-denominator FQHE states, the sample dependence of the nu=5/2 state clearly shows that disorder plays an important role in determining the energy gap at nu=5/2. For the developing nu=19/8 FQHE state the temperature dependence of the Rxx minimum implies an energy gap of ~5mK.The energy gaps of the odd-denominator FQHE states at nu=7/3 and 8/3 also increase with decreasing disorder, similar to the gap at 5/2 state. Unexpectedly and contrary to earlier data on lower mobility samples, in this ultra-high quality specimen, the nu=13/5 state is missing, while its particle-hole conjugate state, the nu=12/5 state, is a fully developed FQHE state. We speculate that this disappearance might indicate a spin polarization of the nu=13/5 state. Finally, the temperature dependence is studied for the two-reentrant integer quantum Hall states around nu=5/2 and is found to show a very narrow temperature range for the transition from quantized to classical value., to be published

We have observed a rapid decrease in the rate by which two-dimensional electrons in very high-mobility (\ensuremath{\mu}\ensuremath{\approxeq}${10}^{7}$ ${\mathrm{cm}}^{2}$/V sec) GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructures are scattered by acoustic phonons as the temperature approaches T=0. This precipitous drop in scattering rate is caused by strong phase-space restrictions for electron-phonon scattering. The characteristic temperature for this transition is not the Debye temperature of the GaAs host material, but the temperature at which phonons of twice the Fermi wave vector cease to be thermally excited.

We have observed quantization of the diagonal resistance, R{sub xx}, at the edges of several quantum Hall states. Each quantized R{sub xx} value is close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, R{sub xy}. Peaks in R{sub xx} occur at different positions in positive and negative magnetic fields. Practically all R{sub xx} features can be explained quantitatively by a 1%/cm electron density gradient. Therefore, R{sub xx} is determined by Rxy and unrelated to the diagonal resistivity {rho}{sub xx}. Our findings throw an unexpected light on the empirical resistivity rule for 2D systems.

We report on measurements of quantum many-body modes in ballistic wires and their dependence on Coulomb interactions, obtained from tunneling between two parallel wires in a GaAs/AlGaAs heterostructure while varying electron density. We observe two spin modes and one charge mode of the coupled wires, and map the dispersion velocities of the modes down to a critical density, at which spontaneous localization is observed. Theoretical calculations of the charge velocity agree well with the data, although they also predict an additional charge mode that is not observed. The measured spin velocity is found to be smaller than theoretically predicted., There are minor textual differences between this version and the version that has been published in Science (follow the DOI link below to obtain it). In addition, here we have had to reduce figure quality to save space on the server

Magnetotransport measurements were performed in a ultra-high mobility GaAs/AlGaAs quantum well of density $\sim 3.0 \times 10^{11}$ $cm^{-2}$. The temperature dependence of the magnetoresistance $R_{xx}$ was studied in detail in the vicinity of $\nu={9/2}$. In particular, we discovered new minima in $R_{xx}$ at filling factor $\nu\simeq 4{1/5}$ and $4{4/5}$, but only at intermediate temperatures $80\lesssim T\lesssim 120$ mK. We interpret these as evidence for a fractional quantum Hall liquid forming in the N=2 Landau level and competing with bubble and Wigner crystal phases favored at lower temperatures. Our data suggest that a magnetically driven insulator-insulator quantum phase transition occurs between the bubble and Wigner crystal phases at T=0., Comment: Phys. Rev. Lett.93 266804 (2004)

We determine the density-dependent electron mass m(*) in two-dimensional electron systems of GaAs/AlGaAs heterostructures by performing detailed low-temperature Shubnikov-de Haas measurements. Using very high-quality transistors with tunable electron densities we measure m(*) in single, high mobility specimens over a wide range of r(s) (6 to 0.8). Toward low densities we observe a rapid increase of m(*) by as much as 40%. For 2r(s)0.8 the mass values fall approximately 10% below the band mass of GaAs. Numerical calculations are in qualitative agreement with our data but differ considerably in detail.

Molecular beam epitaxy of GaN and related alloys is becoming a rival to the more established metalorganic vapor phase epitaxy. Excellent control of impurity, interface abruptness, and in situ monitoring of the growth are driving the increase in quality of MBE epilayers. We have developed nucleation schemes with plasma-assisted MBE on three types of substrates, consisting of sapphire, semi-insulating (SI-) SiC, and HVPE SI-GaN templates on sapphire. While sapphire and SI-SiC are established substrates for the growth of AlGaN/GaN HEMT epilayers, HVPE GaN templates may provide a path to low-cost large-diameter substrates for electronic devices. We compare device results of HEMTs fabricated on these substrates. As a metric for device performance, the saturated RF power output in class A operation is measured at 2 GHz. We achieved a saturated power density of 2.2 W/mm from HEMTs on sapphire, 1.1 W/mm from HEMTs on HVPE GaN templates on sapphire, and 6.3 W/mm. from HEMTs on semi-insulating 6H-SiC substrates. The difference in output power can be attributed to self-heating due to insufficient thermal conductivity of the sapphire substrate, and to trapping in the compensation-doped HVPE template.

We study the scattering properties of an interface between a one-dimensional (1D) wire and a two-dimensional (2D) electron gas. Experiments were conducted in the highly controlled geometry provided by molecular bean epitaxy overgrowth onto the cleaved edge of a high quality GaAs /AlGaAs quantum well. Such structures allow for the creation of variable length 1D-2D coupling sections. We find ballistic 1D electron transport through these interaction regions with a mean free path as long as 6mgr;m. Our results explain the origin of the puzzling nonuniversal conductance quantization observed previously in such 1D wires.