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Strong interactions and topology drive a wide variety of correlated ground states. Some of the most interesting of these ground states, such as fractional quantum Hall states and fractional Chern insulators, have fractionally charged quasiparticles. Correlations in these phases are captured by the binding of electrons and vortices into emergent particles called composite fermions. Composite fermion quasiparticles are randomly localized at high levels of disorder and may exhibit charge order when there is not too much disorder in the system. However, more complex correlations were predicted when composite fermion quasiparticles cluster into a bubble, then these bubbles order on a lattice. Such a highly correlated ground state was termed the bubble phase of composite fermions. Here we report the observation of this bubble phase of composite fermions, evidenced by the reentrance of the fractional quantum Hall effect. We associate this reentrance with a bubble phase with two composite fermion quasiparticles per bubble. Our results demonstrate the existence of a new class of strongly correlated topological phases driven by clustering and charge ordering of emergent quasiparticles.

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.

In a two-dimensional electron system subject to microwaves and a magnetic field, photovoltages emerge. They can be separated into two components originating from built-in electric fields and electric field domains arising from spontaneous symmetry breaking. The latter occurs in the zero resistance regime only and manifests itself in pulsed behavior, synchronous across the sample. The pulses show sign reversal. This implies a flip of the field in each domain, consistent with the existence of two equally probable electric field domain configurations due to the spontaneous symmetry breaking.

Using a double wire geometry we probe the quantum many-body modes in ballistic wires and their dependence on Coulomb interactions. The wires are fabricated using the cleaved edge overgrowth method. We clearly observe two spin modes and one charge mode of the coupled wires. We 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. Surprisingly, the suppression of the spin velocities does not depend on density. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we review low temperature electronic transport results in high quality two-dimensional electron systems. We discuss the quantization of the diagonal resistance, R x x , at the edges of several quantum Hall states. Each quantized R x x value turns out to be close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, R x y . Moreover, peaks in R x x occur at different positions in positive and negative magnetic fields. All three R x x features can be explained quantitatively by a ∼1% cm electron density gradient. Furthermore, based on this observation, the well known but still enigmatic resistivity rule, relating R x x to d R x y / d B , finds a simple interpretation in terms of this gradient. In another sample, at 1.2 K, R x x we observe a strongly linear magnetic field dependence. Surprisingly, this linear magnetoresistance also originates from the density gradient. Our findings throw an unexpected light on the relationship between the experimentally measured R x x and the diagonal resistivity ρ x x .

We determined the spin susceptibility Χ and the effective mass m* towards the high density limit. Using a tunable GaAs/AlGaAs heterostructure, we can vary the 2D electron density from n = 1 x 10 10 cm -2 to 4 x 10" cm -2 . From ∼5 x 10 10 cm -2 to our highest densities the mass values fall ∼10% below the band mass of GaAs. The enhancement of Χ decreases monotonically from a factor of 3 to 0.88 with increasing density. It continues to follow a previously observed power law, which leads to an unphysical limit for n → oo. Band structure effects affecting mass and g-factor become appreciable for large n and, when taken into account, lead to the correct limiting behavior of Χ. Numerical calculations are in qualitative agreement with our data but differ in detail.

A fundamental, yet underexplored, materials system is the interface between biological molecules and inorganic surfaces. In an elemental approach to this problem, we have systematically examined the adhesion of amino acids to a series of inorganic surfaces including metals, insulators, and semiconductors. Significant differential adhesion is observed over the full complement of amino acids, determined largely by amino acid side-chain charge. Extensive mapping of the amino acid adhesion versus materials in multiple solutions is presented, with preliminary mechanisms derived from concentration and pH dependence. These results provide an empirical basis for building peptide to inorganic surface structures, and, using this adhesion data, we design inorganic nanostructures that are shown to selectively bind to prescribed primary peptide sequences.

We have measured the collective excitation spectrum of interacting electrons in one-dimension. The experiment consists of controlling the energy and momentum of electrons tunneling between two clean and closely situated, parallel quantum wires in a GaAs/AlGaAs heterostructure while measuring the resulting conductance. We measure excitation spectra that clearly deviate from the non-interacting spectrum, attesting to the importance of Coulomb interactions. Notable is an observed 30% enhancement of the velocity of the main excitation branch relative to non-interacting electrons with the same density. In short wires, finite size effects resulting from broken translational invariance are observed. Spin - charge separation is manifested through moire patterns, reflecting different spin and charge excitation velocities., 14 pages, 6 eps figures. To be published in NANOWIRE, a special issue of Solid State Communications

We determine the spin susceptibility in a variable-density two-dimensional electron system in GaAs/AlGaAs. Our measurements extend over a wide range of low densities from 2×10 9 to 4×10 10 cm −2 . Our data can be well described by an empirical equation which incorporates both the density dependence and the magnetic field dependence of the spin susceptibility. It can explain the anomalous g -factors reported recently in GaAs electron and hole systems. The extrapolated paramagnetic spin susceptibility increases with decreasing density as expected from theoretical calculations.

In the second Landau level around v = 5/2 filling of an extremely high quality 2D electron system and at temperatures T down to 9 mK we observe a very strong even-denominator fractional quantum Hall effect at Landau level filling v = 5/2 and its energy gap is large and Δ ∼ 0.45K. A clear FQHE state is seen at v = 2 + 2/5, with well-quantized R xy . A novel, even-denominator FQHE state at v = 2 + 3/8 seems to develop, as deduced from the T-dependence of dR xy /dB. In addition, four fully developed re-entrant integral quantum Hall effect (RIQHE) states are also observed. At low temperatures, the wide RIQHE plateau around at v = 2 + 2/7 is interrupted by a dip, indicating an additional reentrance. Finally, the tilted magnetic field experiment at an ultra-low temperature of 10 mK was carried out to examine the spin-polarization of the v = 5/2 FQHE state.

This letter describes the use of nanotransfer printing (nTP) for forming three-dimensional (3D) structures with feature sizes between tens of nanometers and tens of microns over areas of several square millimeters. We demonstrate three different approaches-deep etching through printed hard masks, direct transfer of three-dimensional structures, and purely additive fabrication of multilayer stacks-for using nTP to fabricate a range of complex 3D nanostructures, including closed channels, suspended beams, and nanochannel stacks, that would be difficult or impossible to build with other methods.

Soft contact lamination of source/drain electrodes supported by gold-coated high-resolution rubber stamps against organic semiconductor films can yield high-performance organic transistors. This article presents a detailed study of the electrical properties of these devices, with an emphasis on the nature of the laminated contacts with the p- and n-type semiconductors pentacene and copper hexadecafluorophthalocyanine, respectively. The analysis uses models developed for characterizing amorphous silicon transistors. The results demonstrate that the parasitic resistances related to the laminated contacts and their coupling to the transistor channel are considerably lower than those associated with conventional contacts formed by evaporation of gold electrodes directly on top of the organic semiconductors. These and other attractive features of transistors built by soft contact lamination suggest that they may be important for basic and applied studies in plastic electronics and nanoelectronic systems based ...

Soft, conformable electrical contacts provide efficient, noninvasive probes for the transport properties of chemically and mechanically fragile, ultrathin organic semiconducting films. When combined with high-resolution printing and lamination techniques, these soft contacts also form the basis of a powerful technique for fabricating flexible plastic circuits. In this approach, a thin elastomeric film on a plastic substrate supports the electrodes and interconnections; laminating this substrate against another plastic substrate that supports the gate, dielectric and semiconductor levels establishes effective electrical contacts and completes the circuits. In addition to eliminating many of the problems associated with traditional layer-by-layer fabrication strategies, this lamination scheme possesses other attractive features: the transistors and circuit elements are naturally and efficiently encapsulated, and the active organic semiconductor layer is placed near the neutral mechanical plane. We demonstrate the features of soft, laminated contacts by fabricating large arrays of high-performance thin film transistors on plastic substrates by using a wide variety of organic semiconductors.

Magnetotransport measurements on two-dimensional electrons confined to wide GaAs quantum wells reveal a remarkable evolution of the ground state at filling factor $\nu=1/2$ as we tilt the sample in the magnetic field. Starting with a compressible state at zero tilt angle, a strong $\nu=1/2$ 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.

Electronic systems that use rugged lightweight plastics potentially offer attractive characteristics (low-cost processing, mechanical flexibility, large area coverage, etc.) that are not easily achieved with established silicon technologies. This paper summarizes work that demonstrates many of these characteristics in a realistic system: organic active matrix backplane circuits (256 transistors) for large (≈5 × 5-inch) mechanically flexible sheets of electronic paper, an emerging type of display. The success of this effort relies on new or improved processing techniques and materials for plastic electronics, including methods for ( i ) rubber stamping (microcontact printing) high-resolution (≈1 μm) circuits with low levels of defects and good registration over large areas, ( ii ) achieving low leakage with thin dielectrics deposited onto surfaces with relief, ( iii ) constructing high-performance organic transistors with bottom contact geometries, ( iv ) encapsulating these transistors, ( v ) depositing, in a repeatable way, organic semiconductors with uniform electrical characteristics over large areas, and ( vi ) low-temperature (≈100°C) annealing to increase the on/off ratios of the transistors and to improve the uniformity of their characteristics. The sophistication and flexibility of the patterning procedures, high level of integration on plastic substrates, large area coverage, and good performance of the transistors are all important features of this work. We successfully integrate these circuits with microencapsulated electrophoretic “inks” to form sheets of electronic paper.

This paper presents some recent results on the anisotropic electronic transport at half-filling factors in the second and third Landau levels. In the third Landau level at ν= 9 2 and 11 2 , the electronic transport is already strongly anisotropic in perpendicular magnetic field. This anisotropic phase is believed to be due to the formation of a unidirectional charge density wave. Applying an in-plane field overcomes the initial pinning and reorients the direction of this anisotropy. In the second Landau level, the even-denominator fractional quantum Hall state at ν= 5 2 and 7 2 is destroyed and transformed into an anisotropic phase by an in-plane magnetic field. This result provides additional evidence that the ground state at ν= 5 2 and 7 2 is spin-polarized.

Scanning force microscopy was used to examine the surfaces of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on GaN templates prepared by hydride vapor phase epitaxy (HVPE). Away from dislocations, the MBE growth replicates the surface morphology of the HVPE film, with monolayer steps clarly visible in topographic images. However, the surface morphology near dislocations depends strongly on the MBE growth conditions. Under Ga rich growth the dislocations appear as hillocks, while under stoichiometric growth they appear as pits. A dependence on Al concentration is also observed. Surface contact potential variation near dislocations is consistent with excess negative charges surrounding by a depletion region, but this was observed only for the film grown under stoichiometric conditions.

We report the results of 3 terminal measurements on a one-dimensional (1D) wire. By utilizing cleaved-edge overgrowth, we were able to vary the coupling between a 1D conductor and an adjacent two-dimensional electron gas (2DEG) strip attached to the central part of the wire. We study the 1D-2DEG coupling by controlling the width of the 2DEG strip. Ballistic transport along the wire persists for interaction regions as long as 6 μm . Thus, with our shortest interaction region (narrowest strip) of 2 μm , the wire remains almost undisturbed by the presence of the nearby 2DEG allowing a true 3 terminal measurement on a clean wire. Our observations also explain the origin of the non-universal conductance quantization of such 1D wires.

Experimental data on the sequence of fractional quantum Hall effects (FQHE) states around Landau level filling factor ν =1/2 are well explained within the framework of the composite fermion model. Around other even-denominator fractions the situation is less clear, experimentally as well as theoretically. In this paper we report experimental studies at even-denominator fillings at ν=1/4, 3/4,5/2 , etc. We show that the composite fermion picture can be applied successfully to the fractions at ν =1/4 and 3/4. Their effective mass, when normalized to B 1/2 (where B is the magnetic field) and their effective g factor are very similar to the composite fermions at ν =1/2 and 3/2. Our ultra-low-temperature study at ν =5/2 shows that this fraction is a true FQHE state, showing vanishing R xx and a Hall resistance R xy precisely quantized to h /(5/2 e 2 ).

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are {\em isotropic} in perpendicular magnetic field but become strongly {\em anisotropic} at $\nu$ = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, $B_{ip} \sim$ 2.5T, with the easy-current direction {\em parallel} to $B_{ip}$ but rotates by 90$^{\circ}$ at $B_{ip} \sim$ 10T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model., Comment: 4 pages, 4 figures

We present experimental data showing unambiguously an even-denominator fractional quantum Hall effect (FQHE) state at ν=52. At a bath temperature Tb=8 mK, we observe a Hall plateau quantized to a value of 2h/5e2 with an uncertainty smaller than 2 parts in 106 and a vanishing Rxx (Rxx=1.7±1.7Ω). The thermal activation energy gaps Δ at Landau level filling factors ν=52,73, and 83 are 0.11, 0.10, and 0.055 K, respectively. Adding a disorder broadening (typically ∼2 K) to these values, we deduce that all three FQHE states have probably very similar energy gaps. The electron heating experiment shows that the 2D electrons are efficiently cooled to the bath temperature for Tb⩾8 mK. We also explore the density dependence of the activation gap at ν=52. Preliminary results at Tb⩾25 mK show that the 52 state is very sensitive to disorder.

Recent experiments (Yacoby et al., Phys. Rev. Lett. 77 (1996) 4612; Solid State Commun. 101 (1997) 77; M. Rother et al., ICPS-24, Th-P137) have shown non-universal conductance quantization in one-dimensional wires that are fabricated using the cleaved edge overgrowth technique (L.N. Pfeiffer et al., Microelectronics J. 28 (1997) 817). In one of the papers (Yacoby et al., Phys. Rev. Lett. 77 (1996) 4612), it was speculated that the origin of the reduced conductance lies in the interface between the one-dimensional wire and the two-dimensional electron gas regions, which serve as ohmic contacts and thermal reservoirs. Here we report the results of a systematic study of such 2D–1D interfaces. By embedding a 2D–1D interaction section region of controllable length inside an otherwise isolated wire, we were able to study the properties of the coupling between these two subsystems. Our results show that 2D–1D interface is in fact the origin of the non-universal conductance.

Low-temperature, electronic transport in higher Landau levels (N>1) in a two-dimensional electron system is strongly anisotropic. At half-filling of either spin level of such Landau levels ( ν= 9 2 , 11 2 , 13 2 , 15 2 , etc.) the magnetoresistance either collapses to form a deep minimum or is peaked in a sharp maximum, depending on the in-plane current direction. The anisotropic axis can be reoriented by applying an in-plane magnetic field of ∼1–2 T strength. The magnetoresistance at ν= 5 2 and ν= 7 2 (N=1) is initially isotropic but an in-plane field induces a strong anisotropy. Our observations are strong evidence for a new many-electron phase in higher Landau levels, which forms spontaneously or can be induced by an in-plane field.

We have investigated the influence of an increasing in-plane magnetic field on the states of half filling of Landau levels ( $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}11/2$, 9/2, 7/2, and 5/2) of a two-dimensional electron system. In the electrically anisotropic phase at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9/2$ and 11/2 an in-plane magnetic field of $\ensuremath{\sim}1--2\mathrm{T}$ overcomes its initial pinning to the crystal lattice and reorients this phase. In the initially isotropic phases at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5/2$ and 7/2 an in-plane magnetic field induces a strong electrical anisotropy. In all cases, for high in-plane fields the high-resistance axis is parallel to the direction of the in-plane field.

We investigate the properties of composite fermions with four attached flux quanta through tilted field experiments near Landau level filling factor nu = 3/4. The observed collapse of fractional quantum Hall gaps in the vicinity of this quarter-filling state can be comprehensively understood in terms of composite fermions with mass and spin. Remarkably, the effective mass and g-factor of these four flux quanta composite fermions around nu = 3/4 are very similar to those of two flux quanta composite fermions around nu = 3/2., Comment: 4 pages, 1 table, 3 postscript figures

Due to the current interest in quantum Hall states described by the Moore-Read or Pfaffian wavefunction, we have recently begun experiments involving a series of high-quality wide GaAs quantum well in an effort to search for new Pfaffian states. Here we report the characterization of these samples. The quantum well in our series have differing well widths which range from 50 to 80 nm. The self-consistently calculated electronic band edge and electron distribution function are shown for each sample. We present the magnetoresistance up to 18T where the quantum Hall states at v = 1 and 3 are seen to vanish with increasing L. We also discuss the observation of a fractional quantum Hall state at v = 1/2 in the L = 50 nm sample.

A one-dimensional active region consisting of 20-period T-shaped GaAs/AlGaAs quantum wires of 14×6 nm2 combined with a well-defined current-confinement scheme has been utilized to obtain a low-threshold-current high-efficiency laser. A threshold current as low as 0.27 mA, a single-mode cw output power of 0.13 mW at 1 mA injection current, and a differential quantum efficiency of 12% were achieved at 30 K using a quantum-wire laser with a 500-µm-long Fabry–Perot cavity coated by high-reflectivity thin metals.

The MBE double-growth technique that we call cleaved-edge overgrowth has, over the past several years, proved itself to be especially suitable for making quantum wires of the very highest quality. We will review our recent progress in measuring the transport and quantum optics characteristics of these wires, and the MBE growth issues that arise with cleaved-edge overgrowth fabrication. Our transport experiments have resulted in 250 A wide quantum wires with ballistic mean free paths exceeding 10 μm. We verify the prediction that in the ballistic regime the electron conductivity in a quantum wire is independent of the wire length and shows quantized steps proportional to e 2 h . The deviation of our observed step heights from exactly e 2 h is taken as evidence for correlated electron behaviour. The electrons are tightly confined on three sides by atomically smooth GaAs/AlGaAs heterojunctions and in the fourth direction by an electric field. This results in a quantum wire of nominal square cross-section 250 × 250 A . Magneto-transport measurements reveal quantum wire sub-band separations in excess of 20 meV as well as the symmetries of the wave functions of the one-dimensional modes. For optics studies our quantum wires are made using cleaved-edge overgrowth to form a line junction as two quantum wells are made to intersect with the cross-section forming a letter ‘T’. This line intersection separately forms a quantum wire bound-state for holes, for electrons, and even for excitons. We have characterized our optical wires by PL, by PLE, and by scanning near-field optics. An important application of this work is our demonstration of the first quantum laser using this T-geometry.

One-dimensional structures are expected to show unique electronic transport behavior as a consequence of the Coulomb interaction between carriers. A considerable number of theoretical predictions remain largely untested by experiment due to a lack of a suitable one-dimensional wire. Using a new crystal growth technique a unique tube-like structure with a cross section of 25 nm × 25 nm has been created in a semiconductor. The mean free path of the electrons exceeds 10 μm - more than 400 times the confinement dimension. The energy spacing between one-dimensional modes far exceed any random potential fluctuations and is more than 10 times larger than previously achieved by other techniques. The conductance plateaus of these wires deviate significantly from the expected universal values suggesting the relevance of electron-electron interactions.

We have measured the transport properties of high-quality quantum wires fabricated in GaAs-AlGaAs by using cleaved edge overgrowth. The low temperature conductance is quantized as the electron density in the wire is varied. While the values of the conductance plateaus are reproducible, they deviate from multiples of the universal value of ${2e}^{2}/h$ by as much as 25%. As the temperature or dc bias increases the conductance steps approach the universal value. Several aspects of the data can be explained qualitatively using Luttinger liquid theory although there remain major inconsistencies with such an interpretation.

Two-dimensional electrons in high magnetic field behave like new particles o which an even number of magnetic flux quanta has been attached. They are termed composite fermions. Several properties of composite fermions such as mass and ballistic motion have been observed in the past. This paper presents measurement of their interaction with impurities and acoustic phonons and establishes the spin and g -factor of a composite fermion.

We describe a simple method and device design that enables fast in-plane electro-optic modulation in conventional nematic liquid crystal (NLC) devices. When combined with optimized NLC materials, this approach yields rotational speeds of 1°/μs (independent of rotation angle, over a wide range) at a moderately low voltage. The observed rotational dynamics indicate that even these high speeds may not represent fundamental physical limits. We demonstrate these ideas in a compact tunable NLC waveplate that uses microelectrodes patterned directly on the tips of optical fibers. These devices offer fast, continuously tunable optic axis with low insertion loss and good performance in the near infrared. Modulators that use this design have promising potential applications for polarization control and analysis in optical communication systems.

We describe a method for contact printing metal patterns with nanometer features over large areas. This nanotransfer printing (nTP) technique relies on tailored surface chemistries to transfer metal films from the raised regions of a stamp to a substrate when these two elements are brought into intimate physical contact. The printing is purely additive, fast (

Very fast millimeter wave mixers have been fabricated from a high mobility two-dimensional electron gas (2DEG) in a GaAs–AlGaAs heterostructure. Using a 115 GHz local oscillator, intermediate frequency (IF) bandwidths exceeding 20 GHz have been obtained for channel lengths L of a few microns. Operating at 77 K, the IF bandwidth scales as L−2, indicating that the response speed is determined by the diffusion transit time of hot electrons to the leads.

We report on the growth and transport properties of high-mobility two-dimensional electron gases (2DEGs) confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on GaN templates prepared by hydride vapor phase epitaxy. We have grown samples over a broad range of electron densities ranging from ns=6.9×1011 to 1.1×1013 cm−2, and at T=4.2 K, observe a peak mobility of 53 300 cm2/V s at a density of 2.8×1012 cm−2. Magnetotransport studies on these samples display exceptionally clean signatures of the quantum Hall effect. Our investigation of the dependence of 2DEG mobility on carrier concentration suggests that the low-temperature mobility in our AlGaN/GaN heterostructures is currently limited by the interplay between charged dislocation scattering and interface roughness.

A laser light source for high-resolution near-field optics applications with an output power exceeding 1 mW (104 times the power from previous sources) and small (300 nm square to less than 50 nm square) output beam size is demonstrated. The very-small-aperture laser (VSAL) tremendously expands the range of applications possible with near-field optics and increases the signal-to-noise ratios and data rates obtained in existing applications. As an example, 250-nm-diam marks corresponding to 7.5 Gb/in.2 storage density have been recorded and read back in reflection and transmission on a rewritable phase-change disk at 24 Mb/s with a 250-nm-square aperture VSAL. VSALs potentially enable data storage densities of over 500 Gb/in.2 (up to 100 times today’s magnetic or optical storage densities).

We report the characteristics of complementary organic ring oscillators. The shortest delay time measured is 38 μs per stage that corresponds to a 2.63 kHz oscillation frequency. The active material in the n-channel transistors is copper hexadecafluorophthalocyanine and that in the p-channel transistors is an oligothiophene/oligothiophene derivative.

We report the observation of an even-denominator fractional quantum Hall (FQH) state at $\nu=1/4$ in a high quality, wide GaAs quantum well. The sample has a quantum well width of 50 nm and an electron density of $n_e=2.55\times10^{11}$ cm$^{-2}$. We have performed transport measurements at $T\sim35$ mK in magnetic fields up to 45 T. When the sample is perpendicular to the applied magnetic field, the diagonal resistance displays a kink at $\nu=1/4$. Upon tilting the sample to an angle of $\theta=20.3^o$ a clear FQH state at emerges at $\nu=1/4$ with a plateau in the Hall resistance and a strong minimum in the diagonal resistance., Comment: Accepted for publication in Phys. Rev. Lett

CW single-mode lasing from 30 K to 70 K with the lowest threshold current of 0.27 mA has been demonstrated in 20-period T-shaped quantum-wire laser diodes with parallel p and n doping layers.

We present low-temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide band-gap $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ system. We observed pronounced antilocalization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is ${\ensuremath{\alpha}}_{\mathrm{so}}\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.2em}{0ex}}\mathrm{m}$, while the conduction-band spin-orbit splitting energy amounts to ${\ensuremath{\Delta}}_{\mathrm{so}}\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at ${n}_{e}=1\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$.

We determine the spin susceptibility $\ensuremath{\chi}$ in the weak interaction regime of a tunable, high quality, two-dimensional electron system in a $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ heterostructure. The band structure effects, modifying mass and $g$-factor, are carefully taken into accounts since they become appreciable for the large electron densities of the weak interaction regime. When properly normalized, $\ensuremath{\chi}$ decreases monotonically from 3 to 1.1 with increasing density over our experimental range from 0.1 to $4\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. In the high density limit, $\ensuremath{\chi}$ tends correctly towards $\ensuremath{\chi}\ensuremath{\rightarrow}1$ and compare well with recent theory.

Tunneling characteristics of a two-dimensional lateral tunnel junction (2DLTJ) are reported. A pseudogap on the order of Coulomb energy is detected in the tunneling density of states (TDOS) when two identical two-dimensional electron systems are laterally separated by a thin energy barrier. The Coulombic pseudogap remains robust well into the quantum Hall regime until it is overshadowed by the cyclotron gap in the TDOS. The pseudogap is modified by in-plane magnetic field, demonstrating a non-trivial effect of in-plane magnetic field on the electron-electron interaction., Comment: 5 pages, 4 figures

We report on a magnetic resonance study in the electron solid phase of a low-disorder $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ sample at very high magnetic fields using a resistively detected NMR technique. An anomalous dispersivelike line shape is observed at the onset of the electron solid in the vicinity of the Landau filling $\ensuremath{\nu}=\frac{2}{9},\frac{1}{5}$ fractional quantum Hall state, reminiscent of the line shape observed near filling factor $\ensuremath{\nu}\ensuremath{\sim}1$. In contrast to this filling factor region, the spin-lattice relaxation time ${T}_{1}$ in the electron solid is found to be very long, and does not correlate with either ${R}_{xx}$ or the Landau filling factor. The dispersivelike line shapes observed in this regime suggests that the intricate interplay between the nuclear spin and the electronic transport may be modified by the formation of the electron solid.

The many-body wave-function of an interacting one-dimensional electron system is probed, focusing on the low-density, strong interaction regime. The properties of the wave-function are determined using tunneling between two long, clean, parallel quantum wires in a GaAs/AlGaAs heterostructure, allowing for gate-controlled electron density. As electron density is lowered to a critical value the many-body state abruptly changes from an extended state with a well-defined momentum to a localized state with a wide range of momentum components. The signature of the localized states appears as discrete tunneling features at resonant gate-voltages, corresponding to the depletion of single electrons and showing Coulomb-blockade behavior. Typically 5-10 such features appear, where the one-electron state has a single-lobed momentum distribution, and the few-electron states have double-lobed distributions with peaks at $\pm k_F$. A theoretical model suggests that for a small number of particles (N, 10 pages, 4 figures, 1 table

Tunable dispersion compensators are an essential component for optical networks operating at 40 Gbits/s and beyond. One fiber-based tunable dispersion compensator that has proved to be effective consists of a chirped fiber Bragg grating tuned by a thin-film distributed resistive heating element. We describe several modifications to the heater design that minimize temperature-induced higher-order dispersion, eliminate the need for a second stabilization heater when the device is operated at constant ambient temperature, and significantly lower its maximum operating temperature. We demonstrate a tunable dispersion compensator with a single thin-film heater that provides over 500 ps/nm of tunable dispersion over a fixed 100-GHz bandwidth with a maximum operating temperature of less than 125 degrees C above ambient.

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate $(1/T_{1})$ was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of $^{75}$As is found to relax much more efficiently with $T\to 0$ and when a well developed quantum Hall state with $R_{xx}\simeq 0$ occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e. a Skyrmion crystal.