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We study the superconducting proximity effect in an InAs nanowire contacted by Ta-based superconducting electrodes. Using local bottom gates, we control the potential landscape along the nanowire, tuning its conductance to a quasi-ballistic regime. At high magnetic field ($B$), we observe approximately quantized conductance plateaus associated with the first two spin-polarized one-dimensional modes. For $B < 1$ T, the onset of superconductivity occurs in concomitance with the development of sizeable charge localization leading to a 0.7-type conductance anomaly. In this regime, the proximity supercurrent exhibits an unusual, non-monotonic $B$ dependence. We interpret this finding in terms of a competition between the Kondo effect, dominating near $B=0$, and the Zeeman effect, enforcing spin polarization and the emergence of a $��$ phase shift in the Josephson relation at higher $B$.

We experimentally investigate the temperature (T) dependence of the resistance of a classical ballistic point contact (PC) in a two-dimensional electron system (2DES). The split-gate PC is realized in a high-quality AlGaAs/GaAs heterostructure. The PC resistance is found to drop by more than 10% as T is raised from 0.5 K to 4.2 K. In the absence of a magnetic field, the T dependence is roughly linear below 2 K and tends to saturate at higher T. Perpendicular magnetic fields on the order of a few 10 mT suppress the T-dependent contribution dR. This effect is more pronounced at lower temperatures, causing a crossover to a nearly parabolic T dependence in a magnetic field. The normalized magnetic field dependencies dR(B) permit an empiric single parameter scaling in a wide range of PC gate voltages. These observations give strong evidence for the influence of electron-electron (e-e) scattering on the resistance of ballistic PCs. Our results are in qualitative agreement with a recent theory of the e-e scattering based T dependence of the conductance of classical ballistic PCs [ Phys. Rev. Lett. 101 216807 (2008) and Phys. Rev. B 81 125316 (2010)]., Comment: as published

We observed room-temperature intersubband transitions at 1.26 microns in n-doped type-II In$_{0.3}$Ga$_{0.7}$As/AlAs strained quantum wells. An improved tight-binding model was used to optimize the structure parameters in order to obtain the shortest wavelength intersubband transition ever achieved in a semiconductor system. The corresponding transitions occur between the first confined electronic levels of the well following mid-infrared optical pumping of electrons from the barrier X-valley into the well ground state., To appear in Applied Physics Letters

We use the circularly polarized resonant Rayleigh scattering (RRS) to study the quantum Hall ferromagnet at $��$ = 1. At this filling factor we observe a right handed copolarized RRS which probes the Skyrmion spin texture of the electrons in the photoexcited grounds state. The resonant scattering is not present in the left handed copolarization, and this can be related to the correlation between Skymionic effects, screening and spin wave excitations. These results evidence that RRS is a valid method for the study of the spin texture of the quantum Hall states.

The magnetic-field evolution of Coulomb blockade peaks in lateral In0.75Ga0.25As/In0.75Al0.25As quantum dots in the few-electron regime is reported. Quantum dots are defined by gates evaporated onto a 60-nm-thick hydrogen silsesquioxane insulating film. A gyromagnetic factor g(*)approximate to 4.4 is measured via zero-bias spin spectroscopy and a transition from singlet to triplet spin configuration is found at an in-plane magnetic field B=0.7 T. This observation opens the way to the manipulation of singlet and triplet states at moderate fields and its relevance for quantum information applications will be discussed.

The magnetic-field evolution of Coulomb blockade peaks in lateral InGaAs/InAlAs quantum dots in the few-electron regime is reported. Quantum dots are defined by gates evaporated onto a 60 nm-thick hydrogen silsesquioxane insulating film. A gyromagnetic factor of 4.4 is measured via zero-bias spin spectroscopy and a transition from singlet to triplet spin configuration is found at an in-plane magnetic field B = 0.7 T. This observation opens the way to the manipulation of singlet and triplet states at moderate fields and its relevance for quantum information applications will be discussed., Comment: 4 pages, 3 figures

At a superconductor (S)-normal metal (N) junction pairing correlations can "leak-out" into the N region. This proximity effect [1, 2] modifies the system transport properties and can lead to supercurrent flow in SNS junctions [3]. Recent experimental works showed the potential of semiconductor nanowires (NWs) as building blocks for nanometre-scale devices [4-7], also in combination with superconducting elements [8-12]. Here, we demonstrate an InAs NW Josephson transistor where supercurrent is controlled by hot-quasiparticle injection from normal-metal electrodes. Operational principle is based on the modification of NW electron-energy distribution [13-20] that can yield reduced dissipation and high-switching speed. We shall argue that exploitation of this principle with heterostructured semiconductor NWs opens the way to a host of out-of-equilibrium hybrid-nanodevice concepts [7, 21]., Comment: 6 pages, 6 color figures

The properties of self-assembled InAs/GaAs quantum rings are investigated by conductive atomic force microscopy. Our two-dimensional current maps and current-voltage curves show a lower conductivity of the central ring hole as compared to rim and surrounding planar region. This result is quite surprising if we take into account the compositional profile of quantum rings: being the region with the highest In concentration, one would expect the central hole to be the region with the highest conductivity. However, including the presence of a surface oxide into numerical simulations yields consistent results, which show the same qualitative behavior as the measured conductivities. (C) 2008 American Institute of Physics.

We have investigated the infrared transmission of a two-dimensional (2DEG) electron gas confined in a $Al_{0.25}In_{0.75}As/Ga_{0.25}In_{0.75}As$ single quantum well in order to study the electron optical phonon interaction in a two phonon mode system. Infrared transmission experiments have been performed in both the perpendicular Faraday (PF) and tilted Faraday (TF) configurations for which the growth axis of the sample is tilted with respect to the incident light propagation direction and to the magnetic field direction. The experimental results lead to question the validity of the concept of polaron mass in a real material., Comment: 7 pages, 3 figures

We report the first experimental investigation of the spectral response of a ''bulk microcavity'': a semiconductor system in which the same cavity material is the active medium. Despite the smaller oscillator strength of the bulk exciton, a Rabi splitting even clearer than that displayed by usual quantum-well microcavities is observed. The splitting increases with the cavity layer thickness and the exciton dispersion produces a satellite structure due to quantum confinement. We also show that control of the exciton-photon interaction can be realized by suitably tailoring the photonic wave function.

A great interest in the study of exciton-photon coupling in resonant optical media such as semiconductor microcavities is currently developing both in experiments and in theory. We investigate the spectral response of a new semiconductor quantum system in which the cavity material itself is the active medium. If the Fabry-Perot quasi-mode is carefully tuned on the excitonic transition of the cavity bulk material, strong exciton-photon coupling takes place and produces new features such as a Rabi-like splitting as large as that observed in quantum well microcavities and comparable optical absorption. In addition, the polaritonic spatial dispersion and the quantization of the centre-of-mass motion introduce remarkable fine structures which are absent in the quantum well ease. We demonstrate the above effects from a spectroscopic analysis of GaAs cavities, using standard reflectance and photoluminescence techniques. The experimental results are further clarified with theoretical calculations performed with an adapted transfer-matrix approach. The case of semi-cavities is finally considered and a new way to control the exciton-photon interaction in our system is achieved tailoring the photonic wave function with a change in the reflectivity phase of the mirrors.

Electron-phase modulation in magnetic and electric fields will be presented in In_(0.75)Ga_(0.25)As Aharonov-Bohm (AB) rings. The zero Schottky barrier of this material made it possible to nanofabricate devices with radii down to below 200 nm without carrier depletion. We shall present a fabrication scheme based on wet and dry etching that yielded excellent reproducibility, very high contrast of the oscillations and good electrical gating. The operation of these structures is compatible with closed-cycle refrigeration and suggests that this process can yield coherent electronic circuits that do not require cryogenic liquids. The InGaAs/AlInAs heterostructure was grown by MBE on a GaAs substrate [1], and in light of the large effective g-factor and the absence of the Schottky barrier is a material system of interest for the investigation of spin-related effects [2-4]} and the realization of hybrid superconductor/semiconductor devices [5]., Comment: 7 pages, 4 figures, to appear in Physica E

We report on the achievement of a two-dimensional electron gas in completely undoped In[0.75]Al[0.25]As/In[0.75]Ga[0.25]As metamorphic quantum wells. Using these structures we were able to reduce the carrier density, with respect to reported values in similar modulation-doped structures. We found experimentally that the electronic charge in the quantum well is likely due to a deep-level donor state in the In[0.75]Al[0.25]As barrier band gap, whose energy lies within the In[0.75]Ga[0.25]As/In[0.75]Al[0.25]As conduction band discontinuity. This result is further confirmed through a Poisson-Schroedinger simulation of the two-dimensional electron gas structure., Comment: 17 pages, 6 figures, to be published in J. Vac. Sci. Technol. B

We explore the defect distribution and the degradation in electron beam pumped Zn1-xCdxSe/ZnSe laser structures by combining cathodoluminescence measurements in a scanning electron microscope with transmission electron microscopy. We found that degradation occurs via the formation of [100]-oriented dark line defects, and that it involves the formation of a characteristic type of defect, namely dislocation loops within the ZnCdSe quantum well, which grow to form a characteristic network as degradation processes.

The conduction band offset in ZnSe/GaAs n-p heterodiodes was determined from measurements of the low-temperature tunneling current of photoinjected carriers, We found widely different discontinuities for heterojunctions fabricated with different Zn/Se flux ratios, with conduction band offsets as high as 0.75 eV for Se-rich interfaces, and as low as 0.26 eV for Zn-rich interfaces. (C) 1996 American Institute of Physics.

In this study, cross-sectional transmission electron microscopy (XTEM) was used to investigate the defect structure at the interface between CdTe(001) and GaAs(001) as well as CdTe(1 11) and GaAs(001). The heterostructures were fabricated by molecular beam epitaxy on GaAs(001) buffer layers grown in-situ by molecular beam epitaxy. The defect structure at the as-deposited CdTe(001)/GaAs(001) interface consists of both dislocations and planar faults. The planar faults are both microtwins and stacking faults. It is found that annealing of the film ex-situ causes a restructuring of the CdTe near the interface, with the microtwins being completely removed upon annealing to 450°C for 100 hours. The CdTe(111)/GaAs(001) thin film structure consists of a large number of microtwins parallel to the growth direction. This twinned structure is shown to be related to the relaxation of a residual misfit strain normal to the twin direction. Possible mechanisms for the relaxation are discussed.

Internal photoemission phototransport measurements revealed 0.27+/-0.04 eV conduction- and valence-band discontinuities induced by a Si intralayer at p-GaAs(100)/n-GaAs(100) homojunctions. The interface dipole originating from the heterovalent character of the Si-GaAs bonds raises the bands of the GaAs overlayer above that of the GaAs subtrate.

We have recently shown that in II-VI/III-V heterojunctions and related devices fabricated by molecular beam epitaxy, the II/VI flux ratio employed during the early stages of II-VI growth can be used to control the local interface composition and the band alignment. Here we demonstrate that the local interface composition in pseudomorphic, strained ZnSe/GaAs(001) heterostructures as well as lattice-matched ZnSe/In0.04Ga0.96As(001) heterostructures also have a dramatic effect on the nucleation of native stacking fault defects. Such extended defects have been associated with the early degradation of blue-green lasers. We found, in particular, that Se-rich interfaces consistently exhibited a density of Shockley stacking fault pairs below our detection limit and three to four orders of magnitude lower than those encountered at interfaces fabricated in Zn-rich conditions. (C) 1997 American Vacuum Society. [S0734-211X(97)09104-X].

We report transmission electron microscopy studies of native extended defects in pseudomorphic ZnSe/GaAs (001) and lattice-matched ZnSe-In0.04Ga0.96As (001) heterostructures. The dominant defects present in the layers were identified as Shockley stacking fault pairs lying on (111) and fault planes and single Frank stacking faults lying on or fault planes by comparing experimental images with the predictions obtained with the g.b = 0 rule as well as with simulated images.

We demonstrate that the emission characteristics of site-controlled InGaAs/GaAs single quantum dots embedded in photonic crystal slab cavities correspond to single confined excitons coupled to cavity modes, unlike previous reports of similar systems based on self-assembled quantum dots. By using polarization-resolved photoluminescence spectroscopy at different temperatures and a theoretical model, we show that the exciton-cavity interaction range is limited to the phonon sidebands. Photon-correlation and pump-power dependence experiments under nonresonant excitation conditions further establish that the cavity is fed only by a single exciton.

Degradation defects induced by electron-beam bombardment in Zn1-xCdxSe/ZnSe (001) graded-index separate-confinement blue-green laser structures have been characterized by transmission electron microscopy. Most defects appear as tree-like arrangements of small extrinsic and intrinsic dislocation loops opening in +/-[100] or +/-[010] directions and are located within the quantum well or at its boundaries. The Burgers vectors of the loops were identified as b = 1/2[01(1) over bar] and b = 1/2[10(1) over bar] for defects opening in the +/-[100] and +/-[010] directions respectively.

We explored degradation in electron-beam-pumped Zn1-xCdxSe/ZnSe laser structures by combining cathodoluminescence (CL) measurements in a scanning electron microscope with transmission electron microscopy. The rate of degradation, measured as the decrease of the emitted CL intensity under electron bombardment, depends critically on the threading dislocation density and on the strain in the quantum well. Degradation occurs via the formation of dark spot defects, which are related to bombardment-induced networks of dislocation loops in the quantum well. These degradation defects often initiate where threading dislocations cross the quantum well. We propose a self-supporting dislocation climb mechanism activated by nonradiative recombination to explain the formation and propagation of the degradation defects. (C) 1998 American Institute of Physics.

We explored the degradation in electron beam pumped Zn1-xCdxSe/ZnSe laser structures by combining cathodoluminescence measurements in a scanning electron microscope with transmission electron microscopy. We found that degradation occurred via the formation of dark spot and dark line defects, and that it involved the formation of a characteristic type of defect, namely dislocation loops within the ZnCdSe quantum well, which grew to form a characteristic network as degradation progressed.

The broad, multi-terahertz spectral window ranging from 10 to 100 THz (1 THz = 1 · 10 Hz) contains a plethora of fundamental excitations in solid state systems, including plasmons, phonons, magnons and interlevel transitions in excitons [1]. Field-resolved detection of phase-stable multi-terahertz probe pulses by electro-optic sampling (EOS) provides a direct measure of the polarization response of such phenomena. It thus reveals their evolution with a temporal resolution much faster than a single oscillation cycle of the multi-terahertz probe [2]. However, the diffraction limit restricts the spatial resolution of conventional, far-field multi-terahertz spectroscopy to half the probe wavelength (µm - mm). Consequently, in ensembles of nano-sized objects like nanowires or quantum dots, the measured dielectric function is averaged and important information can be obscured completely. Yet, accessing the optical properties of these nanostructures is crucial for understanding how their properties differ from those of a bulk material. Scattering-type near-field scanning optical microscopy (s-NSOM) can be used to overcome the diffraction limit [3].

We present results obtained both with scanning electron microscopy (SEM) in the cathodoluminescence (CL) mode and transmission electron microscopy (TEM) on ZnCdSe/ZnSe electron-beam-pumped laser structures. This investigation aims at establishing a relationship between different parameters measurable in electron microscopy and the lasing activity of the structures.