Charge localization and reentrant superconductivity in a quasi-ballistic InAs nanowire coupled to superconductors.

A semiconductor nanowire with strong spin-orbit coupling in proximity to a superconductor is predicted to display Majorana edge states emerging under a properly oriented magnetic field. The experimental investigation of these exotic states requires assessing the one-dimensional (1D) character of the nanowire and understanding the superconducting proximity effect in the presence of a magnetic field. Here, we explore the quasi-ballistic 1D transport regime of an InAs nanowire with Ta contacts. Fine-tuned by means of local gates, the observed plateaus of approximately quantized conductance hide the presence of a localized electron, giving rise to a lurking Coulomb blockade effect and Kondo physics. When Ta becomes superconducting, this local charge causes an unusual, reentrant magnetic field dependence of the supercurrent, which we ascribe to a 0 - π transition. Our results underline the relevant role of unintentional charge localization in the few-channel regime where helical subbands and Majorana quasi-particles are expected to arise.

Mapping the Coulomb Environment in Interference-Quenched Ballistic Nanowires.

The conductance of semiconductor nanowires is strongly dependent on their electrostatic history because of the overwhelming influence of charged surface and interface states on electron confinement and scattering. We show that InAs nanowire field-effect transistor devices can be conditioned to suppress resonances that obscure quantized conduction thereby revealing as many as six sub-bands in the conductance spectra as the Fermi-level is swept across the sub-band energies. The energy level spectra extracted from conductance, coupled with detailed modeling shows the significance of the interface state charge distribution revealing the Coulomb landscape of the nanowire device. Inclusion of self-consistent Coulomb potentials, the measured geometrical shape of the nanowire, the gate geometry and nonparabolicity of the conduction band provide a quantitative and accurate description of the confinement potential and resulting energy level structure. Surfaces of the nanowire terminated by HfO2 are shown to have their interface donor density reduced by a factor of 30 signifying the passivating role played by HfO2.

Magnetically-driven colossal supercurrent enhancement in InAs nanowire Josephson junctions.

The Josephson effect is a fundamental quantum phenomenon where a dissipationless supercurrent is introduced in a weak link between two superconducting electrodes by Andreev reflections. The physical details and topology of the junction drastically modify the properties of the supercurrent and a strong enhancement of the critical supercurrent is expected to occur when the topology of the junction allows an emergence of Majorana bound states. Here we report charge transport measurements in mesoscopic Josephson junctions formed by InAs nanowires and Ti/Al superconducting leads. Our main observation is a colossal enhancement of the critical supercurrent induced by an external magnetic field applied perpendicular to the substrate. This striking and anomalous supercurrent enhancement cannot be described by any known conventional phenomenon of Josephson junctions. We consider these results in the context of topological superconductivity, and show that the observed critical supercurrent enhancement is compatible with a magnetic field-induced topological transition.

Crystal Phases in Hybrid Metal-Semiconductor Nanowire Devices.

We investigate the metallic phases observed in hybrid metal-GaAs nanowire devices obtained by controlled thermal annealing of Ni/Au electrodes. Devices are fabricated onto a SiN membrane compatible with transmission electron microscopy studies. Energy dispersive X-ray spectroscopy allows us to show that the nanowire body includes two Ni-rich phases that thanks to an innovative use of electron diffraction tomography can be unambiguously identified as Ni3GaAs and Ni5As2 crystals. The mechanisms of Ni incorporation leading to the observed phenomenology are discussed.

Heterogeneous nucleation of catalyst-free InAs nanowires on silicon.

We report on the heterogeneous nucleation of catalyst-free InAs nanowires on Si(111) substrates by chemical beam epitaxy. We show that nanowire nucleation is enhanced by sputtering the silicon substrate with energetic particles. We argue that particle bombardment introduces lattice defects on the silicon surface that serve as preferential nucleation sites. The formation of these nucleation sites can be controlled by the sputtering parameters, allowing the control of nanowire density in a wide range. Nanowire nucleation is accompanied by unwanted parasitic islands, but careful choice of annealing and growth temperature allows us to strongly reduce the relative density of these islands and to realize samples with high nanowire yield.

Tunable Esaki Effect in Catalyst-Free InAs/GaSb Core-Shell Nanowires.

We demonstrate tunable bistability and a strong negative differential resistance in InAs/GaSb core-shell nanowire devices embedding a radial broken-gap heterojunction. Nanostructures have been grown using a catalyst-free synthesis on a Si substrate. Current-voltage characteristics display a top peak-to-valley ratio of 4.8 at 4.2 K and 2.2 at room temperature. The Esaki effect can be modulated-or even completely quenched-by field effect, by controlling the band bending profile along the azimuthal angle of the radial heterostructure. Hysteretic behavior is also observed in the presence of a suitable resistive load. Our results indicate that high-quality broken-gap devices can be obtained using Au-free growth.

Length distributions of Au-catalyzed and In-catalyzed InAs nanowires.

We present experimental data on the length distributions of InAs nanowires grown by chemical beam epitaxy with Au catalyst nanoparticles obtained by thermal dewetting of Au film, Au colloidal nanoparticles and In droplets. Poissonian length distributions are observed in the first case. Au colloidal nanoparticles produce broader and asymmetric length distributions of InAs nanowires. However, the distributions can be strongly narrowed by removing the high temperature annealing step. The length distributions for the In-catalyzed growth are instead very broad. We develop a generic model that is capable of describing the observed behaviors by accounting for both the incubation time for nanowire growth and secondary nucleation of In droplets. These results allow us to formulate some general recipes for obtaining more uniform length distributions of III-V nanowires.

Local noise in a diffusive conductor.

The control and measurement of local non-equilibrium configurations is of utmost importance in applications on energy harvesting, thermoelectrics and heat management in nano-electronics. This challenging task can be achieved with the help of various local probes, prominent examples including superconducting or quantum dot based tunnel junctions, classical and quantum resistors, and Raman thermography. Beyond time-averaged properties, valuable information can also be gained from spontaneous fluctuations of current (noise). From these perspective, however, a fundamental constraint is set by current conservation, which makes noise a characteristic of the whole conductor, rather than some part of it. Here we demonstrate how to remove this obstacle and pick up a local noise temperature of a current biased diffusive conductor with the help of a miniature noise probe. This approach is virtually noninvasive for the electronic energy distributions and extends primary local measurements towards strongly non-equilibrium regimes.

Nucleation and growth mechanism of self-catalyzed InAs nanowires on silicon.

We report on the nucleation and growth mechanism of self-catalyzed InAs nanowires (NWs) grown on Si (111) substrates by chemical beam epitaxy. Careful choices of the growth parameters lead to In-rich conditions such that the InAs NWs nucleate from an In droplet and grow by the vapor-liquid-solid mechanism while sustaining an In droplet at the tip. As the growth progresses, new NWs continue to nucleate on the Si (111) surface causing a spread in the NW size distribution. The observed behavior in NW nucleation and growth is described within a suitable existing theoretical model allowing us to extract relevant growth parameters. We argue that these results provide useful guidelines to rationally control the growth of self-catalyzed InAs NWs for various applications.

Suspended InAs nanowire Josephson junctions assembled via dielectrophoresis.

We present a novel technique for the realization of suspended Josephson junctions based on InAs semiconductor nanowires. The devices are assembled using a technique of drop-casting guided by dielectrophoresis, which allows one to finely align the nanostructures on top of the electrodes. The proposed architecture removes the interaction between the nanowire and the substrate which is known to influence disorder and the orientation of the Rashba vector. The relevance of this approach in view of the implementation of hybrid Josephson junctions based on semiconducting nanowires coupled with high-temperature superconductors is discussed.

Catalyst-free growth of InAs nanowires on Si (111) by CBE.

We investigate a growth mechanism which allows for the fabrication of catalyst-free InAs nanowires on Si (111) substrates by chemical beam epitaxy. Our growth protocol consists of successive low-temperature (LT) nucleation and high-temperature growth steps. This method produces non-tapered InAs nanowires with controllable length and diameter. We show that InAs nanowires evolve from the islands formed during the LT nucleation step and grow truly catalyst-free, without any indium droplets at the tip. The impact of different growth parameters on the nanowire morphology is presented. In particular, good control over nanowire aspect ratio is demonstrated. A better understanding of the growth process is obtained through the development of a theoretical model combining the diffusion-induced growth scenario with some specific features of the catalyst-free growth mechanism, along with the analysis of the V/III flow ratio influencing material incorporation. As a result, we perform a full mapping of the nanowire morphology versus growth parameters which provides useful general guidelines on the self-induced formation of III-V nanowires on silicon.

Pb/InAs nanowire josephson junction with high critical current and magnetic flux focusing.

We have studied mesoscopic Josephson junctions formed by highly n-doped InAs nanowires and superconducting Ti/Pb source and drain leads. The current-voltage properties of the system are investigated by varying temperature and external out-of-plane magnetic field. Superconductivity in the Pb electrodes persists up to ∼7 K and with magnetic field values up to 0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and the critical current is measured to be as high as 615 nA. The supercurrent suppression as a function of the magnetic field reveals a diffraction pattern that is explained by a strong magnetic flux focusing provided by the superconducting electrodes forming the junction.

Nanowire-based field effect transistors for terahertz detection and imaging systems.

The development of self-assembled nanostructure technologies has recently opened the way towards a wide class of semiconductor integrated devices, with progressively optimized performances and the potential for a widespread range of electronic and photonic applications. Here we report on the development of field effect transistors (FETs) based on semiconductor nanowires (NWs) as highly-sensitive room-temperature plasma-wave broadband terahertz (THz) detectors. The electromagnetic radiation at 0.3 THz is funneled onto a broadband bow-tie antenna, whose lobes are connected to the source and gate FET electrodes. The oscillating electric field experienced by the channel electrons, combined with the charge density modulation by the gate electrode, results in a source-drain signal rectification, which can be read as a DC signal output. We investigated the influence of Se-doping concentration of InAs NWs on the detection performances, reaching responsivity values higher than 100 V W⁻¹, with noise-equivalent-power of ∼10⁻9 W Hz(⁻½). Transmission imaging experiments at 0.3 THz show the good reliability and sensitivity of the devices in a real practical application.

Modeling of InAs-InSb nanowires grown by Au-assisted chemical beam epitaxy.

Interesting phenomena during the Au-assisted chemical beam epitaxy of InAs-InSb nanowire heterostructures have been observed and interpreted within the framework of a theoretical model. An unusual, non-monotonous diameter dependence of the InSb nanowire growth rate is demonstrated experimentally within a range of deposition conditions. Such a behavior is explained by competition between the Gibbs-Thomson effect and different diffusion-induced material fluxes. Theoretical fits to the experimental data obtained at different flux pressures of In and Sb precursors allow us to deduce some important kinetic coefficients. Furthermore, we discuss why the InAs nanowire stem forms in the wurtzite phase while the upper InSb part has a pure zinc blende crystal structure. It is hypothesized that the 30° angular rotation of nanowire when passing from InAs to the InSb part is driven by the lowest surface energy of (1100) wurtzite and (110) zinc blende facets.

Unilateral retinal detachment as the initial sign of lung adenocarcinoma.

Intraocular metastases, especially to the retina, are uncommon in cancer patients and generally occur in an advanced phase of the disease. In patients with lung cancer, uveal metastases, in particular to the choroid, are the most frequent, and are associated mainly with small cell carcinoma or undifferentiated carcinoma. We report a case of unilateral retinal detachment as first sign of a moderately differentiated lung adenocarcinoma in a 55-year-old non-smoker that was admitted to the hospital for the first time complaining of a sudden visual loss in the superior fields of the left eye. A CT revealed a slight retinal enlargement of the left eye and a solid mass of about 3 centimeters behind the right pulmonary hilus. Bronchoscopic biopsies were performed with diagnosis of adenocarcinoma of the lung. The patient died after 2 months for rapid progression of the disease despite of combined chemotherapy treatment.

External ophthalmomyiasis. Report of four cases.

Four cases of external ocular myiasis caused by larvae of Oestrus ovis are reported. Each patient showed some degree of inflammation in the lids and conjunctiva. Regression was only subsequent to removal of parasites from the eye. Two patients had respiratory signs, i.e. rhinitis. For one of these two cases, a short-time admission in the resuscitation unit was recommended following a noticeable bronchospasm. The pathogenetic mechanism of this respiratory distress is not known. Two hypotesis are made.

La vitrectomia en el manejo del sindrome de Terson. / Vitrectomy in the management of Terson's syndrome

Se practico vitrectomia a traves de la pars plana en tres pacientes (cinco ojos) con Sindrome de Terson. La agudeza visual mejoro significativamente en todos los ojos intervenidos.Se sugiere el tratamientos quirurgico de los ojos de estos pacientes, especialmente en casos con hemorragia vitrea bilateral, despues de un periodo de observacion razonable