Kondo physics in tunable semiconductor nanowire quantum dots

We have observed the Kondo effect in strongly coupled semiconducting nanowire quantum dots. The devices are made from indium arsenide nanowires, grown by molecular beam epitaxy, and contacted by titanium leads. The device transparency can be tuned by changing the potential on a gate electrode, and for increasing transparencies the effects dominating the transport changes from Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics appearing in the intermediate region.Comment: 4 pages, 4 figure

Single nanowire solar cells beyond the Shockley-Queisser limit

Light management is of great importance to photovoltaic cells, as it determines the fraction of incident light entering the device. An optimal pn-junction combined with an optimal light absorption can lead to a solar cell efficiency above the Shockley-Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core-shell p-i-n junction GaAs nanowire solar cell grown on a silicon substrate. At one sun illumination a short circuit current of 180 mA/cm^2 is obtained, which is more than one order of magnitude higher than what would be predicted from Lambert-Beer law. The enhanced light absorption is shown to be due to a light concentrating property of the standing nanowire as shown by photocurrent maps of the device. The results imply new limits for the maximum efficiency obtainable with III-V based nanowire solar cells under one sun illumination.Comment: 19 pages, 3 figure

Shadow epitaxy for in-situ growth of generic semiconductor/superconductor devices

Uniform, defect-free crystal interfaces and surfaces are crucial ingredients for realizing high-performance nanoscale devices. A pertinent example is that advances in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices are currently built on the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminium heterostructures. Fabrication of devices requires selective etch processes; these exist only for InAs/Al hybrids, precluding the use of other, potentially superior material combinations. We present a crystal growth platform -- based on three-dimensional structuring of growth substrates -- which enables synthesis of semiconductor nanowire hybrids with in-situ patterned superconductor shells. This platform eliminates the need for etching, thereby enabling full freedom in choice of hybrid constituents. We realise and characterise all the most frequently used architectures in superconducting hybrid devices, finding increased yield and electrostatic stability compared to etched devices, along with evidence of ballistic superconductivity. In addition to aluminium, we present hybrid devices based on tantalum, niobium and vanadium. This is the submitted version of the manuscript. The accepted, peer reviewed version is available from Advanced Materials: http://doi.org/10.1002/adma.201908411 Previous title: Shadow lithography for in-situ growth of generic semiconductor/superconductor device

Advances in the theory of III-V Nanowire Growth Dynamics

Nanowire (NW) crystal growth via the vapour_liquid_solid mechanism is a complex dynamic process involving interactions between many atoms of various thermodynamic states. With increasing speed over the last few decades many works have reported on various aspects of the growth mechanisms, both experimentally and theoretically. We will here propose a general continuum formalism for growth kinetics based on thermodynamic parameters and transition state kinetics. We use the formalism together with key elements of recent research to present a more overall treatment of III_V NW growth, which can serve as a basis to model and understand the dynamical mechanisms in terms of the basic control parameters, temperature and pressures/beam fluxes. Self-catalysed GaAs NW growth on Si substrates by molecular beam epitaxy is used as a model system.Comment: 63 pages, 25 figures and 4 tables. Some details are explained more carefully in this version aswell as a new figure is added illustrating various facets of a WZ crysta

Kondo-enhanced Andreev tunneling in InAs nanowire quantum dots

We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with sub-gap-peaks at $|V_{sd}|=\Delta/e$ markedly stronger(weaker) than the quasiparticle tunneling peaks at $|V_{sd}|=2\Delta/e$ for odd(even) occupation. We attribute the enhanced $\Delta$-peak to an interplay between Kondo-correlations and Andreev tunneling in dots with an odd number of spins, and substantiate this interpretation by a poor man's scaling analysis

Clinical Description of a Completed Outbreak of SARS in Vietnam, February–May, 2003

We investigated the clinical manifestations and course of all probable severe acute respiratory syndrome (SARS) patients in the Vietnam outbreak. Probable SARS cases were defined by using the revised World Health Organization criteria. We systematically reviewed medical records and undertook descriptive statistical analyses. All 62 patients were hospitalized. On admission, the most prominent symptoms were malaise (82.3%) and fever (79.0%). Cough, chest pain, and shortness of breath were present in approximately one quarter of the patients; 79.0% had lymphopenia; 40.3% had thrombocytopenia; 19.4% had leukopenia; and 75.8% showed changes on chest radiograph. Fever developed on the first day of illness onset, and both respiratory symptoms and radiographic changes occurred on day 4. On average, maximal radiographic changes were observed on day 10, and fevers subsided by day 13. Symptoms on admission were nonspecific, although fever, malaise, and lymphopenia were common. The complications of SARS included invasive intubation and ventilation (11.3%) and death (9.7%)

Impact of the liquid Phase shape on the structure of III-V nanowires

The III-V nanowire structure (zinc blende or wurtzite) grown by the vapor-liquid-solid process is shown to be highly dependent on the parameters which shape the droplet at the top of the nanowire. Under conditions that the droplet volume does not exceed a certain value, it is demonstrated that when the nucleation of the solid starts at the solid-liquid-vapor triple line, a relatively large droplet volume and low wetting angle favor the formation of the wurtzite structure. We show that the effective V/III flux ratio is the primary parameter controlling the structure. Most of the III-V semiconductor nanowires (NW) are grown by the vapor-liquid-solid (VLS) process [1], in which a supersaturated liquid droplet initiates NW growth in the (111) direction of the stable zinc blende (ZB) structure or in the equivalent (0001) direction of the metastable wurtzite (WZ) structure. It is now well accepted that the liquid-solid transformation is 2D nucleation limited and that the nucleation events take place preferentially at the vapor-liquid-solid triple phase line (TL) [2,3]. However, for sufficiently small liquid surface energies [4] and sufficiently large contact angles between the top-facet and liquid surface, the probability of nucleating away from the TL may be energetically possible. Assuming a cylindrical shaped NW with a spherical shaped droplet, Glas et al. [2] used classical nucleation theory to show that Au-catalyzed NWs should adopt the metastable hexagonal wurtzite (WZ) structure under conditions of a large difference of chemical potentials between the liquid and the solid, Á, which acts as a driving force for solidification from the supersaturated liquid phase. Taking into account that the NW top facet has a hexagonal shape, we show that the droplet shape will change as a function of its relative size, implying a huge change in the nucleation statistics. We show that the important parameters determining the growth structure are those that determine the shape of the droplet. In agreement with many experiments [5–10], we show that this is determined primarily by the effective V/III current ratio, defined to be the ratio of currents of group V and III elements sorbed in the droplet. The VLS growth can generally be divided into two regimes. Regime I: the droplet is solely in contact with the (111) top surface. Regime II: a part of the TL has expanded onto the side-facet. In this study we focus on regime I which is the most common growth regime. The nucleation barrier is given by the maximum value of the free energy increase upon nucleation, ÁG Ã ZBðWZÞ ð!Þ ¼ cð!Þ À 2 ZBðWZÞ ð!Þ Á ZBðWZÞ , where cð!Þ and À ZBðWZÞ ð!Þ are the nucleus shape factor and the effective specific surface energy of the nucleus, respectively, and ! is the angle between the middle of the side facet and the nucleation site, as measured from the center of the top facet, see Fig. 1. At a given !, the nucleation probability for the ZB (WZ) phase is dominated by the factor exp Àcð!Þ À 2 ZBðWZÞ ð!Þ Á ZBðWZÞ kT : (1) Thus, the driving force Á ZBðWZÞ is in principle the dominating parameter which determines the preferred growth structure at ! for a given droplet shape. Because the free energy of the volume WZ structure is higher than that of the ZB volume [11] we have Á ZB > Á WZ for the same liquid supersaturation. A calculation of the normalized probabilities of nucleating ZB and WZ at a given !, is shown in Fig. 2(a) for Au-assisted GaAs NW growth. Because of the exponential factor in Eq. (1), the total probability of nucleation is practically zero below a critical driving force Á c and increases sharply above, see Fig. 2(b), which means that the driving force will oscillate around Á c due to lowering of the liquid concentrations c III and primarily c V , upon forming each ML. As the equilibrium shape of a NW top facet and a droplet on a 2D surface has a hexagonal shape and is a truncated sphere, respectively, the nucleation conditions are different along the TL. Thus, the total probabilities of WZ or ZB nuclea-tion at the TL can be approximated by integrating over ! P ZBðWZÞ / Z 30 0 exp Àcð!Þ À 2 ZBðWZÞ ð!Þ Á ZBðWZÞ kT d!; (2) where only the interval 0 ! 30 is considered because of the sixfold symmetry of the NW top facet. In order to understand why growth conditions have a huge effect on the structure, we have to examine how cð!ÞÀ 2 ZBðWZÞ ð!Þ depends on changes in the shape of the drop. The nucleus effective step energy can be written as PR