The 0.7 (2e 2 /h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e 2 /h) at finite bias. Varying the gate voltage the transition between the 1.0 and the 0.85 (2e 2 /h) plateaus occurs for definite bias voltages, which defines a gate voltage dependent energy difference ∆. This energy difference is compared with the activation temperature Ta extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find ∆ = kBTa which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge ∆ below the chemical potential becomes thermally depopulated as the temperature is increased.PACS 73.61.-r, 73.23.-b

We study electron decoherence by measuring the temperature dependence of Aharonov-Bohm (AB) oscillations in quasi-1D rings, etched in a high-mobility GaAs/GaAlAs heterostructure. The oscillation amplitude is influenced both by phase-breaking and by thermal averaging. Thermal averaging is important when the temperature approaches the energy scale, on which the AB oscillations shift their phase. For the phase-breaking, it is demonstrated that the damping of the oscillation amplitude is proportional to the length of the interfering paths. For temperatures T from 0.3 to 4 K we find the phase coherence length The understanding of decoherence in quantum mechanical systems gives valuable insight into the cross-over from quantum to classical behavior. Quantum phenomena like weak localization, universal conductance fluctuations and the Aharonov-Bohm effect, that are observed in mesoscopic electronic systems, make these systems well suited for studying decoherence. The loss of electron phase coherence is interesting in its own right because it reveals information about the fundamental physics of the electron scattering mechanisms. Moreover, from the perspective of possible phase-coherent mesoscopic electronic devices 1 , knowledge of phase-breaking length and time scales is crucial.At low temperatures, electron-electron scattering is usually the dominating source of phase-breaking. In disordered 1D and 2D conductors, the loss of phase coherence at low temperatures has been studied intensively, both theoretically and experimentally 2 . In clean electron systems, the number of investigations are fewer 3-6 . In 2D, experiments consistent with the expected 7 electronelectron scattering time τ φ ∼ (T 2 ln T ) −1 has been carried out 3,4 . In open quantum dots (a 0D system), an unexpected T −1 contribution was found 5,6 . In general, phase breaking mechanisms in ballistic, mesoscopic systems of dimensionality less than 2, are presently not well understood.Aharonov-Bohm (AB) rings are obvious systems for probing phase coherence. Here, the interference of two electron paths leads to conductance oscillations of period h/e [or frequency e/h] in the magnetic flux enclosed by the paths. The oscillation amplitude is a direct measure of the interference strength, and it has been used to study decoherence in disordered systems 8 . For AB rings with a 2DEG elastic mean free path longer than the circumference of the device (e.g. 9-15 and references therein), systematic studies of phase-breaking have been scarce.In this paper, we report measurements of the phase coherence length L φ via the temperature dependence of AB conductance oscillations in quasi-1D rings, made by shallow etching in GaAs/GaAlAs heterostructures. Two mechanisms are important for the temperature dependence of the oscillation amplitude: phase-breaking, and thermal averaging. At finite temperature, the measured conductance is a weighted average over an energy interval of finite width, proportional to the temperature. We discuss, how thermal averaging influences the AB os...

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.

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

The perspectives offered by vertical arrays of nanowires for biosensing applications in living cells depend on the access of individual nanowires to the cell interior. Recent results on electrical access and molecular delivery suggest that direct access is not always obtained. Here, we present a generic approach to directly visualize the membrane conformation of living cells interfaced with nanowire arrays, with single nanowire resolution. The method combines confocal z-stack imaging with an optimized cell membrane labelling strategy which was applied to HEK293 cells interfaced with 2-11 μm long and 3-7 μm spaced nanowires with various surface coatings (bare, aminosilane-coated or polyethyleneimine-coated indium arsenide). We demonstrate that, for all commonly used nanowire lengths, spacings and surface coatings, nanowires generally remain enclosed in a membrane compartment, and are thereby not in direct contact with the cell interior.

We present an investigation of the morphology and composition of novel types of axial nanowire heterostructures where Ga(x)In(1-x)As is used as barrier material in InAs nanowires. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray analysis we demonstrate that it is possible to grow junctions by changing the group III elements, and we find that a substantial fraction of Ga can be incorporated in axial InAs/Ga(x)In(1-x)As/InAs, retaining straight nanowire configurations. We explain how the adatoms are transferred to the incorporation site at the growth interface via two different routes, (1) interface diffusion and (2) volume diffusion through the catalyst particle.

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

The incorporation paths of Be in GaAs nanowires grown by the Ga-assisted method in molecular beam epitaxy have been investigated by electrical measurements of nanowires with different doping profiles. We find that Be atoms incorporate preferentially via the nanowire side facets, while the incorporation path through the Ga droplet is negligible. We also show that Be can diffuse into the volume of the nanowire giving an alternative incorporation path. This work is an important step towards controlled doping of nanowires and will serve as a help for designing future devices based on nanowires.