133 research outputs found
Noisy fractions
No full textFractional charges are one of the hallmarks of topological matter and the building blocks of various topological devices. Now, there are indications that their fingerprint in terms of electrical noise is less obvious, but more universal, than expected
Measurement of the Thermoelectric Properties of Individual Nanostructures
No full textThe advent of nanotechnology and nanomaterials is opening new perspectives for the achievement of efficient solid-state heat converters. After decades of slow progress, in recent years innovative ideas have been put forward to improve the thermodynamic conversion efficiency and, as a consequence, new thermoelectric nanomaterials have been developed. A key and challenging ingredient for the progress of this research ambit is today the refinement of precise methods for the measurement of the thermoelectric parameters of nanostructures
Impact of electron heating on the equilibration between quantum Hall edge channels
No full textWhen two separately contacted quantum Hall (QH) edge channels are brought into interaction, they can equilibrate their imbalance via scattering processes. In this work, we use a tunable QH circuit to implement a junction between copropagating edge channels, the length of which can be controlled with continuity. Such a variable device allows us to investigate how current-voltage characteristics evolve when the junction length d is changed. Recent experiments with fixed geometry reported a significant reduction of the threshold voltage for the onset of photon emission, the origin of which is still under debate. Our spatially resolved measurements reveal that this threshold shift depends on the junction length. We discuss this unexpected result on the basis of a model that demonstrates that a heating of electrons is the dominant process responsible for the observed reduction of the threshold voltage
Formation of axial metal-semiconductor junctions in GaAs nanowires by thermal annealing
No full textWe demonstrate the formation of nanoscale axial Schottky contacts in GaAs nanowires by thermal annealing of NiGeAu multilayers. Energy dispersive x-ray spectroscopy indicates that in specific annealing conditions a sharply-defined Au-rich phase can be obtained in the vicinity of the metallic contacts. Charge transport characteristics are analyzed for different degrees of diffusion of the Au-rich phase in the wire and indicate that it has a metallic nature. The mechanism behind this local post-growth modification of the nanowire composition and its potential impact on device applications are discussed
Perfecting the Growth and Transfer of Large Single-Crystal CVD Graphene: A Platform Material for Optoelectronic Applications
No full textIn this work, we demonstrate the synthesis of millimetre-sized single-crystals of graphene, achievable in a commercially available cold-wall CVD reactor, and several different approaches to transfer it from the growth substrate to a target substrate of choice. We confirm the high crystal quality of this material using various characterisation techniques, including optical and scanning electron microscopy as well as Raman spectroscopy. By performing field effect and quantum Hall effect measurements, we demonstrate that the electronic properties of such single crystals are comparable to those of ideal mechanically exfoliated flakes of graphene. Several applications of this high-quality material are also reviewed
Cascaded quantum hall bisection and applications to quantum metrology
No full textWe demonstrate a programmable quantum Hall circuit that implements an iterative voltage bisection scheme and allows any binary fraction (k/2n) of the fundamental resistance quantum RK/2=h/2e2 to be obtained. The circuit requires a number n of bisection stages that only scales logarithmically with the resolution of the set of possible output fractions. The value of k can be set to any integer between 1 and 2n by proper and easily predictable gate configuration. The architecture exploits gate-controlled routing, mixing, and equilibration of edge modes of robust quantum Hall states. The device does not contain internal Ohmic contacts and is thus naturally robust towards stray-resistance effects. Our scheme offers an alternative way to obtain custom quantum Hall resistance standards, and its potential advantages are discussed. The basic viability of the approach is demonstrated in a proof-of-principle two-stage bisection circuit built on a high-mobility GaAs/(Al,Ga)As heterostructure operating at a temperature of 260mK and a magnetic field of 4.1T. Our prototype achieves a relative quantization precision of the order of 10-4, which is limited by the experimental setup rather than by the circuit itself
Selective control of edge-channel trajectories by scanning gate microscopy RID B-4406-2011 RID C-6303-2008 RID C-5465-2009
No full textElectronic Mach-Zehnder interferometers in the quantum Hall (QH) regime are currently discussed for the realization of quantum information schemes. A recently proposed device architecture employs interference between two co-propagating edge channels. Here we demonstrate the precise control of individual edge-channel trajectories in quantum point contact devices in the QH regime. The biased tip of an atomic force microscope is used as a moveable local gate to pilot individual edge channels. Our results are discussed in light of the implementation of multi-edge interferometers. (C) 2009 Elsevier B.V. All rights reserved
Giant Thermovoltage in Single InAs Nanowire Field-Effect Transistors
No full textMillivolt range thermovoltage is demonstrated in single InAs nanowire based field effect transistors. Thanks to a buried heating scheme, we drive both a large thermal bias Delta T > 10 K and a strong field-effect modulation of electric conductance on the nanostructures. This allows the precise mapping of the evolution of the Seebeck coefficient S as a function of the gate-controlled conductivity sigma between room temperature and 100 K. Based on these experimental data a novel estimate of the electron mobility is given. This value is compared with the result of standard field-effect based mobility estimates and discussed in relation to the effect of charge traps in the devices
Coherent edge mixing and interferometry in quantum Hall bilayers
No full textWe discuss the implementation of a beam splitter for electron waves in a quantum Hall bilayer. Our architecture exploits interlayer tunneling to mix edge states belonging to different layers. We discuss the basic working principle of the proposed coherent edge mixer, possible interferometric implementations based on existing semiconductor-heterojunction technologies, and advantages with respect to canonical quantum Hall interferometers based on quantum point contacts
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