We investigate phase-coherent transport in InN nanowires of various diameters and lengths. The nanowires were grown by means of plasma-assisted molecular beam epitaxy. Information on the phase-coherent transport is gained by analyzing the characteristic fluctuation pattern in the magneto-conductance. For a magnetic field oriented parallel to the wire axis we found that the correlation field mainly depends on the wire cross section, while the fluctuation amplitude is governed by the wire length. In contrast, if the magnetic field is oriented perpendicularly, for wires longer than approximately 200 nm the correlation field is limited by the phase coherence length. Further insight into the orientation dependence of the correlation field is gained by measuring the conductance fluctuations at various tilt angles of the magnetic field.
A phase-stable superposition of femtosecond pulses and their second harmonic induces ultrashort microampere current bursts in single unbiased GaAs nanowires. Current injection relies on quantum interference of one- and two-photon absorption pathways.
Hybrid InSb nanowire-superconductor devices are promising for investigating Majorana modes and topological quantum computation in solid-state devices. An experimental realisation of ballistic, phase-coherent superconductor-nanowire hybrid devices is a necessary step towards engineering topological superconducting electronics. Here, we report on a low-temperature transport study of Josephson junction devices fabricated from InSb nanowires grown by molecular-beam epitaxy and provide a clear evidence for phase-coherent, ballistic charge transport through the nanowires in the junctions. We demonst…
A longstanding goal of spintronics is to inject, coherently transport, and detect spins in a semiconductor nanowire where a SINGLE quantized subband is occupied at room temperature. Here, we report achieving this goal in 50-nm diameter InSb nanowires by demonstrating both the spin-valve and the Hanle effect. The spin relaxation time in the nanowires was found to have increased by an order of magnitude over what has been reported in bulk and quantum wells due to the suppression of D'yakonov-Perel' spin relaxation as a result of single subband occupancy. These experiments raise hopes for the rea…
In this paper we employ a combined {\it ab initio} and tight-binding approach to obtain the electronic and optical properties of hydrogenated InN nanowires. We first discuss InN band structure for the wurtzite structure calculated at the LDA+U level and use this information to extract the parameters needed for an empirical tight-binging implementation. These parameters are then employed to calculate the electronic and optical properties of InN nanowires in a diameter range that would not be affordable by {\it ab initio} techniques. The reliability of the large nanowires results is assessed by …
We report on low-temperature magnetotransport measurements on InN nanowires, grown by plasma-assisted molecular beam epitaxy. The characteristic fluctuation pattern observed in the conductance was employed to obtain information on phase-coherent transport. By analyzing the root-mean-square and the correlation field of the conductance fluctuations at various temperatures the phase-coherence length was determined.
We experimentally studied the Josephson supercurrent in Nb/InN-nanowire/Nb junctions. Large critical currents up to 5.7 $μ$A have been achieved, which proves the good coupling of the nanowire to the superconductor. The effect of a magnetic field perpendicular to the plane of the Josephson junction on the critical current has been studied. The observed monotonous decrease of the critical current with magnetic field is explained by the magnetic pair-breaking effect in planar Josephson junctions of ultra-narrow width [J. C. Cuevas and F. S. Bergeret, Phys. Rev. Lett. 99, 217002 (2007)]
We report on gigahertz acoustic phonon waveguiding in free-standing single copper nanowires studied by femtosecond transient reflectivity measurements. The results are discussed on the basis of the semianalytical resolution of the Pochhammer and Chree equation. The spreading of the generated Gaussian wave packet of two different modes is derived analytically and compared with the observed oscillations of the sample reflectivity. These experiments provide a unique way to independently obtain geometrical and material characterization. This direct observation of coherent guided acoustic phonons i…
Using a one-dimensional tight-binding Anderson model, we study a disordered nanowire in the presence of an external gate which can be used for depleting its carrier density (field effect transistor device configuration). In this first paper, we consider the low temperature coherent regime where the electron transmission through the nanowire remains elastic. In the limit where the nanowire length exceeds the electron localization length, we derive three analytical expressions for the typical value of the thermopower as a function of the gate potential, in the cases where the electron transport …
Despite the fact that GeTe is known to be a very interesting material for applications in thermoelectrics and for phase-change memories, the knowledge on its low-temperature transport properties is only limited. Here, we report on phase-coherent phenomena in the magnetotransport of GeTe nanowires. From universal conductance fluctuations, a phase-coherence length of about 200nm at 0.5K is determined for the hole carriers. The distinct phase-coherence is confirmed by the observation of Aharonov--Bohm type oscillations for magnetic fields applied along the nanowire axis. We interpret the occurren…
We report low-temperature electrical transport studies of molecule-scale silicon nanowires. Individual nanowires exhibit well-defined Coulomb blockade oscillations characteristic of charge addition to a single nanostructure with length scales up to at least 400 nm. Further studies demonstrate coherent charge transport through discrete single particle quantum levels extending the whole device, and show that the ground state spin configuration follows the Lieb-Mattis theorem. In addition, depletion of the nanowires suggests that phase coherent single-dot characteristics are accessible in a regim…
With a one-band envelope function theory, we investigate the dynamics of a finite nanowire superlattice driven by an ac electric field by solving numerically the time-dependent Schroedinger equation. We find that for an ac electric field resonant with two energy levels located in two different minibands, the coherent dynamics in nanowire superlattices is much more complex as compared to the standard two-level description. Depending on the energy levels involved in the transitions, the coherent oscillations exhibit different patterns. A signature of barrier-well inversion phenomenon in nanowire…
Classically coherent dynamics analogous to those of quantum two-level systems are studied in the setting of force sensing. We demonstrate quantitative control over the coupling between two orthogonal mechanical modes of a nanowire cantilever, through measurement of avoided crossings as we deterministically position the nanowire inside an electric field. Furthermore, we demonstrate Rabi oscillations between the two mechanical modes in the strong coupling regime. These results give prospects of implementing coherent two-mode control techniques for force sensing signal enhancement.
Strong coupling between electronic and mechanical degrees of freedom is a basic requirement for the operation of any nanoelectromechanical device. In this Review we consider such devices and in particular investigate the properties of small tunnel-junction nanostructures that contain a movable element in the form of a suspended nanowire. In these systems, electrical current and charge can be concentrated to small spatial volumes resulting in strong coupling between the mechanics and the charge transport. As a result, a variety of mesoscopic phenomena appear, which can be used for the transduct…
Low-temperature magnetotransport measurements are performed on GaAs/InSb core-shell nanowires. The nanowires were self-catalyzed grown by molecular beam epitaxy. The conductance measurements as a function of back-gate voltage show an ambipolar behavior comprising an insulating range in between the transition from the p-type to the n-type region. Simulations based on a self-consistent Schrödinger--Poisson solver revealed that the ambipolar characteristics originate from a Fermi level dependent occupation of hole and electron states within the approximately circular quantum well formed in the In…
A highly tunable linear triple quantum dot (TQD) device is realized in a single-crystalline pure-phase InAs nanowire using a local finger gate technique. The electrical measurements show that the charge stability diagram of the TQD can be represented by three kinds of current lines of different slopes and a simulation performed based on a capacitance matrix model confirms the experiment. We show that each current line observable in the charge stability diagram is associated with a case where a QD is on resonance with the Fermi level of the source and drain reservoirs. At a triple point where t…
We report on observation of coherent electron transport in suspended high-quality InAs nanowire-based devices. The InAs nanowires were grown by low-temperature gold-assisted vapor-liquid-solid molecular-beam-epitaxy. The high quality of the nanowires was achieved by removing the typically found stacking-faults and reducing possible Au incorporation. Minimizing substrate-induced scattering in the device was achieved by suspending the nanowires over predefined grooves. Coherent transport involving more than a single one-dimensional mode transport, was observed in the experiment, manifested by Fa…
When a local and attractive potential is quenched in a nanowire, the spectrum changes its topology from a purely continuum to a continuum and discrete portion. We show that, under appropriate conditions, this quench leads to stable coherent oscillations in the observables time evolution. In particular, we demonstrate that ballistic nanowires with spin-orbit coupling (SOC) exposed to a uniform magnetic field are especially suitable to observe this effect. Indeed, while in ordinary nanowires the effect occurs only if the strength $U_0$ of the attractive potential is sufficiently strong, even a w…
We demonstrate evidence of coherent magnetic flux tunneling through superconducting nanowires patterned in a thin highly disordered NbN film. The phenomenon is revealed as a superposition of flux states in a fully metallic superconducting loop with the nanowire acting as an effective tunnel barrier for the magnetic flux, and reproducibly observed in different wires. The flux superposition achieved in the fully metallic NbN rings proves the universality of the phenomenon previously reported for InOx. We perform microwave spectroscopy and study the tunneling amplitude as a function of the wire w…
The coherence of polaritons plays a fundamental role in numerous recent experimental observations, from strong coupling to Bose-Einstein condensation, but accessing the coherence of polaritons is a difficult task due to the high energy of plasmons and excitons. However, surface phonon polaritons offer similar promises at much lower energy. Here, we demonstrate the possibility to use visible ultrafast lasers to generate and time resolve coherent surface phonon polaritons in nanowires. We show that these modes are generated through the ultrafast screening of the surface depletion field. By compa…
Quantum transmission spectra of a twisted electron waveguide expose the coupling between traveling and quasi-bound states. Through a direct numerical solution of the open-boundary Schrödinger equation we single out the effects of the twist and show how the presence of a localized state leads to a Breit-Wigner or a Fano resonance in the transmission. We also find that the energy of quasi-bound states is increased by the twist, in spite of the constant section area along the waveguide. While the mixing of different transmission channels is expected to reduce the conductance, the shift of localiz…
InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowire-based photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps.
The thermal conductance by phonons of a quasi-one-dimensional solid with isotope or defect scattering is studied using the Landauer formalism for thermal transport. The conductance shows a crossover from localized to Ohmic behavior, just as for electrons, but the nature of this crossover is modified by delocalization of phonons at low frequency. A scalable numerical transfer-matrix technique is developed and applied to model quasi-one-dimensional systems in order to confirm simple analytic predictions. We argue that existing thermal conductivity data on semiconductor nanowires, showing an unex…
Physical implementations of large-scale quantum processors based on solid-state platforms benefit from realizations of quantum bits positioned in regular arrays. Self-assembled quantum dots are well-established as promising candidates for quantum optics and quantum information processing, but they are randomly positioned. Site-controlled quantum dots, on the other hand, are grown in pre-defined locations, but have not yet been sufficiently developed to be used as a platform for quantum information processing. In this letter we demonstrate all-optical ultrafast complete coherent control of a qu…
Quantum coherent transport of Dirac fermions in a mesoscopic nanowire of the 3D topological insulator Bi2Se3 is studied in the weak-disorder limit. At very low temperatures, many harmonics are evidenced in the Fourier transform of Aharonov-Bohm oscillations, revealing the long phase-coherence length of surface states. Remarkably, from their exponential temperature dependence, we infer an unusual 1/T power law for the phase coherence length. This decoherence is typical for quasi-ballistic fermions weakly coupled to the dynamics of their environment.
We review a series of works describing thermoelectric effects in gated disordered nanowires (field effect transistor device configuration). After considering the elastic coherent regime characterizing sub-Kelvin temperatures, we study the inelastic activated regime occurring at higher temperatures, where electronic transport is dominated by phonon-assisted hops between localised states (Mott variable range hopping). The thermoelectric effects are studied as a function of the location of the Fermi level inside the nanowire conduction band, notably around its edges where they become very large. …
We experimentally study the electrical transport properties of Ge/Si core/shell nanowire device with two superconducting leads in the Coulomb blockade regime. Anomalous zero field magnetoconductance peaks are observed for the first time at the gate voltages where Coulomb blockade oscillation peaks present. Many evidences indicate this feature is due to Andreev reflection enhanced phase coherent single hole tunneling through the quantum dot, which can be suppressed by an external magnetic field without destroying the superconducting states in the electrodes.
Dispersive sensing is a powerful technique that enables scalable and high-fidelity readout of solid-state quantum bits. In particular, gate-based dispersive sensing has been proposed as the readout mechanism for future topological qubits, which can be measured by single electrons tunneling through zero-energy modes. The development of such a readout requires resolving the coherent charge tunneling amplitude from a quantum dot in a Majorana-zero-mode host system faithfully on short time scales. Here, we demonstrate rapid single-shot detection of a coherent single-electron tunneling amplitude be…
Nanowires with helical surface states represent key prerequisites for observing and exploiting phase-coherent topological conductance phenomena, such as spin-momentum locked quantum transport or topological superconductivity. We demonstrate in a joint experimental and theoretical study that gated nanowires fabricated from high-mobility strained HgTe, known as a bulk topological insulator, indeed preserve the topological nature of the surface states, that moreover extend phase-coherently across the entire wire geometry. The phase-coherence lengths are enhanced up to 5 $μ$m when tuning the wires…
We investigate the quantum transport properties of surface electrons on a topological insulator Bi2Te3 nanowire in a magnetotransport study. Although the nanowires are synthesized by using a relatively coarse method of electrochemical deposition, clear Aharonov-Bohm oscillations of phases 0 and pi are observed, owing to the highly coherent surface electron channel. The oscillation amplitude exhibits exponential temperature dependence, suggesting that the phase coherence length L_phi is inversely proportional to the temperature, as in quasi-ballistic systems. In addition, a weak antilocalizatio…
Engineering of the cross-section shape and size of ultra-scaled Si nanowires (SiNWs) provides an attractive way for tuning their structural properties. The acoustic and optical phonon shifts of the free-standing circular, hexagonal, square and triangular SiNWs are calculated using a Modified Valence Force Field (MVFF) model. The acoustic phonon blue shift (acoustic hardening) and the optical phonon red shift (optical softening) show a strong dependence on the cross-section shape and size of the SiNWs. The triangular SiNWs have the least structural symmetry as revealed by the splitting of the d…
We report phase coherent electron transport in PbTe nanowire networks with a loop geometry. Magneto-conductance shows Aharonov-Bohm (AB) oscillations with periods of $h/e$ and $h/2e$ in flux. The amplitude of $h/2e$ oscillations is enhanced near zero magnetic field, possibly due to interference between time-reversal paths. Temperature dependence of the AB amplitudes suggests a phase coherence length $\sim$ 8 - 12 $μ$m at 50 mK. This length scale is larger than the typical geometry of PbTe-based hybrid semiconductor-superconductor nanowire devices.
We propose a hybrid system composed of a Majorana qubit and a nanomechanical resonator, implemented by a spin-orbit-coupled superconducting nanowire, using a set of static and oscillating ferromagnetic gates. The ferromagnetic gates induce Majorana bound states in the nanowire, which hybridize and constitute a Majorana qubit. Due to the oscillation of one of these gates, the Majorana qubit can be coherently rotated. By tuning the gate voltage to modulate the local spin-orbit coupling, it is possible to reach the resonance of the qubit-oscillator system for relatively strong couplings.
Long, cylindrical metal nanowires have recently been observed to form and be stable for seconds at a time at room temperature. Their stability and structural dynamics is well described by a continuum model, the nanoscale free-electron model, which predicts cylinders in certain intervals of radius to be linearly unstable. In this paper, I study how a small, localized perturbation of such an unstable wire grows exponentially and propagates along the wire with a well-defined front. The front is found to be pulled, and forms a coherent pattern behind it. It is well described by a linear marginal s…
The Josephson supercurrent in a Nb-InAs nanowire-Nb junction was studied experimentally. The nanowire goes superconducting due to the proximity effect, and can sustain a phase coherent supercurrent. An unexpected modulation of the junction critical current in an axial magnetic field is observed, which we attribute to a novel form of Josephson interference, due to the multi-band nature of the nanowire. Andreev pairs occupying states of different orbital angular momentum acquire different superconducting phases, producing oscillations of the critical current versus magnetic flux. We develop a se…
A semiconductor transmon with an epitaxial Al shell fully surrounding an InAs nanowire core is investigated in the low $E_J/E_C$ regime. Little-Parks oscillations as a function of flux along the hybrid wire axis are destructive, creating lobes of reentrant superconductivity separated by a metallic state at a half-quantum of applied flux. In the first lobe, phase winding around the shell can induce topological superconductivity in the core. Coherent qubit operation is observed in both the zeroth and first lobes. Splitting of parity bands by coherent single-electron coupling across the junction …
Electric control of individual atoms or molecules in a solid-state system offers a promising way to bring quantum mechanical functionalities into electronics. This idea has recently come into the reach of the established domain of silicon technology, leading to the realization of single-atom transistors and to the first measurements of electron spin dynamics in single donors. Here we show that we can electrically couple two donors embedded in a multi-gate silicon transistor, and induce coherent oscillations in their charge states by means of microwave signals. We measure single-electron tunnel…
We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by InAs nanowires coupled to superconducting Nb terminals. The use of Si-doped InAs-nanowires with different bulk carrier concentrations allowed to tune the properties of the junctions. We have studied the junction characteristics as a function of temperature, gate voltage, and magnetic field. In junctions with high doping concentrations in the nanowire Josephson supercurrent values up to 100\,nA are found. Owing to the use of Nb as superconductor the Josephson coupling persists at temperatures up to …
Spin torque from spin current applied to a nanoscale region of a ferromagnet can act as negative magnetic damping and thereby excite self-oscillations of its magnetization. In contrast, spin torque uniformly applied to the magnetization of an extended ferromagnetic film does not generate self-oscillatory magnetic dynamics but leads to reduction of the saturation magnetization. Here we report studies of the effect of spin torque on a system of intermediate dimensionality - a ferromagnetic nanowire. We observe coherent self-oscillations of magnetization in a ferromagnetic nanowire serving as the…
Suspended nanowires are shown to provide mechanically-controlled coherent mixing/splitting of the spin states of transmitted electrons, caused by the Rashba spin-orbit interaction. The sensitivity of the latter to mechanical bending makes the wire a tunable nano-electro-mechanical (NEM) weak link between reservoirs. When the reservoirs are populated with misbalanced "spin up/down" electrons, the wire becomes a source of split spin currents, which are not associated with electric charge transfer and which do not depend on temperature or driving voltages. The mechanical vibrations of the bended …
We investigate the properties of a weak link between two Rashba-based superconducting nanowires with geometric misalignment. By applying an external magnetic field the system can be driven into a topological non-trivial regime. We demonstrate that the Josephson current can be modulated in amplitude and sign through the variation of the applied field and, remarkably, via the angle controlling the spin-orbit locking mismatch at the interface of the nanowires. The proposed setup with misaligned coplanar nanowires provides the building block configuration for the manipulation of coherent transport…
Strong spin-orbit semiconductor nanowires coupled to a superconductor are predicted to host Majorana zero modes. Exchange (braiding) operations of Majorana modes form the logical gates of a topological quantum computer and require a network of nanowires. Here, we develop an in-plane selective-area growth technique for InSb-Al semiconductor-superconductor nanowire networks with excellent quantum transport properties. Defect-free transport channels in InSb nanowire networks are realized on insulating, but heavily mismatched InP substrates by 1) full relaxation of the lattice mismatch at the nano…
The point-like nature and exquisite magnetic field sensitivity of the nitrogen vacancy (NV) center in diamond can provide information about the inner workings of magnetic nanocircuits in complement with traditional transport techniques. Here we use a single NV in bulk diamond to probe the stray field of a ferromagnetic nanowire controlled by spin transfer (ST) torques. We first report an unambiguous measurement of ST tuned, parametrically driven, large-amplitude magnetic oscillations. At the same time, we demonstrate that such magnetic oscillations alone can directly drive NV spin transitions,…
We present a circuit model to describe the electron transport through a domain wall in a ferromagnetic nanowire. The domain wall is treated as a coherent 4-terminal device with incoming and outgoing channels of spin up and down and the spin-dependent scattering in the vicinity of the wall is modelled using classical resistances. We derive the conductance of the circuit in terms of general conductance parameters for a domain wall. We then calculate these conductance parameters for the case of ballistic transport through the domain wall, and obtain a simple formula for the domain wall magnetores…
We study electron transport through a domain wall in a ferromagnetic nanowire subject to spin-dependent scattering. A scattering matrix formalism is developed to address both coherent and incoherent transport properties. The coherent case corresponds to elastic scattering by static defects, which is dominant at low temperatures, while the incoherent case provides a phenomenological description of the inelastic scattering present in real physical systems at room temperature. It is found that disorder scattering increases the amount of spin-mixing of transmitted electrons, reducing the adiabatic…
We theoretically study the electronic transport of a nanowire partly irradiated under an external terahertz (THz) electromagnetic field. Although the electrons in the ballistic nanowires only suffer lateral collision with photons the reflection of electrons also takes place in this partly irradiated case. Using free-electron model and scattering matrix approach we showed that at resonance there exists a step decrement of 50 percent for the transmission probability as the amplitude of field increases to a certain volume. And the coherent structure of transmission for the system apparently appea…
We propose an all-electrically controlled nanodevice - a gated semiconductor nanowire - capable of generating a coherent state of a single electron trapped in a harmonic oscillator or superposition of such coherent states - the Schrödinger cat state. In the proposed scheme, electron in the ground state of the harmonic potential is driven by resonantly oscillating Rashba spin-orbit coupling. This allows for the creation of the Schrödinger cat state with superposition amplitudes depending on the initial electron spin state. Such a method can be used for initialization of a single spin qubit defi…
Systematic electrical transport characterizations were performed on high-quality Bi2O2Se nanowires to illustrate its great transport properties and further application potentials in spintronics. Bi2O2Se nanowires synthesized by chemical vapor deposition method presented a high field-effect mobility up to 1.34*104 cm2V-1s-1, and exhibited ballistic transport in the low back-gate voltage (Vg) regime where conductance plateaus were observed. When further increasing the electron density by increasing Vg, we entered the phase coherent regime and weak antilocalization (WAL) was observed. The spin re…
We calculate the conductance of quasi-one-dimensional nanowires with electronic states confined to a surface charge layer, in the presence of a uniform magnetic field. Two-terminal magnetoconductance (MC) between two leads deposited on the nanowire via tunnel barriers is dominated by density-of-states (DOS) singularities, when the leads are well apart. There is also a mesoscopic correction due to a higher-order coherent tunneling between the leads for small lead separation. The corresponding MC structure depends on the interference between electron propagation via different channels connecting…
We investigate proximity induced superconductivity in micrometer-long bismuth nanowires con- nected to superconducting electrodes with a high critical field. At low temperature we measure a supercurrent that persists in magnetic fields as high as the critical field of the electrodes (above 11 T). The critical current is also strongly modulated by the magnetic field. In certain samples we find regular, rapid SQUID-like periodic oscillations occurring up to high fields. Other samples ex- hibit less periodic but full modulations of the critical current on Tesla field scales, with field-caused ext…
Quasi-one-dimensional superconductors or nanowires exhibit a transition into a nonsuperconducting regime, as their diameter shrinks. We present measurements on ultrashort nanowires (~40-190 nm long) in the vicinity of this quantum transition. Properties of all wires in the superconducting phase, even those close to the transition, can be explained in terms of thermally activated phase slips. The behavior of nanowires in the nonsuperconducting phase agrees with the theories of the Coulomb blockade of coherent transport through mesoscopic normal metal conductors. Thus it is concluded that the qu…
We investigate transmon qubits made from semiconductor nanowires with a fully surrounding superconducting shell. In the regime of reentrant superconductivity associated with the destructive Little-Parks effect, numerous coherent transitions are observed in the first reentrant lobe, where the shell carries 2π winding of superconducting phase, and are absent in the zeroth lobe. As junction density was increased by gate voltage, qubit coherence was suppressed then lost in the first lobe. These observations and numerical simulations highlight the role of winding-induced Andreev states in the junct…
