Single-electron Dynamics Research Articles

Using acoustic waves to induce high-frequency current oscillations in superlattices

We show that GHz acoustic waves in semiconductor superlattices can induce THz electron dynamics that depend critically on the wave amplitude. Below a threshold amplitude, the acoustic wave drags electrons through the superlattice with a peak drift velocity overshooting that produced by a static electric field. In this regime, single electrons perform drifting orbits with THz frequency components. When the wave amplitude exceeds the critical threshold, an abrupt onset of Bloch-like oscillations causes negative differential velocity. The acoustic wave also affects the collective behavior of the electrons by causing the formation of localised electron accumulation and depletion regions, which propagate through the superlattice, thereby producing self-sustained current oscillations even for very small wave amplitudes. We show that the underlying single-electron dynamics, in particular the transition between the acoustic wave dragging and Bloch oscillation regimes, strongly influence the spatial distribution of the electrons and the form of the current oscillations. In particular, the amplitude of the current oscillations depends non-monotonically on the strength of the acoustic wave, reflecting the variation of the single-electron drift velocity.

Coherence of single spins coupled to a nuclear spin bath of varying density

The dynamics of single electron and nuclear spins in a diamond lattice with different $^{13}\text{C}$ nuclear spin concentration is investigated. It is shown that coherent control of up to three individual nuclei in a dense nuclear spin cluster is feasible. The free-induction decays of nuclear spin Bell states and single nuclear coherences among $^{13}\text{C}$ nuclear spins are compared and analyzed. Reduction in a free-induction-decay time ${T}_{2}^{\ensuremath{\ast}}$ and a coherence time ${T}_{2}$ upon increase in nuclear spin concentration has been found. For pure diamond, ${T}_{2}^{\ensuremath{\ast}}$ as long as $30\text{ }\ensuremath{\mu}\text{s}$ and ${T}_{2}$ of up to 0.65 ms for the electron spin has been observed. The $^{13}\text{C}$ concentration dependence of ${T}_{2}^{\ensuremath{\ast}}$ is explained by Fermi contact and dipolar interactions with nuclei in the lattice. It has been found that ${T}_{2}$ decreases approximately as $1/n$, where $n$ is $^{13}\text{C}$ concentration, which corresponds to the reported theoretical line of ${T}_{2}$ for an electron spin interacting with a nuclear spin bath.

Coherent Time Evolution of a Single-Electron Wave Function

Observation of coherent single-electron dynamics is severely limited by experimental bandwidth. We present a method to overcome this using moving quantum dots defined by surface acoustic waves. Each dot holds a single electron, and travels through a static potential landscape. When the dot passes abruptly between regions of different confinement, the electron is excited into a superposition of states, and oscillates unitarily from side to side. We detect these oscillations by using a weak, repeated measurement of the current across a tunnel barrier, and find close agreement with simulations.

Resonant optical electron transfer in one-dimensional multiwell structures

We consider coherent single-electron dynamics in one-dimensional nanostructures under aresonant electromagnetic pulse. The structure is composed of two deep quantumwells positioned at the edges of the structure and separated by a sequence ofshallow internal wells. We show that complete electron transfer between the stateslocalized in the edge wells through one of the excited delocalized states can takeplace at a discrete set of times provided that the pulse frequency matches one ofresonant transition frequencies. The transfer time varies from several tens to severalhundreds of picoseconds and depends on the structure and pulse parameters. Theresults obtained in this paper can be applied to the development of quantumnetworks used in quantum communications and/or quantum information processing.

Investigation of single-electron dynamics in tunnelling between zero- and one-dimensional states

We present an experimental technique for investigating the dynamics of single-electron tunnelling on a sub-nanosecond timescale without using fast gate pulses. We use surface acoustic waves (SAWs) to form dynamic quantum dots in a depleted one-dimensional channel. Each dynamic dot carries a single electron at SAW velocity ∼ 2800 m / s . These dynamic dots are coupled to an open channel through a tunnel barrier for a length of ∼ 1 μ m . This allows each electron to tunnel out for a duration of ∼ 350 ps . This fixed tunnel duration allows us to examine the dynamic nature of electron tunnelling without the need for gate pulses. Oscillations with ∼ 1 % visibility are observed in the tunnel current as a function of gate voltage. We argue that these oscillations cannot be explained in the adiabatic limit, and that dynamic processes must be considered.

Quantum effects in small-capacitance high temperature superconducting tunneling junctions

We investigated the effects of single electron charging energy in high temperature superconductors. Various phenomena originating from Coulomb blockade were observable in superconducting tunnel junctions. High quality tunneling junctions were fabricated from c-axis oriented NdBa2Cu3O7−δ∕PrBa2Cu3O7−δ∕NdBa2Cu3O7−δ thin film multilayers by the pulsed laser deposition method. The current-voltage characteristics (CVCs) show a Coulomb gap for Cooper pair tunneling when the charging energy exceeds the Josephson coupling energy. We found a regime in which the CVCs exhibit sharply defined Coulomb steps due to single electron dynamics and nonlinear tunneling rates. From the obtained Coulomb staircase, the tunneling resistance shows a quantum effect: It is modulated by the tunneling current in the form h∕4e2RT∼[sin(πI∕I0)2∕(πI∕I0)]. We suggest an interpretation involving the quantum resistance h∕e2 and the competition between the charging, Josephson, and thermal energies of the system. Our results give a perspective on a solid-state quantum system with considerable interest for direct application in quantum computing.

Single-electron dynamics in a tightly focused laser beat wave: acceleration in vacuum

The fields of two linearly-polarized, co-propagated laser beams differing slightly in frequency are modelled by those of a tightly focused Gaussian beam. When a single electron is injected sideways into or near the common focus of the two beams, it gets captured and violently accelerated, reaching a maximum energy gain in excess of 2 GeV from beams of present-day power. Our simulations show that acceleration takes place typically over a fraction of a millimeter and that a maximum energy gradient of over 12 TeV m−1 may be achieved.

Collective and single-particle dynamics in time-resolved two-photon photoemission

A general model for time-resolved two-photon photoemission from solids is presented comprising both collective and single-electron dynamics. In combination with interferometric time-resolved two-photon photoemission, this allows one to determine the shape of the collective response function across the excitation spectrum and the single-particle lifetime of excited electrons. Ag nanoparticles supported on graphite exhibit a strong collective resonance and serve as model system to demonstrate experimentally this separation of collective and single-particle dynamics in two-photon photoemission.

Investigation on Single-Electron Dynamics in Coupled GaAs–AlGaAs Quantum Wires

The aim of this paper is the study of the single-electron coherent propagation in a quantum-computing gate made of coupled quantum wires. The structure under investigation is based on a two-dimensional (2-D) electron gas realized in a modulation-doped GaAs-AlGaAs heterostructure. A number of surface electrodes are used to form one-dimensional channels. The profile of the conduction band at the heterojunction has been computed numerically by solving the three-dimensional Poisson equation on the whole structure at 300 mK. Finally, a single-electron wavefunction is propagated within the so-formed quantum wire geometry by means of a 2-D, time-dependent Schro/spl uml/dinger solver. Results are shown for a single-qubit rotation gate implementing a quantum-NOT transformation. This work is part of a feasibility study on a solid-state realization of a universal set of quantum gates.

Efficient acceleration of electrons with counterpropagating intense laser pulses in vacuum and underdense plasma.

We propose that efficient acceleration of electrons in vacuum and underdense plasmas by an intense laser pulse can be triggered in the presence of another counterpropagating or intersecting laser pulse. This mechanism works when the laser fields exceed some threshold amplitudes for stochastic motion of electrons, as found in single-electron dynamics. Particle-in-cell simulations confirm that electron heating and acceleration in the case with two counterpropagating laser pulses can be much more efficient than with one laser pulse only. Two different diagnoses show that the increased heating and acceleration are caused mainly by direct laser acceleration rather than by plasma waves. In plasma at moderate densities such as a few percent of the critical density and when the underdense plasma region is large enough, the Raman backscattered and side-scattered waves can grow to a sufficiently high level to serve as the second counterpropagating or intersecting pulse and trigger the electron stochastic motion. As a result, even with a single intense laser pulse only in plasma, electrons can be accelerated to an energy level much higher than the corresponding laser ponderomotive potential.

Electrical pulse measurement and single-electron dynamics in quantum dots

Electrical pulse measurement and single-electron dynamics in quantum dots

Stochastic heating and acceleration of electrons in colliding laser fields in plasma.

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.

Chaos and interactions in quantum dots

Quantum dots are small conducting devices containing up to several thousand electrons. We focus here on closed dots whose single-electron dynamics are mostly chaotic. The mesoscopic fluctuations of the conduction properties of such dots reveal the effects of one-body chaos, quantum coherence and electron-electron interactions.

Scanning-tunneling-microscope observations of Coulomb blockade and oxide polarization in small metal droplets.

The I-V characteristics of two serially coupled small tunnel junctions (about ${10}^{\mathrm{\ensuremath{-}}18}$--${10}^{\mathrm{\ensuremath{-}}19}$ F capacitances) are measured at 4 K. The junctions are formed using a scanning-tunneling microscope to probe a metal droplet deposited on an oxidized metal substrate. Sharply defined Coulumb steps due to single-electron dynamics, oxide polarization, and nonlinear (voltage dependent) tunneling rates are observed. The results show very good quantitative agreement with theoretical calculations based on the semiclassical picture.