Transporting and manipulating single electrons in surface-acoustic-wave minima

A surface acoustic wave (SAW) can produce a moving potential wave that can trap and drag electrons along with it. We review work on using a SAW to create moving quantum dots containing single electrons, with the aims of developing a current standard, emitting single photons, transferring single electrons between static quantum dots, and investigating non-adiabatic effects

Possible evidence of a spontaneous spin polarization in mesoscopic two-dimensional electron systems

We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero B, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at B=0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems

Kondo effect from a tunable bound state within a quantum wire

We investigate the conductance of quantum wires with a variable open quantum dot geometry, displaying an exceptionally strong Kondo effect and most of the 0.7 structure characteristics. Our results indicate that the 0.7 structure is not a manifestation of the singlet Kondo effect. However, specific similarities between our devices and many of the clean quantum wires reported in the literature suggest a weakly bound state is often present in real quantum wires

Effects of a piezoelectric substrate on phonon-drag thermopower in monolayer graphene

The phonon-drag thermopower is studied in a monolayer graphene on a piezoelectric substrate. The phonon-drag contribution [Formula: see text] from the extrinsic potential of piezoelectric surface acoustic (PA) phonons of a piezoelectric substrate (GaAs) is calculated as a function of temperature T and electron concentration n s. At a very low temperature, [Formula: see text] is found to be much greater than [Formula: see text] of the intrinsic deformation potential of acoustic (DA) phonons of the graphene. There is a crossover of [Formula: see text] and [Formula: see text] at around ~5 K. In graphene samples of about  >10 µm size, we predict S (g) ~ 20 µV at 10 K, which is much greater than the diffusion component of the thermopower and can be experimentally observed. In the Bloch-Gruneisen (BG) regime T and n s dependence are, respectively, given by the power laws [Formula: see text] ([Formula: see text]) ~ T (2)(T (3)) and [Formula: see text], [Formula: see text] ~ [Formula: see text]. The T(n s) dependence is the manifestation of the 2D phonons (Dirac phase of the electrons). The effect of the screening is discussed. Analogous to Herring's law (S (g) μ p ~ T (-1)), we predict a new relation S (g) μ p ~ [Formula: see text], where μ p is the phonon-limited mobility. We suggest that the n s dependent measurements will play a more significant role in identifying the Dirac phase and the effect of screening.Private Fellowship

Zero-bias anomaly and kondo-assisted quasiballistic 2D transport

Nonequilibrium transport measurements in mesoscopic quasiballistic 2D electron systems show an enhancement in the differential conductance around the Fermi energy. At very low temperatures, such a zero-bias anomaly splits, leading to a suppression of linear transport at low energies. We also observed a scaling of the nonequilibrium characteristics at low energies which resembles electron scattering by two-state systems, addressed in the framework of two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an intrinsic phenomenon in unconfined 2D systems in the low electron-density regime

Microscopic metallic air-bridge arrays for connecting quantum devices

We present a single-exposure fabrication technique for a very large array of microscopic air-bridges using a tri-layer resist process with electron-beam lithography. The technique is capable of forming air-bridges with strong metal-metal or metal-substrate connections. This was demonstrated by its application in an electron tunneling device consisting of 400 identical surface gates for defining quantum wires, where the air-bridges are used as suspended connections for the surface gates. This technique enables us to create a large array of uniform one-dimensional channels that are open at both ends. In this article, we outline the details of the fabrication process, together with a study and the solution of the challenges present in the development of the technique, which includes the use of water-IPA (isopropyl alcohol) developer, calibration of the resist thickness, and numerical simulation of the development.</jats:p

Energy-dependent tunneling from few-electron dynamic quantum dots

We measure the electron escape rate from surface-acoustic-wave dynamic quantum dots (QDs) through a tunnel barrier. Rate equations are used to extract the tunneling rates, which change by an order of magnitude with tunnel-barrier-gate voltage. We find that the tunneling rates depend on the number of electrons in each dynamic QD because of Coulomb energy. By comparing this dependence to a saddle-point-potential model, the addition energies of the second and third electron in each dynamic QD are estimated. The scale (similar to a few meV) is comparable to those in static QDs as expected

Nonlinear spectra of spinons and holons in short GaAs quantum wires.

One-dimensional electronic fluids are peculiar conducting systems, where the fundamental role of interactions leads to exotic, emergent phenomena, such as spin-charge (spinon-holon) separation. The distinct low-energy properties of these 1D metals are successfully described within the theory of linear Luttinger liquids, but the challenging task of describing their high-energy nonlinear properties has long remained elusive. Recently, novel theoretical approaches accounting for nonlinearity have been developed, yet the rich phenomenology that they predict remains barely explored experimentally. Here, we probe the nonlinear spectral characteristics of short GaAs quantum wires by tunnelling spectroscopy, using an advanced device consisting of 6000 wires. We find evidence for the existence of an inverted (spinon) shadow band in the main region of the particle sector, one of the central predictions of the new nonlinear theories. A (holon) band with reduced effective mass is clearly visible in the particle sector at high energies.This work was supported by the UK EPSRC [Grant Nos. EP/J01690X/1 and EP/J016888/1].This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/NCOMMS12784

Single-electron population and depopulation of an isolated quantum dot using a surface-acoustic-wave pulse

We use a pulse of surface acoustic waves (SAWs) to control the electron population and depopulation of a quantum dot. The barriers between the dot and reservoirs are set high to isolate the dot. Within a time scale of similar to 100 s the dot can be set to a nonequilibrium charge state, where an empty (occupied) level stays below (above) the Fermi energy. A pulse containing a fixed number of SAW periods is sent through the dot, controllably changing the potential, and hence the tunneling probability, to add (remove) an electron to (from) the dot

Experimental verification of electrostatic boundary conditions in gate-patterned quantum devices

In a model of a gate-patterned quantum device it is important to choose the correct electrostatic boundary conditions (BCs) in order to match experiment. In this study, we model gated-patterned devices in doped and undoped GaAs heterostructures for a variety of BCs. The best match is obtained for an unconstrained surface between the gates, with a dielectric region above it and a frozen layer of surface charge, together with a very deep back boundary. Experimentally, we find a 0.2V offset in pinch-off characteristics of one-dimensional channels in a doped heterostructure before and after etching off a ZnO overlayer, as predicted by the model. Also, we observe a clear quantised current driven by a surface acoustic wave through a lateral induced n-i-n junction in an undoped heterostructure. In the model, the ability to pump electrons in this type of device is highly sensitive to the back BC. Using the improved boundary conditions, it is straightforward to model quantum devices quite accurately using standard software