R. Newbury

Reversible quantum brownian heat engines for electrons.

Brownian heat engines use local temperature gradients in asymmetric potentials to move particles against an external force. The energy efficiency of such machines is generally limited by irreversible heat flow carried by particles that make contact with different heat baths. Here we show that, by using a suitably chosen energy filter, electrons can be transferred reversibly between reservoirs that have different temperatures and electrochemical potentials. We apply this result to propose heat engines based on mesoscopic semiconductor ratchets, which can quasistatically operate arbitrarily close to Carnot efficiency.

Patterning of conducting polymer nanowires on gold/platinum electrodes

We demonstrate a technique for the patterning of conducting polyaniline nanowires on metallic substrates. Electrochemical synthesis of template-free polyaniline nanowires is initially preceded by the formation of a thin compact 2D layer, which we show spreads horizontally from the edge of a gold electrode across a SiO2 surface. This encumbers nanowire patterning during the fabrication of nanowire devices since it leads to electrical short-circuiting of the metal contacts before the nanowires have fully developed. In this paper we demonstrate the use of amine-terminated self-assembled monolayers, 3-aminopropyltriethoxysilane, to inhibit the spread of polyaniline growth onto adjacent SiO2 surfaces. The technique has important applications in the fabrication of nanowire devices and area-selective patterning of nanowires on metallic electrodes.

Geometry-induced fractal behaviour in a semiconductor billiard

We report geometry-induced fractal behaviour in the low-field magneto-conductance fluctuations of a mesoscopic semiconductor billiard. Such fractal behaviour was recently predicted to be induced by the mixed (chaotic/regular) phase space generated by the soft-walled billiard potential, and our results constitute a possible experimental observation of the infinite hierarchical nature of this mixed phase space. Preliminary investigations of the effects of temperature and gate bias, which directly control the electron coherence and billiard potential profile, are presented.

Pumping heat with quantum ratchets

Abstract We describe how adiabatically rocked quantum electron ratchets can act as heat pumps. In general, ratchets may be described as non-equilibrium systems in which directed particle motion is generated using spatial or temporal asymmetry. In a rocked ratchet, which may also be described as a non-linear rectifier, an asymmetric potential is tilted symmetrically and periodically. The potential deforms differently during each half-cycle, producing a net current of particles when averaged over a full period of rocking. Recently, it was found that in the quantum regime, where tunnelling contributes to transport, the net current may change sign with temperature. Here we show that a Landauer model of an experimental tunnelling ratchet (Linke et al., Science 286 (1999) 2314) predicts the existence of a net heat current even when the net particle current goes through zero. We quantify this heat current and define a coefficient of performance for the ratchet as a heat pump, finding that more heat is deposited in each of the two electron reservoirs due to the process of rocking than is pumped from one reservoir to the other by the ratchet.

Giant magnetoresistance and possible miniband effects in periodic magnetic fields

We have investigated the magnetoresistance (MR) of high mobility, near surface two-dimensional electron gases that are subject to a periodic magnetic field of amplitude up to 0.3 T. The fields are produced by sub-micron arrays of ferromagnetic stripes on the surface of the heterostructures. For small applied external magnetic fields we observe an MR of up to 1500% in applied fields of ∼100 mT. At low temperatures we observe both commensurability and Shubnikov-de Haas oscillations in the same field range. The SdH oscillations are strongly modulated by the 1D periodic magnetic potential. The observed behaviour is not consistent with semiclassical models. A new type of MR is observed on increasing the amplitude of the modulation. In this case, the MR amplitude increases as T2. This provides evidence of an electron–electron scattering contribution to the resistivity possibly due to the presence of minibands.

Quantum transport in open mesoscopic cavities

Abstract In this review we describe the results of magneto-transport studies in open quantum dots, in which electronic motion is expected to be predominantly ballistic in nature. The devices themselves are realized in different semiconductor materials, using quite distinct fabrication techniques. Electron interference is an important process in determining the electrical properties of the devices at low temperatures and is manifested through the observation of periodic magneto-conductance fluctuations. These are found to result from selective excitation of discrete cavity eigenstates by incoming electrons, which are directed into a collimated beam by the input point contact. Under conditions of such restricted injection, quantum mechanical simulations reveal highly characteristic wavefunction scarring, associated with the remnants of a few classical orbits. The scarring is built up by interference between electrons, confined within the cavities over very long time scales, suggesting the underlying orbits are highly stable in nature. This characteristic is also confirmed by the results of experiment, which reveal the discrete components dominating the interference to be insensitive to changes in lead opening or temperature. The fluctuations decay with increasing temperature, although they can nonetheless still be resolved at a few degrees kelvin. This characteristic is confirmed by independent studies of devices, fabricated using very different techniques, further demonstrating the universal nature of the behavior we discuss here. These results therefore demonstrate that the correct description of electron interference in open quantum cavities, is one in which only a few discrete orbits are excited by the collimating action of the input lead, giving rise to striking wavefunction scarring with measurable magneto-transport results.

A low-temperature insulating phase at for 2D holes in high-mobility heterostructures with Landau level degeneracy

Magneto-transport measurements of the 2D hole system (2DHS) in p-type Si-Si1-xGex heterostructures identify the integer quantum Hall effect (IQHE) at dominantly odd-integer filling factors v and two low-temperature insulating phases (IPs) at v = 1.5 and v less than or similar to 0.5, with re-entrance to the quantum Hall effect at v = 1. The temperature dependence, current-voltage characteristics, and tilted field and illumination responses of the IP at v = 1.5 indicate that the important physics is associated with an energy degeneracy of adjacent Landau levels of opposite spin, which provides a basis for consideration of an intrinsic, many-body origin.

Electronic transport in conducting polymer nanowire array devices.

We report on the temperature dependent conductivity and current-voltage (I-V) properties of novel polyaniline nanowire array devices. Below 60 K, I-V measurements show a transition to non-linear behaviour, leading to the onset at 30 K of a threshold voltage, for potentials below which little current flows. By considering an intrinsic morphology of small conducting regions separated by tunnel junctions, we show that charging of the conducting regions leads to Coulomb blockade effects that can account for this behaviour.

Dependence of fractal conductance fluctuations on soft-wall profile in a double-layer semiconductor billiard

We present a semiconductor system featuring two billiards located one on top of the other. We use this system to study the dependence of fractal conductance fluctuations on soft-wall potential profile and show the fluctuations to be surprisingly robust to changes in profile.

Self-similar conductance fluctuations in a Sinai billiard with a mixed chaotic phase space

Abstract We have investigated ballistic transport through a sub-micron-sized Sinai billiard fabricated using a high-mobility two-dimensional electron gas. The geometry of the billiard is defined by a circular surface gate surrounded by an outer square gate with two quantum point contacts. When both gates are biased negatively, the Sinai billiard is formed and collisions with the confining potential walls generate chaotic classical motion. Recent experiments have revealed novel ‘self-similar’ fluctuation patterns in the conductance of the Sinai billiard, measured as a function of magnetic field perpendicular to the plane of the electron gas. We have undertaken quantum mechanical calculations of the device conductance using a detailed model for the potential landscape under the surface gates. We compare these results with predictions for a hard-walled billiard and show that the surface gate geometry can have a large effect on the observed transport properties.