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We study the magnetic field dependence of the correlation field ΔB c and amplitude δg of the conductance fluctuations, observed in the low-temperature magnetoresistance of a quasi-ballistic split-gate wire. With increasing the fields, there exists a crossover from diffusive to ballistic motions in the quasi-ballistic transport. Scattering processes have been discussed by analyzing a linear field dependence of ΔB c above the crossover. The crossover behaviors are considered to be a precursor effect on the formation of the edge channel accompanied with a rapid increase of inelastic scattering length l in .

We demonstrate that low-temperature illumination of a GaAs/AlGaAs Hall bar, partially covered by a split metal gate structure, results in the formation of a shallow barrier across the entire device, even in the point contact openings not directly covered by the gates. The minimum carrier density inhomogeneity is determined from the position of the anomalous Hall steps, which are observed even at zero gate voltage as a result of barrier formation. Our results suggest a substantial contribution to the carrier density inhomogeneity, implying that low-temperature illumination may considerably increase disorder in split gate nanostructures.

We use a novel technique to determine the phase breaking time τ Φ of electrons in ballistic quantum dots, from the aperiodic fluctuations observed in their low-temperature magnetoconductance. At temperatures below 100 mK, roughly similar values of τ Φ are obtained in both sizes of dot studied, suggesting a strong materials-related influence on τ Φ . Analysing the temperature-dependent characteristics of the fluctuations, τ Φ is found to scale roughly inversely with temperature close to 1 K, reminiscent of electron-electron scattering in two-dimensional disordered systems. At much lower temperatures, however, a saturation in τ Φ is observed, and is thought to result from a transition from two- to zero-dimensional transport, which occurs as the discrete level structure of the dot becomes resolved.

We study the magnetic field dependence of the correlation field ΔBcand amplitude δgof the conductance fluctuations, observed in the low temperature magnetoresistance of GaAs/AlGaAs split-gate wires. Near zero field, universality of quantum interference is retained and the magnetoresistance shows universal conductance fluctuations. At high magnetic fields, although the discrete Landau level quantization becomes resolved. ΔBcand δgare found to increase linearly with magnetic field, with a slope which depends upon the nature of electron scatterings in the wire.

We report a clear and sudden period doubling in the Aharonov-Bohm oscillations (ABO), observed in the low-temperature magneto-resistance of a small quantum dot. The doubling occurs over a well-defined range of magnetic field, outside of which the ABO revert back to their single period. The precise nature of the doubling is reflected in the Fourier spectra of the oscillations, and suggests an interpretation in terms of the single-particle energy-level spectrum of the dot.

We study carrier scattering processes in GaAs/AlGaAs quantum wires by a correlation field analysis of the conductance fluctuations observed in their low temperature magnetoresistance. In the diffusive transport regime, the magneto-resistance at low magnetic fields shows universal conductance fluctuations (UCF) and the correlation field Bc is field independent. However, at higher fields, where the Landau level quantization is resolved, Bc is found to increase as a power law of the magnetic field. We compare our results with recent theoretical predictions for the diffusive nature and also with our own semi-classical explanations for the dynamical transport.

The temperature-dependent resistivity of highly disordered Pt/C-Ga composite nanowires is shown to be well described by the interference of electron–phonon scattering and elastic electron scattering from boundaries and defects. The strongly disordered nature of these wires, combined with a high value of their Debye temperature, are responsible for the pronounced nature of the interference effects in their resistivity.

In this paper, we describe our ongoing research on CMOS-compatible semiconductor nanosensors for broadband terahertz (THz) detection. We review the results of earlier work, which reveal the promise of THz rectification by nanoconstrictions, and present proof-of-concept results showing efficient THz detection at room temperature.

Field effect transistors (FETs) whose channel is composed of a network of single wall carbon nanotubes (SWNTs) have been studied to investigate the mechanism of the device operation via scanning gate microscopy (SGM) at room temperature. We observed different SGM response in networks of SWNTs either synthesized by CoMoCAT process or semiconducting enriched by density gradient ultracentrifuge process. In the former case, SGM response was observed at specific inter-tube junctions suggesting a Schottky junction formed with semiconducting and metallic SWNTs in the network. In contrast, multiple concentric rings in the SGM response are observed within the tubes in a network of the latter SWNTs suggesting a possibility of quantum mechanical transport at room-temperature. Different type of SGM responses are confirmed in the two kinds of SWNTs networks, nevertheless such active positions would likely have an important role in the FET operation mechanism in each network.

We demonstrate the generation of zero-current voltage fluctuations in quantum dots at high magnetic fields, where one or more edge states (ES) are trapped within the dot. While currently unable to provide a definitive model for the generation of the noise, we propose a possible mechanism in which time-dependent perturbations to the dot geometry, give rise to corresponding fluctuations in the coupling between the transmitted and confined ES. In other words, the voltage noise is thought to be a direct probe, of the time-dependent motion of the electronic distribution within the dot.

We discuss the application of a novel class of device, the magneto-electric magnetic tunnel junction (ME-MTJ) to realize a variety of computational functions, including majority logic and the XNOR/XOR gate. We also develop a compact model to describe the operation of these devices, which function by utilizing the phenomenon of 'voltage-controlled magnetism' to switch the operational state of MTJs. The model breaks down the switching process into three key stages of operation: electrical-to-magnetic conversion, magnetization transfer, and final-state readout. Estimates for the switching energy and delay of these devices, obtained from this compact model, reveal significant improvements in both of these parameters when compared to conventional MTJs switched by spin-transfer-torque. In fact, the capacity to use the ME-MTJ to implement complex logical operations within a single device allows its energy costs to even approach those of low-power CMOS. The added benefits of non-volatility and compact circuit footprint, combined with their potential for heterogeneous integration with CMOS, make the ME devices of considerable interest for post-CMOS technology.

We report precise measurements of the specific heat of seven samples of superconducting La2-xSrxCuO4 (0.05 ≤ x ≤ 0.27) between 0.09 K and 1.5 K. When phonon and nuclear terms are subtracted, the remaining, electronic, specific heat has the form αTn, where n < 1 for all samples and shows a smooth correlation with Tc, having a minimum value in the optimally doped sample, x = 0.15. We discuss the possibility that this unconventional form of the electronic specific heat arises from localisation of the quasi-particles at low temperatures.

We have fabricated Pt/C composite nanowires and nanowire transistors, using the technique of electron-beam-induced deposition. The current-voltage characteristics of the granular nanowires are strongly nonlinear at 4.2 K, and evidence for this nonlinearity is found to persist to room temperature. A voltage gap of order 0.1–0.2 V is observed at the lowest temperatures, and we suggest that this feature is consistent with single-electron tunneling via Pt nanocrystals that form in the wires during their fabrication. In order to further explore this possibility, we incorporate the nanowires into three-terminal transistor structures and find evidence for a gate-induced modulation of their voltage gap.

We describe how a spin filter may be realized in open quantum-dot systems, by exploiting the Fano resonances that occur in their transmission characteristics. In quantum dots in which the spin degeneracy of carriers is lifted, we show that the Fano resonances may be used as an effective means to generate spin polarization of transmitted carriers, and that electrical detection of the resulting polarization should also be possible.

We study the transport in a system of coupled quantum wires and show evidence for a resonant interaction that occurs whenever one of them is biased close to pinch off. Measuring the conductance of one of the wires, as the width of the other is varied, we observe a resonant peak in the conductance that is correlated to the point at which the other wire pinches off. The origin of this interaction remains undetermined at present, although its characteristics appear consistent with predictions that a correlated many-body state should form in narrow wires as their conductance vanishes.

We study the electrical properties of Pt nanointerconnects, formed on SiO2 substrates by focused-ion-beam deposition. Studies of their temperature-dependent resistivity reveal a small residual-resistivity ratio, and a Debye temperature that differs significantly from that of pure Pt, indicative of the disordered nature of the nanowires. Their magnetoresistance shows evidence for weak antilocalization at temperatures below 10 K, with a phase-breaking length of ∼100 nm, and a temperature dependence suggestive of quasi-one-dimensional interference.

Magneto-transport measurements are performed on two-dimensional GaAs electron systems to probe the quantum Hall (QH) effect at low magnetic fields. Oscillations following the Shubnikov-de Haas (SdH) formula are observed in the transition from the insulator to QH liquid when the observed almost temperature-independent Hall slope indicates insignificant interaction correction. Our study shows that the existence of SdH oscillations in such a transition can be understood based on the non-interacting model., 17 pages

Open quantum dots provide a natural system in which to study both classical and quantum features of transport. From the classical point of view these dots possess a mixed phase space which yields families of closed, regular orbits as well as an expansive sea of chaos. An important question concerns the manner in which these classical states evolve into the set of quantum states that populate the dot in the quantum limit. In the reverse direction, the manner in which the quantum states evolve to the classical world is governed strongly by Zurek?s decoherence theory. This was discussed from the quantum perspective in an earlier review?(Ferry et?al 2011 Semicond. Sci. Technol. 26 043001). Here, we discuss the nature of the various classical states, how they are formed, how they progress to the quantum world, and the signatures that they create in magnetotransport and general conductance studies of these dots.

We demonstrate a fully tunable realization of a multistate Fano resonance, in which a pair of remote quantum states experience an effective coupling due to their mutual overlap with a continuum. Our mesoscopic implementation of this system exploits the ability of the semiconductor nanostructures known as quantum point contacts (QPCs) to serve, in the low-density limit close to pinch-off, as an on-demand localized state. By coupling the states formed on two separate QPCs, through a two-dimensional electron gas that serves as a continuum, we observe a robust effective interaction between the QPCs. To explain this result, we develop a theoretical formulation, based on the ideas of the Schrieffer-Wolff transformation, which is able to reproduce our key experimental findings. According to this model, the robust character of the interaction between the two remote states arises from the fact that the interaction is essentially mediated by a large number of degenerate continuum states. While the continuum is often viewed as a source of decoherence, our experiment therefore instead suggests the possibility of using this medium to support the interaction of quantum states, a result that may allow new approaches to coherently couple nanostructures in extended geometries.

We show a dramatic deviation from ergodicity for the conductance fluctuations in graphene. In marked contrast to the ergodicity of dirty metals, fluctuations generated by varying magnetic field are shown to be much smaller than those obtained when sweeping Fermi energy. They also exhibit a strongly anisotropic response to the symmetry-breaking effects of a magnetic field, when applied perpendicular or parallel to the graphene plane. These results reveal a complex picture of quantum interference in graphene, whose description appears more challenging than for conventional mesoscopic systems., Comment: Revised version published in Physical Review B, Rapid Communications

We study the effect of Coulomb blockade (CB) on the Aharonov-Bohm (AB) oscillations, observed in the magnetoresistance of small dots in both the quantum Hall and tunneling regimes. In the quantum Hall regime, where one or more edge states pass freely through the dot, the period of oscillation \ensuremath{\Delta}B is well correlated to the dot size, and the temperature-dependent decay of the oscillations is consistent with the expected form of the single-particle magnetospectrum. In the tunneling regime, however, \ensuremath{\Delta}B is enhanced by more than two orders of magnitude over the value expected from the size of the dot, and the resulting oscillations persist to above a degree Kelvin. This behavior can only be explained by considering the effect of CB on the magnetic depopulation of electrons from the isolated dot; these magneto-Coulomb oscillations are the magnetic analogue of the CB oscillations more conventionally observed using electrostatic techniques. These results demonstrate that by varying the coupling of the dot to the source and drain, the resulting AB oscillations can be used to separately probe both the single-particle and CB-related energy scales of small dots.

We are studying quantum interference effects in narrow GaAs/AlGaAs wires of the quasi-ballistic regime, where the wire dimensions are comparable to or less than the phase coherence length of electron transport. By analysing the auto-correlation function of the universal conductance fluctuations in low field magnetoresistances, we obtained the wire-width and the magnetic field dependences of the correlation field and observed phase-coherence behaviors of electron waves. Estimating the boundary correlation field as ω c τ = 1, we can discuss a mean free path in the split-gated wire. Such a boundary field analysis indicates a possibility of determination of the microscopic mean free path of quantum narrow wires.

We have studied the low temperature electrical properties of semiconductor quantum dot structures at high magnetic fields. The devices consisted of lithographically defined aluminium gates deposited on to the surface of a GaAs/AlGaAs heterojunction substrate and the dot structures were formed on the application of a suitable negative bias to the gates. At high magnetic fields electronic transport occurred via edge states and we were able to use the well known properties of these states to study transmission through the dot structures. In particular we observed large resistance resonances superimposed on the quantised magnetoresistance plateaux of the dot. These resonances were found to persist with reduced amplitude to above a degree Kelvin and showed a sensitive dependence on the applied gate voltage. We associate them with the increased edge state interaction introduced by the dot and discuss our results in terms of the various scattering mechanisms inherent to quantum dot structures.

We formulate a many-body model of transport in open quantum dots, which is based upon the idea of an enhanced electron-electron interaction in the vicinity of a quasibound state. Our studies suggest that the effect of including this peaked interaction is to increase the amplitude of the conductance fluctuations, beyond the value expected from a single-particle treatment. While the role of interactions in transport through open dots has attracted little theoretical attention, our results demonstrate the presence of interaction-induced corrections to the transport in these structures. (C) 2002 American Institute of Physics.

We discuss the growth by molecular-beam epitaxy, and studies of the low-temperature electrical properties, of undoped InAs/AlSb quantum wells. The two-dimensional electron gas realized in the wells exhibits high mobility at low temperatures, and an analysis of its Shubnikov–de Haas oscillations suggests this mobility is limited by scattering from remotely located unintentional dopants. Spin splitting of the oscillations is clearly resolved at 4.2 K, revealing a g-factor as large as −60 at high magnetic fields. The size of this enhancement increases with decreasing electron density, and is thought to reflect the associated increase in the strength of the effective Coulomb interaction.

Electron-heating measurements are used to compare the form of the electron–phonon interaction in two-dimensional, and quasi-one-dimensional, InGaAs quantum wires. Evidence for a strongly enhanced interaction is found in the quasi-one-dimensional wire, and is suggested to result from the presence of the singularities in its electronic density of states. The Bloch–Gruneisen criterion is easily violated in this wire, and its energy-loss function is found to show a weak temperature dependence, which is argued to result from a saturation of scattering processes in the uppermost one-dimensional subband.

We study the characteristics of the fluctuations, observed in the low-temperature magnetoconductance of an open quantum-dot molecule formed from a pair of split-gate quantum dots. The evolution of these fluctuations suggests a decrease in the typical area for coherent interference with decreasing dot-coupling strength. We discuss this behavior in terms of a transition from multi- to single-dot interference as a function of the interdot coupling.

We measure a strong enhancement of the nonlinear differential conductance ($g=dI/dV$), the amplitude of which exceeds the universal quantum conductance ($2{e}^{2}/h$), under finite bias voltage in quantum point contacts (QPCs). By developing a spin-based model in the low-electron-density limit, we demonstrate that this resonance is an intrinsic nonequilibrium phenomenon that arises from many-body induced modifications to the QPC potential. A comparison with the linear conductance ($G=I/V$) shows that this phenomenon is driven by many-body dynamics within a single one-dimensional sub-band.

In order to study on THz radiation pick upping by use of graphene nano-structures, we explore the use of quntum dot as nano scale GHz-THz detector with a high sensitive and/or a wide range frequency response. A fundamental possibility in the device application has been studied.

Quasi-periodic conductance fluctuations are observed in the low-temperature magneto-conductance of a bilayer graphene sample. The quasi-periodic nature of the fluctuations is confirmed by their Fourier power spectrum, which consists of just a small number of dominant frequency components. From an experimental study of these features, which are highly reminiscent of those reported previously for ballistic semiconductor quantum dots, we suggest that they are associated with the formation of an open quantum dot in the submicron graphene sample.

The UV light irradiation to C60 fullerene induces [2+2] cyclo‐additional reaction, and creates inter‐molecule bonds between C60 molecules. That reaction makes fullerene polymer from molecules. A fine crystalline wire structure consisting of C60 molecules is called a fullerene nano whisker (FNW), and also is used to fabricate n‐type field effect transistors (FET) as the channel. The UV irradiation to the FNW‐FET for polymerization must achieve the FET operation in ambient atmosphere. Here, a drastic changing of FET parameters has been observed. In the ESR measurement in ambient atmosphere, we have studied on temperature dependence of peak‐to‐peak width (ΔHPP) and ESR magnetic susceptibility ratio. The comparison, the temperature dependence of pristine FNW and UV irradiated FNW, clearly indicates that there exists a clearer difference in electronic state between pristine FNW and UV polymerized FNW.

Transient heat pulse techniques allow specific heat C and thermal conductivity κ to be determined from a single measurement. In the main these parameters have previously been derived using linearizations of simplified solutions to the diffusion equation in restricted time regimes. We present a numerical fitting technique which allows a fit to be made to the full solution of the diffusion equation across all times of the measurement. This technique also accounts for and allows measurement of the boundary resistance at the clamped end of the sample, an effect which can cause significant distortion of the measured temperature profile. This distortion can lead to significant errors in values of C and κ derived using other methods.

We have analysed the variation of the amplitude of the magnetoconductivity peaks of the 2DEG in GaAs/AlGaAs heterojunctions as a function of Landau level index number. We interpret our observations in terms of a scattering change as the magnetic field is varied and the size of the orbit dimensions relative to the scale of long-range potential fluctuations. Furthermore, the experimental values of the conductivity peaks are compared with theoretical predictions. Agreement is possible over a limited range of magnetic field and only when a low areal density of scatterers is used.

We investigate the electrical conductance of long, high-mobility quantum wires formed by the split-gate technique, which allows for adjustment of the wire width and the number of one-dimensional electron subbands, n. In wires with 3 � n � 8, a logarithmic temperature dependence of the conductance is observed for 1

A narrow channel has been defined in a GaAs/AlGaAs heterojunction using metallic split gates formed by electron beam lithography. Resistance measurements have been performed at low temperature (1.2 K) and in magnetic fields of up to 8 T. By varying the negative voltage bias applied to the split gates it is possible to change the regime of electronic transport. In the low resistance regime, Shubnikov-de Haas (SdH) oscillations are observed which disappear as the resistance of the channel is increased. As the resistance is further increased towards the characteristic value of h e 2 , the conductance fluctuations become increasingly pronounced. In this paper, we discuss the experimental features observed in the magnetoresistance. both above and below this characteristic quantum resistance.

We have measured the low temperature magnetoresistance of narrow wires formed as constrictions in the two-dimensional electron gas of a GaAs/AlGaAs heterojunction. In the quasi-ballistic regime, two relaxation times, at high and low magnetic fields, are observed in the amplitude analysis of the Shubnikov-de Haas oscillations. The boundary field which distinguishes these relaxation times is found to be determined by a simple relationship between the conducting width and the corresponding cyclotron radius. We discuss these features in terms of the effect of the magnetic field on sidewall scattering in a quasi-ballistic wire.

We study both the phase-breaking and the energy-relaxation times as a function of both temperature and bias current in arrays of coupled, open quantum dots. These times show a saturation at low temperature, and a decay at higher temperatures. Importantly, the variations of sample resistance as a function of temperature and current cannot be used to infer the electron temperature at a given bias current, which suggests that the energy relaxation process is likely to be phonon based.

One of the key parameters for the observation of quantum interference effects in mesoscopic systems is the phase-breaking time τ φ . While there have been many measurements of this important phase-breaking time in diffusive mesoscopic structures, there have been relatively few studies of phase breaking in zero-dimensional structures. Here, a saturation of the phase-breaking time at low temperatures is found, with a decay at higher temperatures generally of the form of T −2/3 . Variation with dot size suggests a scaling in which the phase-breaking interaction is described by the total number of electrons N = nA within the dot.

We have observed several symmetrical paired peaks against the zero field in the magneto-resistance of the double corrugation gated wires. These symmetrical peaks can be explained by each commensurate trajectory due to an internal magnetic focusing in the ballistic cavity formed in the quantum dot of the corrugation unit.

Bound-state (BS) formation in quantum point contacts (QPCs) may offer a convenient way to localize and probe single spins. In this Rapid Communication, we investigate how such BSs are affected by monitoring them with a second QPC, which is coupled to the BS via wave-function overlap. We show that this coupling leads to a unique detector backaction, in which the BS is weakened by increasing its proximity to the detector. We also show, however, that this interaction between the QPCs can be regulated at will by using an additional gate to control their wave-function overlap.