青木 伸之

A small forbidden gap matched to low-energy photons (meV) and a quasi-Dirac electron system are both definitive characteristics of bilayer graphene (GR) that has gained it considerable interest in realizing a broadly tunable sensor for application in the microwave region around gigahertz (GHz) and terahertz (THz) regimes. In this work, a systematic study is presented which explores the GHz/THz detection limit of both bilayer and single-layer graphene field-effect transistor (GR-FET) devices. Several major improvements to the wiring setup, insulation architecture, graphite source, and bolometric heating of the GR-FET sensor were made in order to extend microwave photoresponse past previous reports of 40 GHz and to further improve THz detection. © 2013 Mahjoub and Ochiai; licensee Springer.

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity xx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility μqof our device. It is found that at the direct I-QH transition, μqBc≈ 0.37 which is considerably smaller than 1. In contrast, at Bc, xxis close to the Hall resistivity xy, i.e., the classical mobility μBcis ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D. © 2013 Chuang et al.

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.

The facile synthesis of an organic electric conducting nanowire is described. The simple oxidation of 9-methylcarbazole by iron(III) perchlorate in a methanol/acetonitrile mixture under atmospheric pressure and temperature produces abundant nanowires without using a template. The nanowire consists of 9,9′-dimethyl-3,3′-dicarbazyl and has a rectangular nanowire shape with an average diameter of 397 ± 50 nm and length of 17 ± 5 μm. The results of the elemental analysis, 1H NMR, FT-IR, XPS, and ESR measurements revealed that the chemical composition of the nanowire is (dicarbazyl)0.12(dicarbazyliuṁClO4-) 0.88̇H2O. This result, combined with the UV-vis-NIR measurement, demonstrates that 9,9′-dimethyl-3,3′-dicarbazyl stacks in a mixed valence state. The nanowire is electroactive and has an electric conductivity of 3.0 × 10-5 S cm-1. © 2012 American Chemical Society.

We discuss how a pair of quantum point contacts (QPCs), which are coupled to each other via their mutual wavefunction overlap with a common continuum, can be used to provide a realization of a multi-continuum Fano resonance. This behavior arises from the multi-subband character of the QPCs, each of whose transverse subbands may be viewed as providing a unique continuum. Reminiscent of the original analysis of Fano, we show that the resonance exhibited by this system can be defined in terms of an asymmetry parameter (q) and characteristic level broadenings (Gamma & Gamma(0)), although these parameters now determine the resonance lineshape through their inclusion in energy integrals, a result that we refer to as the "integral" Fano formula. We also demonstrate how, dependent upon the effective dimensionality of the "detector" QPC that exhibits the Fano resonance, the resonance amplitude can significantly exceed the one-dimensional conductance quantum (2e(2)/h). Our experimental and theoretical results, therefore, provide further support for the scenario of spontaneous bound-state formation in QPCs near pinch-off and suggest that this bound state may be used to study new aspects of Fano-resonance phenomenology. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765026]

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.

We analyze the domain structure of magnetic Y-branch nanojunctions that are formed by the intersection of a pair of orthogonal nanoscale notches. Dependent upon the magnetization history, we show the possibility of localizing various domain-wall (DW) structures at different parts of these junctions, leading to measurable magneto-resistance changes. These changes in turn allow the investigation of DW resistance, separate from the complicating effects of anisotropic magneto-resistance. The capacity to electrically detect the presence of DW structures in different regions of the junction may eventually make these structures of interest for future investigations of DW-enabled logic. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4750240]

The temperature dependence of the mesoscopic conductance fluctuations is investigated for disordered graphene. The fluctuations are generated by varying either magnetic field or carrier density (via a back-gate). Very different temperature cut-offs are found for these two types of fluctuations, with the density-induced features persisting to much higher temperatures (beyond 100 K, even) than those observed when sweeping magnetic field. The robust character of the density-dependent fluctuations may cause them to play an important role in determining the operation of future graphene nanodevices, particularly as device sizes are reduced to the nanoscale. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748167]

We study electric conduction of single GaN nanocolumns, which are synthesized by rf-plasma-assisted molecular beam epitaxy. We attach Ti/Al electrodes to a single GaN nanocolumn on SiO2/Si substrate by using photolithography, and we succeed in creating the ohmic contacts. We observe the nonlinear conduction at large currents and low temperatures. The temperature dependence of resistivity represents the activation-type one with an activation energy of about 11 meV due to donor levels at high temperatures. At low temperatures, it shows the Mott variable-range hopping conduction. We find a strong positive magnetoresistance effect at low temperatures. © 2012 The Surface Science Society of Japan.

We discovered for the first time that light can twist metal to control the chirality of metal nanostructures (hereafter, chiral metal nanoneedles). The helicity of optical vortices is transferred to the constituent elements of the irradiated material (mostly melted material), resulting in the formation of chiral metal nanoneedles. The chirality of these nanoneedles could be controlled by just changing the sign of the helicity of the optical vortex. The tip curvature of these chiral nanoneedles was measured to be <40 nm, which is less than 1/25th of the laser wavelength (1064 nm). Such chiral metal nanoneedles will enable us to selectively distinguish the chirality and optical activity of molecules and chemical composites on a nanoscale and they will provide chiral selectivity for nanoscale imaging systems (e.g., atomic force microscopes), chemical reactions on plasmonic nanostructures, and planar metamaterials. © 2012 American Chemical Society.

A low-molecular-mass molecule with no long alkyl chain, a mixed-valence 9,9′-dimethyl-3,3′-bicarbazyl, is self-assembled into highly-regular straight micro/nanotubes by the one-step electro-oxidation of its precursor, 9-methylcarbazole, in methanol without templates. The driving force of the tube formation is investigated. © The Royal Society of Chemistry 2012.

Field effect transistors (FETs) whose channel is composed of a network of high-quality single wall carbon nanotubes (SWNTs) have been studied to investigate the mechanism of the device operation via scanning gate microscopy (SGM) at room temperature. SWNTs synthesized by CoMoCAT® process was used for the formation of the network. Clear SGM responses were observed only at some points but not uniformly in a whole of the channel. The observed responses correspond to positions where two SWNTs are crossing. Back gate voltage dependence of the SGM images and an electrostatic force microscopy image were also studied. One of the possible mechanisms of the SGM response is considered as a modulation of Schottky barrier formed at junctions between metallic and semiconducting SWNTs. Such junctions suggestively play an important role in the FET operation. © 2012 The Japan Society of Applied Physics.

We present the gate-dependent photoconductivity measurements of single layer graphene ribbons grafted with zinc porphyrin molecules Zn(OEP). The Zn(OEP)-graphene showed a maximum 610% increase in its photo-sensitivity compared to the bare graphene samples. Furthermore, the measured photocurrent exhibited strong dependence on the gate bias, light power, and light wavelength. These dependences showed clear evidence of the excitation of the carriers in Zn(OEP) and their energy transfer to graphene. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704921]

We use scanning gate microscopy to probe the ballistic motion of electrons within an open GaAs/AlGaAs quantum dot. Conductance maps are recorded by scanning a biased tip over the open quantum dot while a magnetic field is applied. We show that, for specific magnetic fields, the measured conductance images resemble the classical transmitted and backscattered trajectories and their quantum mechanical analogue. In addition, we prove experimentally, with this direct measurement technique, the existence of pointer states. The demonstrated direct imaging technique is essential for the fundamental understanding of wave function scarring and quantum decoherence theory. © 2012 American Physical Society.

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.

C60 fullerene nanowhiskers (FNWs) have been investigated for electronics device applications. Nanofabrication for C60 FNWs has been developed for device manufacturing based on simple synthesis and semiconductor microprocessing of C60. Several kinds of disadvantages, including carrier suppression, were overcome under the control of the device structure. Electrical transport is explained and the device working characteristics of the field-effect transistor are also discussed. © 2012 Pan Stanford Publishing Pte. Ltd. All rights reserved.

Cobalt compound nanowires were dispersed in a transparent nonconductive polymer film by merely stirring, and the film's transparency and electrical conductivity were examined. This composite film is a unique system in which the average length of the nanowires exceeds the film's thickness. Even in such a system, a percolation threshold existed for the electric conductivity in the direction of the film thickness, and the value was 0.18 vol%. The electric conductivity value changed from ∼1 × 10-12 S/cm to ∼1 × 10-3 S/cm when the volume fraction exceeded the threshold. The electric conductivity apparently followed the percolation model until the volume fraction of the nanowires was about 0.45 vol %. The visible light transmission and electric conductivity of the composite film of about 1 vol % nanowires were 92% and 5 × 10-3 S/cm, respectively. Moreover, the electric conductivity in the direction parallel to the film surface did not depend on the amount of the dispersed nanowires, and its value was about 1 × 10-14 S/cm. Even in a weak magnetic field of about 100 mT, the nanowires were aligned in a vertical and parallel direction to the film surface, and the electric conductivity of each aligned composite film was 2.0 × 10-2 S/cm and 2.1 × 10-12 S/cm. The relation between the average wire length and the electric conductivity was examined, and the effect of the magnetic alignment on that relation was also examined. © 2011 American Chemical Society.

A quantum dot (QD) sensing device, fabricated from nanoscaled carbon material has been studied using of a bilayer graphene field effect transistor in order to enable its application to the detection of microwave (GHz) and/or terahertz (THz) radiation. Recently, it has been found that there exist several common features in low temperature quantum transport, found in experimental results of conductance quantization in a semiconductor QDs and the magnetoresistance of a graphene QDs. The applicability of a graphene field effect transistor at the GHz/THz range is discussed in terms of the microwave transconductance characteristics up to 40 GHz. © 2011 The Japan Society of Applied Physics.

The first demonstration of a mid-infrared optical parametric oscillator pumped by 1-μm optical vortex pulses is presented. A 0.5-mJ 2-μm fractional vortex pulse having half-integer topological charge is generated. Using this system, 0.24-mJ vortex pulses with a topological charge of 1 can be created. The topological charges of the mid-infrared vortex pulses are observed by an interferometric technique in combination with second-harmonic frequency conversion. © 2011 Optical Society of America.

A direct insulator-quantum Hall (I-QH) transition corresponds to a crossover/transition from the insulating regime to a high Landau level filling factor ν > 2 QH state. Such a transition has been attracting a great deal of both experimental and theoretical interests. In this study, we present three different two-dimensional electron systems (2DESs) which are in the vicinity of nanoscaled scatterers. All these three devices exhibit a direct I-QH transition, and the transport properties under different nanaoscaled scatterers are discussed. © 2011 Liang et al.

A direct insulator-quantum Hall (I-QH) transition corresponds to a crossover/transition from the insulating regime to a high Landau level filling factor nu &gt; 2 QH state. Such a transition has been attracting a great deal of both experimental and theoretical interests. In this study, we present three different two-dimensional electron systems (2DESs) which are in the vicinity of nanoscaled scatterers. All these three devices exhibit a direct I-QH transition, and the transport properties under different nanaoscaled scatterers are discussed.

The UV light irradiation to C 60 fullerene induces [2+2] cyclo-additional reaction, and creates inter-molecule bonds between C 60 molecules. That reaction makes fullerene polymer from molecules. A fine crystalline wire structure consisting of C 60 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. © 2011 American Institute of Physics.

A field effect transistor (FET) whose channel is composed of a network of single wall carbon nanotubes (SWNT) has been studied via scanning gate microscopy (SGM). The SGM is a unique technique using an atomic force microscope tip as a movable-point-gate electrode. It is confirmed that the SGM responses are observed only some part of channel but not uniformly in the network. Moreover, most of them are obtained at junctions of SWNTs. Such junctions would suggestively take an important role in the FET action of the SWNT network. © 2011 American Institute of Physics.

We have performed low-temperature transport measurements on a disordered two-dimensional electron system (2DES). Features of the strong localization leading to the quantum Hall effect are observed after the 2DES undergoes a direct insulatorquantum Hall transition on increasing the perpendicular magnetic field. However, such a transition does not correspond to the onset of strong localization. The temperature dependences of the Hall resistivity and Hall conductivity reveal the importance of the electronelectron interaction effects for the observed transition in our study. © 2010 Elsevier Ltd. All rights reserved.

Terahertz (>1 THz) irradiation of pinched-off quantum point contacts (QPCs) generates a pronounced photo-current due to radiation-induced heating. This response is reproduced by a model of temperature-dependent transmission through a saddle potential, confirming its bolometric nature. © 2010 American Institute of Physics.

Microneedle fabrication on a metal surface based on laser ablation using twisted light with spin was demonstrated, for the first time. The resulting needle showed a height of at least 10 μm above the target surface and a tip diameter of less than 0.3 μm. We also demonstrated the fabrication of a two-dimensional 5 × 6 microneedle array. The needles were uniformly well shaped with an average length and tip diameter of about 10 and 0.5 μm, respectively. © 2010 Optical Society of America.

We have investigated the irradiation effect of ultraviolet light (UV) and supersonic wave (SW) into fullerene nano-whisker (FNW) field-effect transistors (FET). FNW has been fabricated by liquid/liquid interfacial precipitation using a system of C60-saturated m-xylene and isopropyl alcohol. And the morphology of FNW has been observed by a scanning electron microscope. The UV irradiation effect of FNW-FET in vacuum was observed that field effect mobility reduced from 10-2 to 10-3 cm2 V-1 s-1. Also, SW irradiation in solution, the FNW-FET shows a large increase on/off ratio from 10 to 1000. There exists a clear improvement in FET performance by use of both UV and SW irradiation for FNW. Also, the effect of exposure in air has been discussed on their FET performance. © 2010 The Japan Society of Applied Physics.

Quantum point contacts (QPCs) are nanoscale constrictions that are realized in a high-mobility two-dimensional electron gas by applying negative bias to split Schottky gates on top of a semiconductor. Here, we explore the suitability of these nanodevices to THz detection, by making use of their ability to rectify THz signals via the strong nonlinearities that exist in their conductance. In addition to demonstrating the configuration of these devices that provides optimal THz sensitivity, we also determine their noise equivalent power and responsivity. Our studies suggest that, with further optimization, QPCs can provide a viable approach to broadband THz sensing in the range above 1 THz. © 2010 Optical Society of America.

We report here the observation of quasi-periodic conductance fluctuations in the low temperature magneto-resistance of gated bi-layer (BL) field-effect transistors (FETs). Both correlation field analysis and conductance fluctuation behaviour suggests that the quantum transport in the BL-FETs closely resembles that in conventional semiconductor open quantum dots. © 2010 IOP Publishing Ltd.

The low temperature magnetoresistance (MR) has been studied in a nitrogen doped multi-walled carbon nanotube (CNxMWNT) with a four terminal resistance measurement. The phase coherent properties of electron transport in the CNxMWNT have been deduced from results of quantum-transport analysis. A zero-field peak of the weak localization can be observed in the low-temperature MR, however, there exhibits a clear boundary between the phase braking processes around 10 K. The phase coherence has been analyzed and the quasi-one-dimensional properties have been discussed in weakly localized metallic transport with a low temperature MR in terms of quantum interference. © 2010 The Japan Society of Applied Physics.

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.

Hybrid ferromagnet/semiconductor systems have been the focus of considerable attention because of the transport properties of two-dimensional electron systems and their potential applications to magnetic storage and sensing devices. We use the weak localization effect to probe the dephasing mechanism at low temperatures. In our study, the zero-temperature phase-relaxation rate can be enhanced in a hybrid ferromagnet/semiconductor system, which may be due to the inhomogeneous magnetic field emanating from the Ni film. The result may improve understanding of the issue of zero-temperature dephasing in disordered systems.

We report a magneto-transport study on the two-dimensional electron system (2DES) in an Al-GaAs/GaAs heterostructure. The direct insulator to quantum Hall conductor transition is observed at low temperatures by increasing the magnetic field B perpendicular to the 2DES. We can also observe the transition by varying the current I and find a relation Te - Ia between electron effective temperature Te and current. Here, a denotes the exponent for the power law. The exponent a, however, can have different values on the two sides of the transition point, which indicates different inelastic scattering mechanisms in the low-field insulator and in the quantum Hall conductor.

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. © 2009 The American Physical Society.

Carrier injection into a C60 thin film has been studied from characteristics of a field effect transistor and a displacement current measurement (DCM) using three kinds of metals, Mg, Ag, and Au. The Mg sample shows a highest field effect mobility of 0.62 cm2/Vs and a lowest threshold voltage of 6 V among the samples. A carrier injection and the accumulation are confirmed in the DCM curves by sweeping voltage applied on the back gate. The electron number and the density injected from the electrodes are estimated from the integration of the DCM curves. However, only the curve of the Mg sample shows a two-stage transition in the DCM curves. It could be due to a diffusion of Mg atoms into the C60 layer and a transition of releasing electrons from the donor level could be observed at lower voltage region. © 2009 IOP Publishing Ltd.

We have investigated field-effect transistor (FET) application on the photo-irradiated C60 thin film. Carrier mobility can be estimated to be 3.5 X 10-4 cm2/Vs after photo-irradiation and it shows a lower mobility than that before irradiation. Probably, it would be due to introduction of many cracks in the C60 thin film induced by polymerization due to ultraviolet light irradiation. However, considering the decrease in the threshold voltage from 12 to 2 V in the ISD-V G characteristics, carrier injection barrier clearly decreases by photo-irradiated polymerization. Furthermore, the photo-irradiated C 60 FET works even in atmosphere. © 2009 IOP Publishing Ltd.

Low temperature transport of a well-defined open-quantum dot has been studied via scanning gate microscopy (SGM). The open dot was fabricated on AlGaAs/GaAs heterostructure using trenches defined via electron beam lithography and a wet etching. The active size of the quantum cavity is approximately 1 μm × 1 μm. The SGM observation was performed at low temperature with a conductive piezolever, which is lifted up 50 nm above the surface with applied a negative voltage. During the scan of the tip, resistance across the dot is measured in a four-probe configuration and stored in a SPM controller synchronized with the position of the tip. The SGM response at zero-magnetic field shows a high resistance when the tip situates onto a line along the inlet and the outlet of the dot. The image could be understood as that the tip induced potential disturbs the current flow in the dot and it results in the increase of resistance. Such an image can be obtained even at more than 8 K, therefore this image would correspond to a classical transport. On the other hand, the image obtained at less than 2 K shows almost similar image as that observed at higher temperature. However, after subtracting the high temperature image as the back ground structure, which is achieved by a high pass filtering, conductance fluctuations are visualized in the low temperature image. These fluctuations would be attributed to a change of quantum interference condition as well as a manipulation of the wave-function confined in the open quantum dot. © 2009 IOP Publishing Ltd.

We demonstrate the operation of C60 fullerene nanowhisker (FNW) field-effect transistors (FETs), realized by dissolving C60 powder in either m-xylene or chlorobenzene solvents and performing liquid-liquid interface precipitation. There exists a clear dependence on solvent species for the FET performance. In contrast to prior studies, in which the FETs were investigated under vacuum conditions to avoid introducing crystalline disorder, we achieve comparable transistor operation for FETs under a N2 atmosphere. The finding that solvate FNWs may be used to implement FETs could have important technological implications, by allowing practical applications of these materials to be developed. © 2009 IOP Publishing Ltd.

Potential barriers and their contribution have been visualized in an organic field-effect transistor composed of copper-phthalocyanine (CuPc) thin film via scanning gate microscopy (SGM). The SGM response shows a peak when a biased tip situates on both edges of the Au electrode at the lower source-drain voltages. It indicates that the electric field from the tip modulates the Schottky-type potential barrier at the CuPcAu interface and the barriers strongly restrict the carrier (hole) injection and/or emission at the CuPc channel. On the other hand, a significant peak appears only at the source (hole-injection) side at higher bias voltages. The difference indicates that the contribution of the barrier to the transport changes with the bias condition. The electrostatic force microscopy response, which is simultaneously obtained with the SGM image, supports these considerations. Moreover, it is confirmed that the peak height is related to the distribution of current injection into the channel. © 2009 American Vacuum Society.

We perform nonlocal bias spectroscopy of the self-consistent bound state (BS) in quantum point contacts (QPCs), determining the lever arm (γ) that governs the gate-voltage induced shift in its energy. The value of γ allows us to infer an enhanced g factor, and large remnant spin splitting, for the BS. Our results show many similarities with bias spectroscopy of quantum dots and are reproduced by calculations that assume a discrete BS coupled to a reservoir. This study therefore provides independent evidence in support of the notion of BS formation in QPCs. © 2009 American Institute of Physics.

We demonstrate current-voltage spectroscopy of the one-dimensional subband structure of pinched-off quantum point contacts (QPCs). This technique yields the full subband structure and effective barrier of the QPC, without the need to perform an undesirable average over a range of gate voltage. Our measurements reveal strong asymmetry in the potential drop across the QPC, and a significantly enhanced subband spacing, in the pinch-off regime. © 2009 American Institute of Physics.

Magneto-transport measurements are performed on the two-dimensional electron system (2DES) in an AlGaAs/GaAs heterostructure. By increasing the magnetic field perpendicular to the 2DES, magneto-resistivity oscillations due to Landau quantization can be identified just near the direct insulator-quantum Hall (I-QH) transition. However, different mobilities are obtained from the oscillations and transition point. Our study shows that the direct I-QH transition does not always correspond to the onset of strong localization. © 2008 IOP Publishing Ltd.

For many years now, there has been ongoing interest in the manifestations of many body phenomena in the conductance of strongly confined, one-dimensional (1D) electron systems. One important aspect of this research has centered on the study of the so-called '0.7 feature' in the low-temperature conductance of 1D conductors known as quantum points (QPCs). There have been numerous reports in the literature suggesting that the 0.7 feature should be related to some kind of spontaneous spin polarization in the QPCs, which persists even at zero magnetic field. In this report, we review the results of our recent work on this problem, in which we make use of coupled QPCs to probe the properties of transport very close to pinch off. We observe a resonant interaction between two QPCs whenever one of them pinches off, which we believe is associated with the binding of a single spin to the QPC that is pinching off. A phenomenological theoretical model is developed that relates the observed resonance to a tunnel-induced correlation that arises from the interaction between a presumed bound spin on one QPC and conducting states in the other. Building on these ideas, we use this measurement technique to probe the microscopic properties of the bound spin, finding it to be robustly confined and to show a Zeeman splitting in a magnetic field. The spin binding occurs for stronger gate confinement than the 0.7 feature, and we therefore suggest an alternative scenario for understanding the formation of this feature. In this, one considers the evolution of the self-consistent bound state as the gate potential is weakened from pinch off to allow for electron transmission through the QPC. The suggestion of this work is that a QPC may serve as a naturally formed single-spin system with electrical readout, a finding that may be useful for the development of future generations of single-spin electronics. © 2008 IOP Publishing Ltd.

We investigate the behavior of interacting one-dimensional systems using linear (close to equilibrium) and non-linear transport measurements of split-gate quantum wires of varying channel length. Our measurements reveal a remarkable resonance effect in the differential conductance, which exhibits a pronounced peak, for a narrow range of source-drain voltage, at the transition from tunneling to open transport. This peak becomes more pronounced with increase of channel length, but is rapidly suppressed by increase of temperature or (in-plane) magnetic field. We believe that these unique features may arise from the dependence of transport on the electron density of states, and suggest a phenomenological model to account for this transport behavior. © 2008 IOP Publishing Ltd.

We study the magnetoresistance (MR) of a multiwalled carbon nanotube (MWNT) as a function of the angle (θtilt) between the external field (Bext) and the MWNT axis. With Bext oriented along the nanotube axis, its MR exhibits two symmetric (with respect to zero field) sharp minima that shift to higher Bext as this field is rotated away from the axis. We suggest that these minima are associated with a cancellation of the nanotube effective flux as predicted by Nakanishi and Ando [J. Phys. Soc. Jpn. 74, 3027 (2005)]. © 2008 The American Physical Society.

The low temperature magneto-resistance (MR) has been studied in a single thin multi-walled carbon nanotube (MWNT) field effect transistor (FET). The phase coherent properties of electron transport in the FET have been deduced from results of quantum-transport studies. A large zero-field peak is observed in the low-temperature MR, and exhibits superimposed oscillatory components. Phase coherence is discussed, and the carrier concentration is also estimated, by analyzing these results in terms of quantum interference. © 2008 IOP Publishing Ltd.

Scanning gate microscopy (SGM) has been applied for a study of organic thin-film field effect transistor (OFET). In contrast to one-dimensional nano-material such a carbon nanonube or nano-structure such a quantum point contact, visualization a transport characteristic of OFET channel is basically rather difficult since the channel width is much larger than the size of the SGM tip. Nevertheless, Schottky barriers are successfully visualized at the boundary between the metal electrodes and the OFET channel at ambient atmosphere. © 2008 IOP Publishing Ltd.

Low temperature transport has been studied in a graphene open quantum dot and shows evidence of fractal behaviour in the presence of a perpendicular magnetic field. The graphene quantum dot was fabricated by micromechanical cleavage and standard electron-beam lithography. The magneto-conductance of the dot indicated self similarity. From the exact-self-similarity analysis, the fractal dimension was determined to be 1.4. We have also observed a strong suppression of weak localization that is attributed to mesoscopic corrugations of the graphene. The relationship between the fractal behaviour and structural topology of graphene remains so far unclear. © 2008 IOP Publishing Ltd.

Various kinds of field-effect transistor (FET) have been fabricated with C60 nanowhisker (NW) and also studied for nanoelectronics application. Especially, pure and solvated C60 NWs have been synthesized in N2 environment so as to clarify the best device performance of C60 NW-FET. The FET works not only under vacuum but also in N2 environment when kept in the solvated condition. The solvated C60 NW-FET shows a clear improvement of their on/off ratio in the solvated condition, and the highest electron mobility after annealing. Although further study is needed, our results demonstrate the possibility, by appropriate choice of the solvent, of achieving good improvements in FET performance. Moreover, new kinds of C60 NW, such as derivative-based and nanotube-type one, have also been studied with regards to their fundamental FET characteristics. © 2008 The Japan Society of Applied Physics.

The authors present scanning gate microscopy (SGM) measurements of an in-plane gated open quantum dot and relate structures within the measurement to a theoretical calculation of the change in conductance in a similar dot. The dot was fabricated in GaAs/AlGaAs. Electron beam lithography and wet etching were used to isolate the two dimensional electron gas and form the device geometry. The SGM image was then high-pass filtered to show both classical and quantum behavior and similarities to a calculated structure are suggested. © 2008 American Vacuum Society.