青木 伸之

Self-similar conductance fluctuations have been studied by means of low temperature magneto-resistance in order to clarify the fractal transport behavior in quantum coupled dot systems. We have observed a clear three- or four-hold self-similar structure in the magneto-resistance of coupled dot systems. In our discussion based on saddle-point potential model, the self-similar and unstable periodic orbits can be expected by isochronous pitchfork bifurcations due to harmonic saddles in curved walls of the dot. © 2003 Elsevier Science B.V. All rights reserved.

We have clarified carrier spin relaxation process in multi walled carbon nanotubes (MWCNT) fabricated by DC arc discharge and studied the fundamental transport mechanism in MWCNT by means of electron spin resonance (ESR) in order to discuss on the transport properties in the MWCNT. The g-value and the line width for the ESR signal are gradually increased after thermal annealing. This can be explained by a conduction electron system in the MWCNT. © 2002 Elsevier Science B.V. All rights reserved.

A nano-scale bonding process using a multi-wall carbon nanotube (CNT) has been successfully applied in a scanning electron microscope (SEM)/scanning tunneling microscope (STM) combined system. Under the SEM observation, a Ti-coated STM tip was approached onto a CNT bridging Ti pads, which is patterned with a 4 μm gap on a SiO2 surface, to investigate the conduction property of the CNT. A voltage pulse of 12 V peak and 50 μs duration was applied in order to deposit Ti clusters onto the CNT, which is well known as a field evaporation process in the STM regime. The two-terminal current-voltage characteristics of the CNT have been improved from a nonlinear behavior at 2.5 V of 208 kΩ to a linear one of 1.8 kΩ.

We estimate the fractal dimension, Df , deduced from self-similar pattern of the conductance fluctuations in the coupled dot system. The change of Df is clearly correlated with the coupling between dots and should come from the softness of the scattering potential between the coupled dots.

Low temperature transport in multi-wall carbon nano-tubes (MWNT) has been studied by means of three-terminal measurement in order to discuss on the contact resistance between the electrode and the nano-tube. The net resistance of MWNT and the contact resistance between MWNT and electrode have been determined. No evidence for the exponential behavior is found down to 1 K for both resistances, but the power-law temperature dependence is found. We discuss those power-law behaviors so as to try to fit those into Tomonaga-Luttinger model. © 2002 Elsevier Science B.V. All rights reserved.

Thermal annealing effect in multi-wall carbon nanotubes has been studied by electron spin resonance (ESR) in order to discuss transport properties. Before annealing two overlapping narrow signals have been observed in ESR but the higher field signal disappears after thermal annealing. The g-value and the line width for the two signals are determined from computer simulation. © 2002 Elsevier Science B.V. All rights reserved.

A covering of an alumina insulating layer was deposited on the top of a C60 thin-film field-effect transistor (FET) by rf magnetron sputtering with Ar gas, in order to passivate the FET action from the degradation due to oxygen adsorption. The deposited alumina was amorphous and slightly oxygen deficient from the stoichiometry. The stability of FET action in air was considerably improved and no degradation has been detected even for more than one month. © 2002 American Institute of Physics.

We use a network model for modeling the magneto transport in quantum dot array systems. We study numerically the role of Fermi level pinning for the Landau level (LL) depopulation at the quantum point contacts which couple the dots. We find that for the case of Fermi level pinning by the 2DEG the resulting sawtooth behavior of the Fermi level leads to deviations from the linear dependence of the LL energies versus magnetic field, if those energies are obtained from depopulation experiments by sweeping the split gate voltage. © 2002 Elsevier Science B.V. All rights reserved.

We have studied the low temperature conductance fluctuations in double-dot systems. The electron wave coupling between ballistic quantum dots has been investigated in a pair of coupled quantum dots defined by several independently-tunable quantum-point-contacts (QPCs). Reproducible conductance fluctuations are observed in the low temperature magneto-conductance at low magnetic fields and a clear difference between weak and strong couplings has been identified as the gap of the QPC corresponding to the inter-dot coupling is varied. This suggests clear evidence for a specific spread of the coupling orbit over the whole dot region. © 2002 Elsevier Science B.V. All rights reserved.

We discuss the possibility that a Kondo effect may arise in transport through open quantum dots, in which isolated periodic orbits, accessed by non-classical phase-space tunneling, serve as the localized charge that gives rise to the Kondo behavior. Some support for these ideas is shown to be provided by a simple formulation of the Anderson model, in which we assume the electron interaction strength within the dot to be strongly peaked at specific energies. Using this model, we are able to demonstrate behavior reminiscent of the metal-insulator transition, observed in our experimental studies of open quantum dots and dot arrays. The indication of these experiments appears to be that we enter a regime of many-body transport, which becomes effective once the temperature is lowered such that the discrete density of states of the dot becomes energetically resolved. © 2002 Elsevier Science B.V. All rights reserved.

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. © 2002 American Institute of Physics.

The variation of the magnetoconductance of two quantum dot arrays fabricated on GaAs/AlGaAs with temperature and sample current was used to extract carrier heating values at various magnetic fields. These measurements yielded a power law dependence of the electron temperature with the sample current and we study its variation with magnetic field. The power law exponent for the energy-relaxation time for both the dot arrays stays nearly constant over the entire magnetic field range. © 2002 Elsevier Science B.V. All rights reserved.

We have studied transport properties in the magnetoresistance for coupled quantum-dot array systems fabricated by a split-metal-gate technique on GaAs/AlGaAs 2DEG layers. Edge-channel transport has been observed at high magnetic fields in the dot array system and edge state de-population has been analyzed in order to determine the effective g-value and the local carrier concentration at the quantum point contacts. The gate-voltage dependence of the Zeeman splitting is discussed for the lower Landau levels. It can be expected that a large variation of the g-value as a function of the gate-voltage will be interesting for spintronics device applications. ? 2002 Elsevier Science B.V. All rights reserved.

We have studied several quantized levels in the low temperature magneto-conductance of the quantum dot array system fabricated by a split-metal-gate technique on GaAs/AlGaAs 2DEG layer. The gate-voltage dependence of the effective g-value at high fields has been discussed in the dot array. It is noted that the depopulation effect at the quantum point contacts leads to a large decrease of the effective g-value. © 2002 Elsevier Science B.V. All rights reserved.

We have studied the low temperature magneto-resistance in a pair of coupled quantum dots defined by several pairs of independently-tunable quantum point contacts. The electron wave coupling between ballistic quantum dots has been studied in this open dot system. Reproducible fluctuations are observed in the low temperature magneto-conductance at low magnetic fields and a clear difference between weak and strong couplings have been identified as the strength of the inter-dot coupling is varied. This suggests clear evidence for a specific spread of the coupling orbit over the whole dot region.

© 2002 IEEE. The details of electron interference in quantum-dot systems coupled via quantum point contacts (QPCs) is studied via simulation and experiment. In both open and closed coupled systems, one sees transitions from multi- to single-dot behavior, even when the QPCs support several modes. The results also reveal a non-trivial scaling of the conductance fluctuations in quantum-dot arrays, arising from the influence of the inter-dot coupling on energy hybridization.

The non-weak-localization signature in the average conductance of open quantum-dot arrays was discussed. The influence of magnetic field and temperature variations on the gate-voltage averaged conductance of split-gate quantum dot arrays was studied. The study revealed the presence of a quantum correction that was inconsistent with the effect of weak localization.

© 2001 American Institute of Physics. Low temperature magnetoresistance in multi-wall carbon nanotubes prepared by d.c. arc discharge, has been studied. Those tubes show a highly distributed electrical resistance from metallic to semi-insulating conductors. No evidence for the exponential behavior is found down to 1 K, but temperature dependence of the power law is found to be clear as the conductance is reduced. We discuss these observations to try to fit Tomonaga-Luttinger theory for one-dimensional conductor.

The details of electron interference in open quantum-dot arrays are studied in experiment and numerical simulations. Reproducible fluctuations are observed in their low-temperature magnetoconductance and the characteristics of these are suggested to be consistent with a transition from multiple to single-dot interference, which occurs as the strength of the interdot coupling is varied. These results therefore reveal a nontrivial scaling of the conductance fluctuations in quantum-dot arrays, which is thought to arise due to the influence of the interdot coupling on energy hybridization.

The details of electron interference in open quantum-dot arrays are studied in experiment and numerical simulations. Reproducible fluctuations are observed in their low-temperature magnetoconductance and the characteristics of these are suggested to be consistent with a transition from multiple to single-dot interference, which occurs as the strength of the interdot coupling is varied. These results therefore reveal a nontrivial scaling of the conductance fluctuations in quantum-dot arrays, which is thought to arise due to the influence of the interdot coupling on energy hybridization.

We have studied both the phase-relaxation time and the energy-relaxation time in arrays of quantum dots. The variations of phase-breaking time with respect to temperature and current were related, suggesting a common electron temperature dependence, although these times differ by several orders of magnitude.

We study the resonant peaks in the low-temperature magneto-resistance measured in open quantum dot arrays at high magnetic field. The peaks appear at the field where the innermost edge channel passing through a dot array begins to be depopulated. From the gate voltage dependence, the peaks shift to lower magnetic field along the depopulation field at the point contact region in the dot array. We discuss the possible origin considering the charging effect in small puddles at the point contact region. ? 2001 Published by Elsevier Science B.V.

We study the influence of coherent inter-dot coupling on the transport properties of quantum dot arrays. The magneto-conductance oscillations observed in these open devices show a marked reduction in amplitude, and an associated suppression of their high frequency content, as we reduce the strength of the inter-dot coupling. We discuss these observations in the context of a transition from molecular-to atomic-like interference, which is induced as we suppress the strength of the inter-dot coupling.

The influence of a magnetic field on the classical dynamics in ballistic cavities is investigated by analyzing conductance fluctuations that result from quantum interference effects. The magnetic field transforms the exponential decay of the probability distributions of the dwell time and the enclosed area to a power-law decay. The conductance fluctuations correspondingly exhibit a transition from a nonfractal to a fractal behavior. We also identify two additional states that take place when the magnetic field is weaker or stronger than that required to achieve the power-law probability distributions.

The resistance of open quantum-dot arrays is shown to vary logarithmically at low temperatures. No evidence for the logarithmic behavior is found prior to biasing the gates to confine the two-dimensional electron gas, but the amplitude of this effect is found to become larger as the dot size is reduced. We discuss these observations in the context of a confinement-induced enhancement of carrier interactions in the arrays.

We report a strong enhancement and a sudden decrease of the effective g-value observed in the edge channel transport through the quantum point contact in a doubly-corrugated quantum wire. By a precise analysis of the high-field magnetoresistance, we could obtain the gate-voltage dependences of the local barrier height, the carrier concentration and the effective g-value at the quantum point contact region.

We study the properties of the logarithmic variation observed in the temperature-dependent conductance of open quantum-dot arrays. The magnitude of this variation is found to exhibit a well-defined scaling with system size, and is also found to be unaffected by the application of magnetic fields sufficient to break time-reversal symmetry. We suggest that these characteristics are consistent with the logarithmic variation resulting from a confinement-induced enhancement of electron interactions in the arrays. In particular, we note that the size-dependent scaling of this term is similar to that which we would expect for a system whose boundary interactions act to amplify the mutual repulsion of electrons trapped within it. © 2001 The American Physical Society.

The details of electron interference in open quantum-dot arrays are studied in experiment and numerical simulations. Reproducible fluctuations are observed in their low-temperature magnetoconductance and the characteristics of these are suggested to be consistent with a transition from multiple to single-dot interference, which occurs as the strength of the interdot coupling is varied. These results therefore reveal a nontrivial scaling of the conductance fluctuations in quantum-dot arrays, which is thought to arise due to the influence of the interdot coupling on energy hybridization. © 2001 The American Physical Society.

Temperature-dependent studies of the resistance of open quantum-dot arrays reveal a regime of intermediate temperature (∼1–5 K), over which the resistance increases exponentially with decreasing temperature. In this Brief Report, we explore the origins of this unexpected localization by studying its correlation to the temperature-dependent variation of the magnetoconductance. Based on these studies, we suggest that the exponential regime corresponds to that over which we transition from strongly broadened to energetically resolved levels in the dots. In order to provide further support for this interpretation, we perform numerical studies of temperature-dependent transport through the quantum dots, and discuss the role that many-body effects may play in giving rise to the behavior found in experiment. © 2001 The American Physical Society.

The Shubnikov-de Haas oscillations measured in split-gate quantum-dot arrays are found to exhibit a marked beating behavior, which we attribute to a carrier density variation between the quantum-point-contact leads and the component dots of the array. A simple analysis supports these arguments, revealing that the carrier density in the quantum point contacts vanishes as the array is pinched off. The beating may be suppressed by increasing the measurement current, a behavior which we attribute to the injection of energetic electrons in the source into initially unoccupied Landau levels in the quantum point contacts. © 2001 The Japan Society of Applied Physics.

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.

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.

We discuss the results of experimental studies of coherent electron transport in open quantum dot arrays. These studies suggest that the details of this transport remain highly phase coherent over much of the array, allowing features associated with the quantum mechanical coupling of the dots to be observed. Studies of magneto-conductance oscillations in the arrays appear to show the influence of this coupling through a change in frequency content as their gate voltage is varied. We speculate that this change arises as the contribution to interference of long orbits is suppressed by reducing the strength of the inter-dot coupling. Temperature-dependent studies of these devices reveal evidence for an unexpected `metal-insulator' transition, which is manifest as a logarithmic variation of the conductance at low temperatures, whose sign can correspond to either metallic or insulating behaviour. It is speculated that this logarithmic term provides a novel signature of electron-interaction effects in confined nanostructure devices.

Although soft confinement of electrons in ballistic cavities gives rise to a power law of classical probability distributions, the behavior is limited to a narrow range of the variable if the potential is not substantially smooth. The presence of a magnetic field normal to the cavity extends the power-law regime by generating a hierarchical phase space structure. The change in dynamics induced by the magnetic field is confirmed experimentally through analysis of conductance fluctuations in quantum cavities defined by electrostatic gates on a high-mobility heterojunction. Nonfractal conductance fluctuations as a function of the gate bias at zero magnetic field are transformed to be fractal when the cyclotron radius is comparable to the cavity size.

Although soft confinement of electrons in ballistic cavities gives rise to a power law of classical probability distributions, the behavior is limited to a narrow range of the variable if the potential is not substantially smooth. The presence of a magnetic field normal ro the cavity extends the power-law regime by generating a hierarchical phase space structure. The change in dynamics induced by the magnetic field is confirmed experimentally through analysis of conductance fluctuations in quantum cavities defined by electrostatic gates on a high-mobility heterojunction. Nonfractal conductance fluctuations as a function of the gate bias at zero magnetic field are transformed to be fractal when the cyclotron radius is comparable to the cavity size.

We study the low-temperature magneto-resistance of split-gate corrugated wires, in which the application of a suitable gate voltage leads to the formation of an array of coupled dots. At magnetic fields where the cyclotron orbit is comparable to the size of the individual dots, a series of striking peaks is observed in the magneto-resistance. A simple geometric analysis suggests that the peaks arise from a commensurate focusing effect, involving only backscattered orbits within the array. This result is consistent with the behavior found in studies of individual quantum dots, in which the connection between conductance and backscattered orbits has recently been emphasized.