Shuichi Iwabuchi

Effect of Intersubband Impurity Scattering on the Anderson Localization in Two Dimensional Electron Systems

Effect of intersubband impurity scattering on the Anderson localization is studied in the weakly localized regime of two dimensional electron systems. It is shown that it changes not only the prefactor of the quantum correction of conductivity, but also the diffusion constant, depending on the ratio of the intersubband scattering time to the inelastic scattering time. The crossover behavior in magnetoconductivity from one-subband occupation to two-subband occupation is also studied. Since it varies depending on various scattering times, details of scattering processes can be determined for an arbitrary electron density by fitting experimental observations to theoretical expressions. Experimental determination of the inelastic scattering time without considering this effect leads to an incorrect estimation.

Theory of the Structural Phase Transitions in TTF-TCNQ. II

A theory of the superlattice transitions of TTF-TCNQ is presented with strict regard for its quasi-one-dimensional characters. The model investigated is a two-dimensional array of two kinds of linear chains weakly coupled with one another. The fluctuation of phase of charge density waves on each chain is treated carefully, while the interchain couplings are approximated by mean fields. The properties of the transitions at 54 K and 49 K are well explained, and the expressions of the transition temperatures and the temperature dependence of order parameters are derived.

Transport Properties of a Resistively-Coupled Single-Electron Transistor

A resistively-coupled single-electron transistor (R-SET), whose gate was a tunnel resistor, was fabricated using a modulation-doped GaAs/AlGaAs heterostructure and metal Schottky gates. Observed current-voltage characteristics show the Coulomb diamonds which are predicted for an R-SET. This means that the tunnel resistor can be used as an R-SET gate.

Coulomb blockade and enhancement of magnetoresistance change in ultrasmall ferromagnetic tunnel junctions

Abstract Coulomb blockade and negative magnetoresistance of ultrasmall ferromagnetic tunnel junctions are studied based on the Feynman path integral approach. It is shown that the change in magnetoresistance is considerably enhanced in the Coulomb blockade regime. This is due to the nonlinear current–voltage characteristics caused by the higher-order tunneling processes which set in since negative magnetoresistance tends to make Coulomb blockade unstable. Results obtained are qualitatively in good agreement with recent experimental findings. Coulomb blockade and the magnetoresistance in ultrasmall ferromagnetic double junction are also discussed briefly.

Calculation of the Nondivergent Cotunneling Rates in a Double Tunnel Junction Embedded in a Dissipative Electromagnetic Environment

We consider two serially connected tunnel junctions embedded in a dissipative electromagnetic environment. Within the Coulomb blockade region where the sequential single electron tunneling is suppressed, electron transport through the whole system is achieved via higher-order tunneling processes. The standard approach using perturbation theory to fourth order in the tunnel Hamiltonian gives divergent rates of these processes at the Coulomb blockade boundary. By using a diagrammatic technique, we perform an analytic summation over a perturbative expansion of infinite order. In this way, we remove the divergence and derive an expression for the cotunneling current applicable over the whole Coulomb blockade region.

Single electron tunneling device controlled by environmental impedance modulation

Abstract A single electron tunneling device consisting of a double junction with inductance, resistance and capacitance (Z-SET) is considered. Current–voltage characteristics are calculated using a self-consistent treatment of the environmental impedance. It is shown that the single electron tunneling can be controlled by modulating the resistive environmental impedance. It is also shown that the controllability of the device is strongly degraded when the capacitance becomes large. This suggests the importance of the careful design of Z-SET against the stray capacitance inevitable in the actual device. The critical value of stray capacitance for controllability of Z-SET is estimated.

Phase Transitions and Dielectric Constants in TTE-TCNQ

The model which describes essential aspects of three successive structural phase transitions in TTF-TCNQ is introduced. The effects on the dynamical properties of the system caused by the three dimensional ordering is studied based on the model. Energy spectra of excitations, conductivities and dielectric constants in low temperature ordered phases are calculated. It is shown that conductivities and dielcetric constants are determined by the optical phase modes whose gap energies (“pinning energies”) reflect structural details of the ordered phases; the dc conductivity becomes zero even in the absence of impurity etc., and below the third transition temperature large microwave dielectric constant is obtained with anomalous behavior as experimentally reported.

Theory of coulomb blockade in Tomonaga-Luttinger liquid connected with fermi liquids

A self-consistent microscopic theory of Coulomb blockade in Tomonaga-Luttinger liquid island connected with Fermi liquid electrodes is proposed. The charging effect as well as Tomonaga-Luttinger nature are treated in consistent manner by bosonization technique with zero mode under open boundary condition. Tunneling current is calculated up to the lowest order. It is shown that the current is expressed in terms of the local spectral density modified by the open boundary condition as well as charging effect. For the ultrasmall island, the value of power-law exponent is not always definite.

Coulomb blockade and higher order tunneling effect in ultrasmall ferromagnetic double tunnel junction

A microscopic theory for ultrasmall ferromagnetic double tunnel junctions is proposed, which selfconsistently describes the higher order tunneling processes as well as the charged states of the islands for arbitrary environmental impedance. Such a theory is indispensable to understanding of the physics of this system, since stability of Coulomb blockade here is strongly dependent on applied magnetic field. Tunneling currents and enhancement of negative magnetoresistance due to the Coulomb blockade are briefly discussed.

Possible control method for single-electron tunneling based on environmental-impedance modulation

A device that consists of a double tunnel junction and an environmental impedance is considered. The numerical results obtained show that the number of the island charge and the net current of the single-electron-tunneling (SET) device can be controlled by changing the resistive environmental impedance. This suggests a control method for SET devices.