Kuniichi Ohta

Influence of Fringe Fields on Electrostatic Aharonov-Bohm Effects in Mesoscopic Ring

A new method is invented to treat electron transport in the Aharonov-Bohm (AB)-type ring geometry where transport in each branch of the ring can be treated separately in terms of transfer matrix theory. The influence of fringe field due to the potential edge on the AB effect in a mesoscopic ring is thereby formulated. In the case of potential with an abrupt step, it is found by numerical calculations that the standard treatment of AB effects is approximately permissible at a low enough electrostatic potential of practical interest. The phase of quantum oscillation, however, shifts towards low electrostatic potential, and the amplitudes slowly decay as a function of electrostatic potential as it becomes high. In the case of realistic potential with gradual steps, the transfer matrix can be written in an ordered product of infinitesimal matrices causing AB phase shift and electric-field-dependent amplitude modulation. It is shown that the validity of the usual AB treatment can be assessed in terms of the concept of adiabatic condition.

Structure of h/2e oscillations in a one-dimensional mesoscopic ring

Abstract The quantum oscillation in a one-dimensional ring with two leads is investigated with magnetic flux and electrostatic potential applied simultaneously. A transmission coefficient is derived for various conditi oscillation in a mesoscopic ring as a function of either potential are modulated by holding the other constant. At a certain electrostatic potential and Fermi energy, h/2e oscillations are correctly predicted for the magnetostatic Aharonov-Bohm effect. A similar half-periodic oscillation occurs in the electrostatic case at a certain magnetic flux. A theory is formulated from our earlier numerical calculations and phenomenological interpretations, based on an analytical expression, for a fuller understanding of the origin and mechanism of this behavior. We also investigate the influence of scattering at the junctions on the quantum oscillation.

QUANTUM OSCILLATION IN A ONE-DIMENSIONAL RING COUPLED TO A TUNNEL JUNCTION

Abstract It is shown that the transmission coefficient of a one-dimensional ring coupled to a tunnel clean junction can be calculated by using a scattering matrix. The results agree with the oscillation of a one-dimensional ring when the barrier is narrow enough. The oscillatory behavior of such systems exhibits strong resonance behavior, and the oscillation frequency is greatly influenced by the number of electrons in the ring.

Alternate sign reversal in magnetic flux-induced persistent current in an isolated mesoscopic ring by half-the-ring electrostatic potential

We calculate the energy spectrum for an electron in an isolated ideal one-dimensional mesoscopic ring subject to magnetic flux threading the ring and electrostatic potential applied to one half of the ring. The problem is reduced to that of Kronig-Penney potential with Beyers and Yang boundary condition around the ring. The energy band E is periodic in magnetic phase shift θ and approximately periodic in electrostatic phase shift . At low electrostatic potential, E(θ) bands are shifted in half the potential. Where the E(θ) bands are degenerate, band gaps are formed by electrostatic potential. Above some critical strength of electrostatic potential, narrow energy bands with wide gap are formed that are propagating in one half of the ring and decaying in the other half. From the E(θ) band, magnetic-flux-induced persistent current is calculated. Possibility of its alternate sign reversal by increasing the strength of the electrostatic potential is discussed.

Fabry-Perot Resonance in Aharonov-Bohm Conductance Oscillation at Finite Temperatures

The line shape of Fabry-Perot resonance in Aharonov-Bohm conductance oscillation in a mesoscopic ring is calculated as a function of magnetic phase shift at finite temperatures. The case of the maximum coupling between the ring and leads is considered. It is shown that the 2π periodic series of resonance peaks with height e2/h and width zero at temperature T=0 is broadened as a result of energy dispersion. The peak height is almost kept invariant below 10 K. This is because Fermi energy is far larger than thermal breadth at the step of the Fermi function in this temperature range. Implications of these results are discussed in relation to application to small-area-single-flux-quantum devices.

The saddle point optical transitions under crossed electric and magnetic fields

Abstract Optical transitions at saddle points under the crossed electric and magnetic fields are considered based upon one-band effective mass theory. It has been shown by Baldereschi and Bassani that discrete Landau-type levels should be observed through the interband magnetooptic effects, depending upon the direction of the magnetic field. It is shown that the main effect of the crossed electric field is a line-shift with quadratic dependence upon the electric field. When the magnetic field is parallel to an axis with rotational symmetry, the line shift due to the crossed electric field could determine the conduction and valence band mass-parameters separately.