Yosuke Nagaoka

Copper-Oxide Superconductors as Mixed-Valence Heavy Fermions:Spin-Fluctuation Mechanism of Superconductivity

It is proposed that the physics of high- T c superconductors with a CuO 2 network can be understood in a way parallel to that of heavy-fermion superconductors where the spin fluctuations play an important role. On the basis of a periodic Wolff model with aspects specific to copper oxides, it is shown that the charge carriers are the p -like holes when the antibonding band is filled less than half. It is also shown that these charge carriers form relatively heavy Fermi liquid with an effective bandwidth T 0 ∼10 3 K, which is consistent with experiments.

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.

Numerical Study of the Aharonov-Bohm Effect in Mesoscopic Systems

In order to study the Aharonov-Bohm(AB) effect observed in metal rings attached with leads, we adopt the tight-binding model and numerically calculate the conductance by making use of the Landauer formula and the transfer matrix method. It is confirmed that the magnitude of the conductance fluctuations is almost independent of the system size, consistent with the universal conductance fluctuations. We also find that, while the amplitude of the AB oscillation of the conductance is reduced when the ring length increases, it is enhanced when the lead length increases.

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.

Conductance fluctuations in mesoscopic quantum wires near the ballistic regime

Abstract Conductance fluctuations in mesoscopic quantum wires are studied numerically by the transfer matrix method together with the Landauer formula. Results show that the conductance fluctuations never decrease monotonically to zero towards the ballistic regime, but increase initially beyond the universal value and then decrease to zero making peak structures. Peak values are enhanced by the anisotropy of the band structure and the increasing wire width and the strength of impurity potential. The appearance of peak structures is considered due to the competition between the increase in the number of effective channels brought by the breakdown of the Ohms law and the decrease in fluctuations of transmission probabilities. In the very ballistic regime, where the latter dominates fluctuations, a perturbative calculation is also performed to see how the fluctuations decrease. The results are in good agreement with numerical results.

Conductance Fluctuation of Mesoscopic Systems near the Insulating Regime

The conductance fluctuation of mesoscopic systems at zero temperature is studied numerically by using a simple model of random lattices, with emphasis on its behavior when the degree of randomness increases. It is found that the energy dependence of the conductance has a spiky structure near the insulating regime where the conductance completely vanishes. The result is explained qualitatively as a contribution from a small number of energy levels which remain extended.

Electron motion on a curved interface

The effect of the bend of an interface to which electrons are confined is studied. It is shown that there appears an attractive potential due to the curvature in the two-dimensional effective Hamiltonian of an electron. The effect of an external magnetic field can be taken into account by simply replacing the momentum operators Pi, in the effective Hamiltonian with Pi + (ec)Ai, as far as the field is within the range of ordinary strength. Some examples of curved interfaces are studied. An interesting behaviour of electrons is found in the case of a circular cone, that is “fall” of electrons to the vertex.

Conductance fluctuations in mesoscopic quantum wires

The conductance fluctuations in mesoscopic quantum wires at zero temperature are studied by using the tight binding model and the transfer matrix formalism with the Landauer formula. The numerical calculations for the conductance g (scaled by e 2 / h ) are carried out for the overall range from the ballistic to the localized regime, and it is found numerically that a relation between the averaged conductance and the conductance fluctuation δ g is described by the single parameter near the ballistic regime and also near the localized regime. In the ballistic regime, we find analytically that δ g / M is proportional to (1- /2 M ), which agrees well with the result of the numerical calculations ( M being the channel number). By using the conductance formula due to the resonant tunneling, we discuss the behavior of the conductance near the localized regime, and find analytically that the ratio (δ g ) 2 / tends to the value 0.5 as →0, which is consistent with the numerical result.

Structural Energy and Stability Condition of the Peierls State

The structural energy associated with the Peierls instability in 1-D conductors can be expressed as δ U = c A 2 + d A 2 , A being the order parameter. Because of the defendence of c on the Peierls-gap, the minimized δ U still has the quadratic term of A in addition to the usual quartic term. However, it still holds that whether or not the Peierls transition takes place depends on the balance between the band-structure term and the electrostatic term in the pseudopotential theory.

Transition from Semi-Classical to Quantum Transport in Quasi-Ballistic Wires

We have found evidence of a transition from the semi-classical to quantum transport in quasi-ballistic narrow wires constructed in 2DEG system of a GaAs/AlGaAs heterojunction. By increasing the ratio of the mean free path to the wire length while keeping the wire width constant, we find a clear offset between the semi-classical and the quantum behaviors in the two-terminal measurement of the wire resisitance. We can explain such an offset by taking into account the transmission probability of electron waves propagating in the wire.