Koichi Kusakabe

Theory of the Josephson effect in a superconductor/one-dimensional electron gas/superconductor junction

We present a theory for the Josephson effect in an unconventional superconductor/one-dimensional electron gas/unconventional superconductor

A Rigorous Extension of the Kohn-Sham Equation for Strongly Correlated Electron Systems

(s/o/s)

Josephson effect in unconventional superconductor/Luttinger liquid/unconventional superconductor junctions

junction, where the Josephson current is carried by components injected perpendicular to the interface. When superconductors on both sides have triplet symmetries, the Josephson current is enhanced at low temperature due to the zero-energy states formed near the interface. Measuring Josephson current in this

A quantum Monte Carlo study of the Josephson effect in a quantum dot

s/o/s

A theoretical study of a π-junction realized in a quantum dot array

junction, we can identify parity of the superconductor.

On possible surface magnetism in nanographite

By introducing a set of auxiliary equations representing a many-body system, we have derived an extension of the Kohn-Sham scheme for the density functional theory. These equations consist of a Kohn-Sham-type equation determining single-particle orbitals and an eigen-value equation for an effective many-body problem. A variational method similar to the Kohn-Sham technique was utilized to derive effective interactions as well as effective potentials without artificial substitution of a Hubbard-type interaction and a mean-field correction in the energy functional. The second equation is described by an effective many-body Hamiltonian with both 2-body interactions and mean-field terms. Rigorous formulation of the extended Kohn-Sham equation is also given in accordance with the Hadjisavvas-Theophilou formulation. Our formulation can be interpreted as a way to define models of the strongly correlated electron systems, e.g. the Hubbard model.

Partial ferromagnetism in semimetallic systems: numerical calculation and rigorous proof

Abstract We present a theory for the Josephson effect in an unconventional superconductor/Luttinger liquid/unconventional superconductor (s/LL/s) junction where the Josephson current is carried by components injected perpendicular to the interface. We apply the bosonization method by Haldane and Maslov to the cases with arbitrary barrier height of the insulator formed between the superconductor and the LL. When superconductors on both sides have triplet symmetries, the Josephson current is enhanced at low temperatures due to the zero-energy states formed near the interface independent of the strength of electron–electron interaction and the transparency of the junction.

Josephson effect in spin- or charge-gapped one-dimensional systems

Abstract DC Josephson current in a superconductor/a quantum dot/superconductor junction is studied by a quantum Monte Carlo calculation. Dressed temperature Greens functions are evaluated by using the Hirsch–Fye algorithm and the current is obtained when the bias voltage is zero. Our result reproduces that a π-junction is realized when the Coulomb blockade occurs at finite temperatures.

A possible superstructure: Hyper graphite

Abstract We have studied theoretically the Josephson effect through a quantum dot array connected with two superconducting electrodes. The dot array has been studied by two models: (1) a Hubbard chain with boundary pair-potentials and (2) an Anderson impurity model coupled to superconducting leads. The first model shows a parity effect. Namely, the system becomes a π-junction for odd N e or a 0-junction for even N e , where N e is the average number of electrons in the array. By shifting the on-site potential at the dot array, N e increases one by one for finite U . The maximum Josephson current shows peaks of resonant tunneling determined by both the parity effect and the Hubbard gap. In order to consider the Josephson effect at finite temperature, we have generalized the quantum Monte Carlo method by Hirsch and Fye and have applied it to the second model with a quantum dot, which shows occurrence of the Coulomb blockade.

Numerical study of unconventional superconductor/a quantum dot/unconventional superconductor junction

Magnetism of nanographite is examined by consideration based on known rigorous theorems for the Hubbard model. For a bearded nanographite ribbon with the zigzag edge on one side and Kleins decorated edge on the other side, the surface magnetization of O(mu_B) per a carbon atom is exactly shown to appear for the π-electron system described by the Hubbard model. The surface magnetism appears due to existence of the edge states, which are degenerate localized states specific to these edges of nanographite. The same magnetic mechanism holds for network structures called the hypergraphite, when edges are prepared so that completely degenerate edge states appear.