Shun-ichiro Koh
Division of Physics, Faculty of Education
Kochi University
Akebono-cho,2-5-1, 780,Kochi,Japan
Main field
Theoretical Physics:
(a) Field theory
(b) Quantum many-body problem in the solids and the nuclei
Basic Motivation
(1) Cooper pairs and the gas-liquid transition:
There are two types of particles that obey the quantum mechanics: Fermions and Bosons.They form two different condensed states: the BCS state for the Fermions and the Bose condensate dor Bosons. The behaviors of these particles have some common features, but due to different quatum statics, they behave differently in some situations. As the attractive force in the BCS state is nicreased, a Fermion approaches other Fermion, leading to a tightly-bouind boson. How the BCS state is transformed to the Bose condensate is a long-standing and interesting problem.
In statistical mechanics, there is another long-standing problem that deals with the similar situation: the gas-liquid transition. Hence it is natural to ask whether the gas-liquid phase transitionis possible in the many Cooper pair system as the attractive force is increased.
Applying the method of Yang-Lee zeros in the grand partition funtion to the BCS model, its impossibility is proved. Now the extention of this result to more gerneral type of the attrctive interaction is being attempted.
Recent Publications
S.Koh, Cooper pairs and the gas-liquid transition, Physics Letter.A, 229(1997)59
(2) Nuclei:
In the conventional nuclear theories, the origin of the pairing force and the formation of the superfluid state are treated seperately. In real nuclei, however, the nucleon responsible for the attractive force is identical with the nucleon that stabilizes the superfluid state. Hence the superfluid state modifies the attractive force, and this attractive force determines the superfluid state, leading to a feedback effect. Imcorporating the effective interaction and the formation of the superfluid state into a common self-consistency scheme has a crucial importance in elucidating the self interacting nature of the nuclei. From this viewpoint, various properties of the nuclei are now being studied.
Recent Publications
S.Koh, Dynamics of the nucleon pairing and the alpha-correlation in the valence shell, Nuclear Physics.A, 611(1996)1
S.Koh, Pairing by the many-body force: a mechanism of the alpha-particle-like
correlation in nuclei, Nuclear Physics.A560 (1993)797
(3) Heavy-Fermion superconductor:
In the Heavy-Fermion superconductor、attractive interaction is mediated not by the phonon, but by the quasiparticles.
Recent Publications
S.Koh, Quasiparticle effective masss in the superconducting phase of the heavy-fermion systems, Physical Review.B.51(1995)11669
S.Koh, Thermodynamics of the superconductivity beyond the Gorkov decoupling,
Physical Review.B.49(1994)8983