Yasumasa Hasegawa

Mean Field Theory of RVB and Superconductivity

The mean field theory for the coupled fermion (spinon)-boson (holon) system is developed by taking account self-consistently of coherent motions of both spinon and holon and singlet pair of spinon (RVB). The relative phase of RVB order parameters in x - and , y -directions, whose magnitudes are fixed to be the same, is treated as arbitrary. The phase, which minimizes the free energy, is found to be π for wide range of parameters. The bose condensation of holons is also examined in the case of weak interlayer electron transfer integrals. The specific heat, magnetic susceptibility and nuclear relaxation rate are calculated. It is found that their anomalies at the superconducting transition temperature depend markedly on the concentration of holons. The T -linear term in specific heat at low temperatures is shown to be vanishing, which is consistent with the recent experiments in Bi-Sr-Ca-Cu-O by Sera et al . and Kumagai et al .

Zero modes of tight-binding electrons on the honeycomb lattice

Tight binding electrons on the honeycomb lattice are studied where nearest neighbor hoppings in the three directions are

Mean Field Theory of Superconductivity in Strong Correlation

t_a,t_b

Spin-Triplet Superconductivity with Line Nodes in Sr2RuO4

and

A Theory of Phase Transitions in Quasi-One-Dimensional Electrons

t_c

Zero modes and edge states of the honeycomb lattice

, respectively. For the isotropic case, namely for

Two-dimensional Hubbard model at low electron density

t_a=t_b=t_c

Fermi Surface Instability of Quasi-Two-Dimensional Tight Binding Electrons : A Possible Phase Diagram of (La1-xMx)2CuO4

, two zero modes exist where the energy dispersions at the vanishing points are linear in momentum

Effective Hamiltonian for CuO2 Layer

k

NMR Relaxation Rate of the Anisotropic Superconducting State in Quasi-Two-Dimensional Systems

. Positions of zero modes move in the momentum space as