Crossover of Superconducting Properties and Kinetic-Energy Gain in Two-Dimensional Hubbard Model (Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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Superconductivity in the Hubbard model on a square lattice near half filling is studied using an optimization (or correlated) variational Monte Carlo method. Second-order processes of a strongcoupling expansion are considered in wave functions beyond the Gutzwiller projection. Superconductivity of d_<x^2-y^2>-wave symmetry is widely stable, and exhibits a crossover around U = U_<co> 〜 12t from a BCS type to a new type. For U ≳ U_<co> (U ≳ U_<co>), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and ∝ exp (-t/J) [tiny] on the strong [weak] coupling side of U_<co>. With reference to experiments on optical conductivity, cuprates belong to the strong-coupling regime.
社団法人日本物理学会の論文
2004-05-15