Search for novel low-energy states in strongly correlated electron systems

点击翻译按钮获取中文摘要

In this study we planned to study novel low-energy states in strongly correlated electron systems using variational Monte Carlo method and analytical methods. In particular, we take account of the effects of strong correlation which cannot be treated in the mean-field theories and perturbative approaches.We studied the mechanism of high-temperature superconductivity from a viewpoint of condensation energy. We evaluate the ground state energy by variational Monte Carlo method and study the condensation energy by comparing the energies of superconducting state and normal state. As a result, we conclude that the superconductivity is realized by gaining the kinetic energy in the strong correlation regime, which is opposite to the conventional BCS theory. In contrast, in the weak correlation regime, the system behaves similarly to the BCS theory.As another method, we study the microscopic mechanism of the energy gain below Tc in fluctuation-exchange approximation, and obtain similar results. This is a characteristic phenomenon in high-temperature superconductors where the superconducting gap is very large and is due to the change of the renormalization parameters of quasiparticles by the gap. This result also explains the breakdown of the optical sum rule observed experimentally.About the t-J model which is a typical model for strongly correlated electron systems, we study the superconducting correlation functions using high-temperature expansion method which is alternative to the variational Monte Carlo. In this method, the extrapolation to low temperatures is the most difficult part, but we carry out higher order calculations and succeed to evaluate physical quantities in low temperatures by combining a new extrapolation scheme. As a result, we show that the d-wave superconductivity correlation develops in the parameter space corresponding to the actual materials. This supports the results obtained in the variational Monte Carlo method.