High T/sub c/ superconductor as a strongly correlated state

Ichiro Takahashi

Takashi Yanagisawa, Mitake Miyazaki, Yamaji K.

Sudbo Asle

Krzysztof Byczuk, Jan Karbowski, Józef Spałek +1 more

We summarize recent theoretical results of our group concerning the electronic states of the strongly correlated fermions at nonzero temperature. The state of almost localized fermions is discussed within a mean-field approach for auxiliary bosons, both in the narrow-band (Section 2) and in the hybridized band (Section 3) cases. This state represents a metallic state of correlated fermion systems (such as V{sub 2}O{sub 3}) below the Mott-Hubbard localization threshold. The Fermi liquid state does not describe properly the properties of narrow band electrons in the large Hubbard -- U limit beyond the localization threshold. Therefore, in Section 4 and 5 we introduce a phenomenological concept of statistical spin liquid which describes principal features of a crossover from a high-temperature superconductor to the Mott insulator for a two-dimensional system. It also reproduces the main features of the phase diagram for La{sub 2-x}Sr{sub x}CuO{sub 4} system, which is modelled by a planar structure.

Ikuo Ichinose

Since the discovery of cuprate high-temperature superconductors, there has been a lot of interest in theoretical studies 011 the strongly-correlated eleetron systems.It is now accepted that the t-J model is one of the canonical models of the high-Tr.superconductors, and this model has been studied extensively b)' various methods.Most difficult aspect of the t-J model is the existence of a, local constraint 011 the physical states, excluding the double-occupancy of electrons at each site.~/Iany recent

Takashi Yanagisawa

It is very important to elucidate the mechanism of superconductivity for achieving room temperature superconductivity. In the first half of this paper, we give a brief review on mechanisms of superconductivity in many-electron systems. We believe that high-temperature superconductivity may occur in a system with interaction of large-energy scale. Empirically, this is true for superconductors that have been found so far. In the second half of this paper, we discuss cuprate high-temperature superconductors. We argue that superconductivity of high temperature cuprates is induced by the strong on-site Coulomb interaction, that is, the origin of high-temperature superconductivity is the strong electron correlation. We show the results on the ground state of electronic models for high temperature cuprates on the basis of the optimization variational Monte Carlo method. A high-temperature superconducting phase will exist in the strongly correlated region.