Anomalous Metallic State near the Mott Transition

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In the temperature(T) and carrier concentration phase diagram of high-TィイD2cィエD2 Cu oxides, the anomalous metallic phase (AMP), in which various physical properties exhibit unusual behavior can be found in the region near the "half-filled" Mott insulating phase, or in the region T< TィイD20ィエD2 (P) (p*pィイD20ィエD2, T0(pィイD20ィエD2)=ィイD20ィエD2), where p is the concentration of holes doped into the "half-filled" state. The coincidence of the p-region of the AMP with the superconducting p-region clearly indicates that detailed knowledge on this AMP is essentially important for the understanding of the origin(s) of the superconductivity. From this view point experiments have been carried out to study the physical behavior of high- TィイD2cィエD2 oxides and other related systems, where neutron scattering, NMR, angle-resolved photoemission, optical measurements, STM/STS and other various methods have been applied. As the results of the studies, it has been clarified that the anomalous properties in the … More AMP are closely connected with the antiferromagnetic correlation and the singlet formation which begin to grow at around TィイD20ィエD2 with decreasing T. At around the so-called spin-gap temperature TィイD2SGィエD2 (< TィイD20ィエD2), the singlet correlation dominates over the antiferromagnetic one and therefore effects of the spin gap become significant in various quantities. The supercondunting transition is further suppressed down to TィイD2cィエD2 (<TィイD2SGィエD2) in the underdoped region by the fluctuating nature of the orderparaxneter.The physical picture on the singlet formation described above indicates that the origin of the superconductivity is closely related with the large exchange interaction J among the Cu spins and the d-wave symmetry of the order parameter is naturally understood.Besides the studies on high-TィイD2cィエD2 systems, various achievements have been obtained : Superconductivity has been discovered in a doped ladder system under high pressure. Superconductivity which is possibly exotic has also been found is SrィイD22ィエD2RuOィイD24ィエD2. Behavior which indicates the spin-charge separation has been found in quasi one dimensional spin system by angle-resolved photoemission studies. Extensive studies on the metal-insulator transition in transition metal oxides and sulfides have also been carried out. Less