Charge and spin in low-dimensional cuprates

One of the central issues in the study of high-temperature superconducting cuprates which are composed of two-dimensional (2D) CuO2 planes is whether the 2D systems with strong electron correlation behave as a Fermi liquid or a non-Fermi-liquid-like one-dimensional (1D) system with electron correlat ion. In this article, we start with the detailed examination of the electro nic structure in cuprates and study theoretically the spin and charge dynam ics in 1D and 2D cuprates. The theoretical background of spin-charge separa tion in the 1D model systems including the Hubbard and t-J models is presen ted. The first direct observation of collective modes of spin and charge ex citations in a 1D cuprate, which are called spinons and holons respectively , in angle-resolved photoemission spectroscopy (ARPES) experiments is revie wed in the light of the theoretical results based on the numerically exact- diagonalization method. The charge and spin dynamics in 1D insulating cupra tes is also discussed in connection with the spin-charge separation. The ar guments are extended to the 2D cuprates, and the unique aspects of the elec tronic properties of high-temperature superconductors are discussed. Specia l emphasis is placed on the d-wave-like excitations in insulating 2D cuprat es observed in ARPES experiments. We explain how the excitations are caused by the spin-charge separation. The charge stripes observed in the underdop ed cuprates are examined in connection with spin-charge separation in real space.