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The mechanism of the high temperature hole-doped superconductivity was investigated by Raman scattering. The Raman selection rule is unique, so that anisotropic magnetic excitations in a fluctuating spin-charge stripe can be detected as if it is static. We use different Raman selection rules for two kinds of magnetic Raman scattering processes, two-magnon scattering and high-energy electronic scattering. In order to confirm the stripe effect, the Raman spectra of striped La2−x Sr x CuO4 (LSCO) and nonstriped Nd2−x Ce x CuO4 (NCCO) were compared. The main results in LSCO are (1) magnetic excitations are presented by individual energy dispersions for the k∥ stripe and the k⊥ stripe, (2) the charge transfer is allowed only in the direction perpendicular to the stripe. The direction is the same as the Burgers vector of an edge dislocation. Hence, we assume that a charge moves together with the edge dislocation of the charge stripe. The superconducting coherence length is close to the intercharge stripe distance at x

The superconducting transition temperature in the cuprate is pointed to be enhanced by the self-organized spin-charge separated stripes. The magnetic excitations are very different in k∥ and k⊥ stripes. Therefore if one detects the different excitations, it is a clear evidence of the stripe. Neutron scattering, however, always observes the mixed excitations. We first succeeded to detect the k⊥ stripe excitations of the separated dispersion induced by the super-lattice structure of the stripe in distinction from the k∥ stripe excitations by two-magnon Raman scattering. The stripe structure is ubiquitous in the whole carrier density range at low temperatures.

The phonon modes with strong electron–phonon interactions were investigated by two-phonon Raman scattering in p- and n-type high T c superconductors. In p-type superconductors, the strong electron–phonon interaction mode changes from the breathing mode at (π, π) to the half breathing mode at (π, 0) as carrier density increases across the optimum doping in LSCO or the 60 K phase in YBCO. It is in good accordance with the change of the superconducting coherent peak position in k-space. In n-type superconductors, the strong electron–phonon interaction modes change from (0.4π, 0.4π) to (0.4π, 0) at the insulator–superconductor transition. Electron–phonon interactions play an important role in superconductivity.

Magnetic excitations of the resonating valence bond (RVB) states were searched in high temperature superconducting cuprates by Raman scattering. Ho et al. [C.-M. Ho et al., Phys. Rev. Lett. 86 (2001) 1626] disclosed that the spin excitation in the two-dimensional resonating valence bond state has a magnon-like dispersion and a spinon continuum above it. The density of states of the spin excitation has a peak at 1.5 J which is lower than the 2.36 J of the spin wave. We found that the A 1g spectra have a broad peak at 1.5 J in common at the underdoped region of p ⩽ 0.15 in LSCO, Bi2201, and Bi2212 and p ⩽ 0.1 in YBCO. The B 1g peak and the B 2g peak in the two-magnon region have a fine structure of the spin wave. The carrier density dependence is discussed.

The electronic states in La2−xSrxCuO4 are systematically investigated by Raman scattering, infrared‐ultraviolet reflection spectroscopy, and electric resistivity. A narrow quasi‐particle band is created at EF and sharply grows up by collecting the density of states from the high energy region at 1–1.5 eV as temperature decreases near the insulator‐metal transition. The relaxation time of conducting carriers is limited by the quasi‐particle band width near the insulator‐metal transition, but it drastically increases in the overdoped phase. The quasi‐particle band gives the anomalous electronic properties.The electronic states in La2−xSrxCuO4 are systematically investigated by Raman scattering, infrared‐ultraviolet reflection spectroscopy, and electric resistivity. A narrow quasi‐particle band is created at EF and sharply grows up by collecting the density of states from the high energy region at 1–1.5 eV as temperature decreases near the insulator‐metal transition. The relaxation time of conducting carriers is limited by the quasi‐particle band width near the insulator‐metal transition, but it drastically increases in the overdoped phase. The quasi‐particle band gives the anomalous electronic properties.

Electronic Raman scattering has no sum rule in no and weak correlation, but the existence of sum rule has been proposed in strong correlation. The present Raman scattering disclosed that the first moment of the Raman susceptibility is proportional to the generalized moment of the optical conductivity in the metallic phase. The present experiment disclosed that a large amount of electronic states are incoherent states. The incoherent states are the relevant states for the superconductivity, because the gap opens in the incoherent states as well as the coherent states.

In La 2- x Sr x CuO 4 the superconducting gap energy in the B 2g Raman spectra is almost constant from the optimum doping to near the insulator–metal (I–M) transition, indicating that the gap symmetry is s or d ( x y ), because the Fermi arc shrinks into the nodal direction of the d ( x 2 - y 2 ) gap. The reason of the different result from other experiments comes from the dual components of the low-energy electronic states around (π/2, π/2), the resonant peak relating to the I–M transition and the polaron states. The superconducting coherent peak is found to be created only in the polaron states in the underdoped phase.

The present Raman scattering experiment in La2-xSrxCuO4 disclosed that the low-energy conductive electronic states at (π/2, π/2) in the underdoped phase are composed of dual electronic states of the central resonant peak and the electron-phonon coupled polaron states. Both states develop at the lowest-energy end of the B2g electron - two-magnon coupled high-energy peak. The narrow resonant peak is enhanced at the insulator-metal transition, but it is insensitive to the superconducting transition indicating no relation to the superconductivity. On the other hand the polaron peak changes into the superconducting coherent peak (SCP) exactly below Tc. The gap energy determined from the SCP is almost constant from the optimum doping to near the insulator-metal transition, indicating that the gap symmetry is s or d(xy), because the Fermi arc with the large electronic density of states shrinks into the nodal direction of the d(x2 - y2) gap. In the overdoped phase the low-energy conductive electronic states are created at (π, 0) by decreasing the peak energy of the B1g electron - two-magnon coupled states. The gap symmetry in the overdoped phase is consistent with d(x2 - y2).