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The single-particle spectral function of the strongly correlated electron system is composed of a coherent peak and incoherent hills on both sides. The non-resonant electronic scattering of the incoherent spectral function becomes the same in the $B_{\rm 1g}$ and $B_{\rm 2g}$ channels and also correlates to the optical conductivity. The mid-infrared hill in the optical conductivity has the same origin as the electronic Raman susceptibility induced by the hole hopping perpendicular to the stripe. The wide-energy Raman spectra in the hole-doped and electron-doped cuprates are different, because the hole-doped cuprates are in the stripe phase while the electron-doped ones are not.

Optical conductivity measurements on the organic metal theta-(BEDT-TTF)2I3 revealed that the system crosses over rapidly from a coherent quasiparticle state to an incoherent state with temperature. Despite the metallic temperature dependence of resistivity, a well-defined Drude peak at low temperatures turns into a far-infrared peak with temperature. The peak energy shifts to higher frequencies and, simultaneously, the spectral weight is transferred to high frequencies beyond the electron band width. These characteristics imply that theta-(BEDT-TTF)2I3, so far believed to be a typical metal, is in fact a strongly correlated electron system with "bad-metal" character.

We performed the systematic optical and transport experiment for ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ at high temperatures. The in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}(T)$ saturates not at the classical Ioffe-Regel-Mott limit but at much higher value. The in-plane optical conductivity ${\ensuremath{\sigma}}_{\mathrm{ab}}(\ensuremath{\omega})$ exhibits at high temperatures a less characteristic, nearly flat spectrum over a wide energy range up to 1 eV without a Drude peak, irrespective of the positive slope of ${\ensuremath{\rho}}_{\mathrm{ab}}.$ For small $\ensuremath{\omega},$ ${\ensuremath{\sigma}}_{\mathrm{ab}}(\ensuremath{\omega})$ even possesses a broad finite-energy peak at high temperatures, suggesting ``dynamical'' localization. These characteristics are explained in terms of the breakdown of the quasiparticle picture due to strong scattering.

The in-plane charge dynamics of lightly doped La 1 . 9 2 Sr 0 . 0 8 CuO 4 (T c =24 K) is examined and discussed. The in-plane resistivity exhibits a metallic character up to 1000 K without saturation at the Mott criterion, which is a typical behavior of a bad metal. The in-plane optical conductivity exhibits a Drude-like peak below a certain temperature, above which it is characterized by a finite-energy peak. The relationship between the shift of the Drude peak and Mott criterion for dc conductivity and the universality of the observation with respect to other had metals are discussed.

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 a strongly correlated electron system, the single-particle spectral function changes into a coherent peak and incoherent humps which extend over 1 eV. The incoherent parts lose the symmetry and k dependence, so that the Raman spectra with different symmetries become identical and they are expressed by the optical conductivity. We found that the B1g and B2g spectra in La(2-x)Sr(x)CuO4 become identical above 2000 cm(-1) in the underdoped phase, if Fleury-Loudon type B1g two-magnon scattering is removed. The first Raman susceptibility moment correlates with the generalized optical conductivity moment. The good correlation arises from the incoherent states of a hump from 1000 to 4000 cm(-1). The hump is the only structure of the incoherent electronic states in the mid-infrared absorption spectra below 1.4 eV at low carrier densities. The energy is twice the separated dispersion segments of the spin wave in the k(perpendicular) stripe direction. The incoherent state is formed by the magnetic excitations created by the hole hopping in the antiferromagnetic spin stripes in the real space picture.

The in-plane charge dynamics of La1.92Sr0.08CuO4 are examined. The in-plane resistivity ρab(T) is metallic up to 1000 K without saturation at the Mott criterion, whereas the in-plane optical conductivity σab(ω) shows a Drude peak only below a certain temperature T ∗ ∼300 K. Above T ∗ the Drude peak shifts to finite energy. The relation between the shift of the Drude peak and the Mott Criterion indicates the “dynamical” localization of the carriers.