C-AXIS OPTICAL-SPECTRA AND CHARGE DYNAMICS IN LA2-XSRXCUO4

Out-of-plane (c-axis-polarized) optical reflectivity spectra and c-axi s charge transport properties are studied for single crystals of a hig h-T-c system La2-xSrxCuO4 over a wide compositional range 0 less than or equal to x less than or equal to 0.30. The measurements are made at various temperatures in the normal and superconducting states over an energy range from 0.003 to 40 eV. The present study focuses on the ev olution of the c-axis spectrum with doping and provides a full set of the optical and transport data on this single-layer system together wi th the previously published data of the in-plane spectra [S. Uchida et al., Phys. Rev. B 43, 7942 (1991)]. As in the case of the in-plane sp ectrum, the spectral weight is transferred upon doping from the high- to low-energy region. Different from the in-plane spectrum the transfe rred weight forms a band which is centered at relatively high energy ( similar to 2 eV) and does not appreciably move with increasing x. As a consequence, the c-axis optical conductivity [sigma(c)(omega)] is ext remely small in the lowest-energy region (<0.3 eV) until the compound is overdoped. Particularly, in the underdoped regime (x<0.13) the low- energy sigma(c)(omega) is too small to form a Drude peak, and is furth er suppressed with reducing temperature. This is in common with the ps eudogap effect, observed for the bilayer system YBa2Cu3O6+x, and is co nnected to the semiconducting c-axis resistivity. A Drude peak in sigm a(c)(omega) develops only in the highly doped compounds (x greater tha n or equal to 0.18). Only in the overdoped regime is sigma(c)(omega) d ominated by a sharp Drude term and the anisotropic resistivity rho(c)/ rho(ab) almost constant over a wide temperature range, giving strong e vidence for a three-dimensional metallic state. In the superconducting state the c-axis infrared optical response is quite anomalous as it i s characterized by a sharp plasma edge. In the underdoped regime, the strongly suppressed spectral weight in the normal state gives rise to a sharp plasma edge within a gap region, which can be identified as a Josephson plasma in the weakly Josephson-coupled layered superconducto r. In the highly doped superconducting regime, an appreciable Drude-li ke component remains in the spectrum even at temperatures well below T -c, leading to a substantial damping of the Josephson plasma. Such a g apless spectrum is perhaps associated with a crossover from the underd oped to the nonsuperconducting overdoped regime.