Evidence of coherent c-axis charge transport in underdoped,La2-xSrxCuO4 superconductors -: art. no. 092508

We have observed the plasma edge in the normal state c-axis far-infrared (far-IR) reflectivity of La2-xSrxCuO4 (LSCO) single crystals at an unlikely Sr-doping concentration x=0.07 but not in x=0.09. We find that the c-axis scattering rate (Gamma(c)) is surprisingly small, Gamma(c) = 13 +/- 2 cm(-1) for x=0.07 and Gamma(c)=24 +/- 1 cm(-1) for x = 0.09, and nearly temperature-independent but increases linearly with doping up to x = 0.16. We suggest that the apparent absence of the normal state plasma edge in the previous measurements is due to the larger Gamma(c) than the screened plasma frequency of the coherent free carriers. We conclude that the c-axis charge transport in LSCO in the underdoped regime is intrinsically coherent.