Strong charge fluctuations manifested in the high-temperature Hall coefficient of high-T-c cuprates

By measuring the Hall coefficient R-H up to 1000 K in La2CuO4, Pr1.3La0.7CuO4, and La2-xSrxCuO4 (LSCO), we found that the temperature (T) dependence of R-H in LSCO for x=0-0.05 at high temperature undoubtedly signifies a gap over which the charge carriers are thermally activated, which in turn indicates that in lightly doped cuprates strong charge fluctuations are present at high temperature and the carrier number is not a constant. At higher doping (x=0.08-0.21), the high-temperature R-H(T) behavior is found to be qualitatively the same, albeit with a weakened temperature dependence, and we attempt to understand its behavior in terms of a phenomenological two-carrier model where the thermal activation is considered for one of the two species. Despite the crude nature of the model, our analysis gives a reasonable account of R-H both at high temperature and at 0 K for a wide range of doping, suggesting that charge fluctuations over a gap remain important at high temperature in LSCO deep into the superconducting doping regime. Moreover, our model gives a perspective to understand the seemingly contradicting high-temperature behavior of R-H and the in-plane resistivity near optimum doping in a consistent manner. Finally, we discuss possible implications of our results for such issues as the scattering-time separation and the large pseudogap.