Evidence of Fermi surface reconstruction and the formation of small hole pockets in underdoped La$_{2-x}$Sr$_x$CuO$_4$: Far Infrared Hall measurements

The Hall Effect in La$_{2-x}$Sr$_x$CuO$_4$ films is measured from 3 to 100 meV as a function of temperature from 5K to 300K and carrier doping ranging from severely underdoped ($x=0.03$) to optimal doped ($x=0.15$). The behavior of the infrared Hall angle with temperature and frequency is found to be consistent with a simple extended Drude model at all dopings. A significant reduction of the Hall mass is observed when the hole doping level is reduced from optimal doping, which is consistent with a drastic reduction of the Fermi surface volume. These results are similar to earlier mid-IR Hall measurements obtained in underdoped YBCO, [1] and related to the recent observations of quantum oscillations reported in YBCO. [2] \newline \newline [1] L. B. Rigal, et al., Phys. Rev. Lett. 93, 137002 (2004). \newline [2] N. Doiron-Leyraud, et al., Nature, 447 565 (2007); A. F. Bangura, et al., Cond-mat/07074461 and E. A. Yelland, et al., Cond mat/07070057