Nature of the electronic gap in stripe-ordered cuprates*

The {\it ab}-plane optical properties of single crystals of the high-temperature superconductor La$_{2-x}$Ba$_x$CuO$_4$, with chemical dopings of $x=0.095$ (slightly underdoped) and $0.125$ ($1/8$ doping) and critical temperatures ($T_c$'s) of 32 and $\simeq 2.4$~K, respectively, have been measured over a wide frequency and temperature range. The optical conductivity has been determined from a Kramers-Kronig analysis. In the slightly underdoped material, the reflectance increases monotonically over the far-infrared frequency range, with an abrupt increase in the reflectance below $T_c$ below about 200~cm$^{-1}$ (about 25~meV) signaling the formation of a superconducting energy gap; the suppression of the conductivity for $T\ll T_c$ occurs below this energy. This is close to the estimate of the gap maximum $2\Delta_0$ determined from angle resolved photoemission spectroscopy. In contrast, the 1/8 doping shows a dramatically different behavior.\footnote{C.C.~Homes {\it et al.}, Phys. Rev. Lett. {\bf 96}, 257002 (2006).} The reflectance increases monotonically with decreasing temperature. Below $\simeq 60$~K, corresponding to the onset of charge-stripe order, the far-infrared reflectance continues to increase; however, the reflectance over much of the infrared is suppressed. The conductivity, Drude-like above the ordering temperature, shows a rapid loss of spectral weight below about 40~meV for $T < 60$~K. This behavior is quite different from that typically associated with the pseudogap in the normal state of the cuprates. Instead, the gapping of the normal-state single-particle excitations looks surprisingly similar to that observed in superconducting La$_{2-x}$Sr$_{x}$CuO$_4$, including the presence of a residual Drude peak with reduced weight.