Visible-pump, THz-probe measurement of the photocarrier mobility in an undoped cuprate

We present new experimental results on the mobility of photoexcited carriers in Sr$_2$CuCl$_2$O$_2$, an undoped cuprate. We use ultrafast laser pulses to excite photocarriers in the antiferromagnetic insulating lattice. We then probe the low-frequency dynamical conductivity of the resulting nonequilibrium state with time-domain terahertz spectroscopy. We observe the prompt onset of photoconductivity followed by a non-exponential decay on ps timescales. Assuming all photoexcited carriers are free we observe a peak mobility of $\sim$0.1 cm$^2$/Vs, much lower than the Hall mobility in chemically doped systems with similar carrier concentrations \footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)}. Such a low mobility suggests the formation of polarons or excitons after photoexcitation. We will also discuss the temperature dependence of the nonequilibrium state.