Ultrafast photoconductivity of undoped cuprates

Using a visible pump-THz probe technique, we studied the ultrafast transient photoconductivity of the insulating cuprate La$_2$CuO$_4$, and compared it with earlier measurements of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$. In all these compounds, we observe a rapid onset of photoconductivity that is followed by a non-exponential relaxation on a picosecond time scales, the dynamics of which are independent of photocarrier concentration ranging from 0.2 to 1.5 percent. The photoconductivity decay is qualitatively similar to the decay of the photoinduced gap absorption in Sr$_2$CuO$_2$Cl$_2$,\footnote{J. S. Dodge, arXiv:0910.5048} indicating a common origin for the two effects. Assuming a quantum efficiency of unity, the estimated peak mobility for all three compounds is within 0.1-0.4~$\mathrm{cm}^2/\mathrm{V}\cdot\mathrm{s}$; this is lower than the Hall mobility in chemically doped systems with similar carrier concentrations,\footnote {Ando et al, Phys. Rev. Lett. 87, 017001 (2001)} but orders of magnitude larger than earlier DC photoconductivity results.\footnote{Thio et al, Phys. Rev. B 42, 10800 (1990)} The similarity of the peak photoconductivity across three different compounds indicates that it is an intrinsic feature of the copper oxide plane.