Spatially-resolved relaxation dynamics of photoinduced quasiparticles in   underdoped YBa$sub 2$Cu$sub 3$O$sub {7-delta}$
  The spatially-resolved relaxation characteristics of photoinduced quasiparticles (QPs) in CuO$sub 2$ planes of underdoped YBCO are disclosed by polarized fs time-resolved spectroscopy. The relaxation time (tau) along b axis diverges at Tc, and appears to be governed by a temperature-dependent gap Delta(T) at T < Tc. Furthermore, for T > Tc, a monotonic increase of tau with decreasing T along the b axis and ab diagonal was observed and can be attributed to a temperature-independent gap Delta$sub p$. The results lend support to recombination dominant scenario of QP dynamics. However, the QP thermalization may take part along the nodal direction in the highly underdoped samples. 
C. W. Luo, P. T. Shih, Y.-J. Chen, M. H. Chen, K. H. Wu, J. Y. Juang,   J.-Y. Lin, T. M. Uen and Y. S. Gou