Critical phenomena and femtosecond ordering dynamics associated with electronic and spin-ordered phases in YVO3 and GdVO3

We present a systematic study of the electronic- and spin-excitation dynamics associated with the sequence of spin- and orbital-ordering phase transformations in the complex vanadates YVO3 and GdVO3 with ultrafast optical pump-probe reflectance spectroscopy. Relaxation dynamics occurs on a different time scale for each of the ordering transitions, which enables us to unambiguously associate the critical behavior in the dynamics with the observed ordering phenomena. Spin-ordering dynamics is observed on two time scales: tau(r) similar or equal to 2 ps (rise time) and tau(s) similar or equal to 300-3000 ps (decay time), observed below T-N in both compounds. In contrast, the relaxation dynamics associated with orbital ordering occurs in YVO3 on a time scale of tau(o) similar or equal to 20-50 ps. From the temperature dependence of the dynamics, we observe that in both the G-type and C-type orbitally ordered (OO) phases of YVO3 spin order develops in a second-order mean-field fashion with the Neel temperature of the C-OO phase found from our data T-N(C) = 83 +/- 7 K. In GdVO3 we identify the emergence of a new ordered phase within the phase-separated state below 60 K. A new response component with a lifetime of similar to 60 ps is observed below 60 K together with other anomalies in the Delta R/R(Delta t) data. This new phase is not resolved in x-ray diffraction and is not present in YVO3.