YbInCu$_4$ undergoes a first order structural phase transition near $T_v$=40 K associated with an abrupt change of Yb valence state. We perform ultrafast pump-probe measurement on YbInCu$_4$ and find that the expected heavy fermion properties arising from the emph{c-f} hybridization exist only in a limited temperature range above $T_v$. Below $T_v$, the compound behaves like a normal metal though a prominent hybridization energy gap is still present in infrared measurement. We elaborate that those seemingly controversial phenomena could be well explained by assuming that the Fermi level suddenly shifts up and becomes far away from the flat emph{f}-electron band as well as the indirect hybridization energy gap in the mixed valence state below $T_v$.
Download