Anomalous Femtosecond Quasiparticle Dynamics of Hidden Order State in   URu$_2$Si$_2$
  At T$_0$ = 17.5 K an exotic phase emerges from a heavy fermion state in {\ur}. The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at HO transition, however, experiments and ab-initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time- and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector $q_{<110>} = 0.56 \pm 0.08$ {\an}. Measurements of the temperature-time response reveal that upon entering the HO the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO. 
Georgi L. Dakovski, Yinwan Li, Steve M. Gilbertson, George Rodriguez,   Alexander V. Balatsky, Jian-Xin Zhu, Krzysztof Gofry...
cond-mat.str-el