标题： 隐序态在URu$_2$Si$_2$中的异常飞秒准粒子动力学 显示英文标题

标题： Anomalous Femtosecond Quasiparticle Dynamics of Hidden Order State in URu$_2$Si$_2$

摘要： 在温度$_0$为17.5 K时，一种奇异相从重费米子态中出现于{\ur }中。 这种隐藏序（HO）相的性质到目前为止仍未得到解释。 人们认为形成一种未知准粒子（QP）结构是导致HO转变时熵大幅消除的原因，然而实验和从头算计算尚未揭示QP的本质特征。 在这里，我们使用飞秒泵浦-探测时间-角度分辨光电子能谱（tr-ARPES）来阐明QP的超快动力学。 我们展示了费米面如何通过在特定位置将状态移离费米能级而被重整化，该位置由矢量$q_{<110>} = 0.56 \pm 0.08$ {\an } 表征。 温度-时间响应的测量显示，进入HO相后，这些位置的QP寿命从42 fs增加到几百fs。 此处确认长寿命QP的形成是HO的主要因素。 显示英文摘要

摘要： 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.