Hastatic order in the heavy-fermion compound URu_2Si_2

摘要

The development of collective long-range order by means of phase transitions occurs by the spontaneous breaking of fundamental symmetries. Magnetism is a consequence of broken time-reversal symmetry, whereas superfluidity results from broken gauge invariance. The broken symmetry that develops below 17.5 kelvin in the heavy-fermion compound URu_2Si_2 has long eluded such identification. Here we show that the recent observation of Ising quasiparticles in URu_2Si_2 results from a spinor order parameter that breaks double time-reversal symmetry, mixing states of integer and half-integer spin. Such 'hastatic' order hybridizes uranium-atom conduction electrons with Ising 5f~2 states to produce Ising quasiparticles; it accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Hastatic order predicts a tiny transverse moment in the conduction-electron 'sea', a colossal Ising anisotropy in the nonlinear susceptibility anomaly and a resonant, energy-dependent nematicity in the tunnelling density of states.%在低于17.5K的温度下，重费米子铀化合物URu_2Si_2存在于一种神秘的“rlidderl-order”相，这种相25年来都未能被定性。在这项研究中，本文作者用各种不同的实验证据确定了在17.5K发生的自然相变的性质，并得出结论认为：“单时间逆转对称”和“双时间逆转对称”都会出现，导致在铀原子的5f~2轨道中巡游传导电子和局部化的“Ising”状态的混合。从根本上来说这是一个新型序，作者将其称之为“rlastatic”(来自拉丁文的hasta，意思是长矛)，他们说它有可能是一个现象，适用于在其中发生与f-轨道的混合的其他系统。