The development of collective long-range order at phase transitions occurs by the spontaneous breaking of fundamental symmetries. For example, magnetism is a consequence of broken time-reversal symmetry, while nematicity results from broken rotational symmetry. The broken symmetry that develops below 17.5K in the heavy fermion material URu₂Si₂ has eluded identification for over thirty years – while there is a clear and large specific heat anomaly, the absence of any large observable order parameter has given the problem the name "hidden order."

In this talk, I will show how the observation of highly Ising anisotropic electronic quasiparticles in the hidden order phase provided the missing puzzle piece. To form Ising quasiparticles, the conduction electrons must hybridize with a local Ising moment - a 5f² state of the uranium atom with integer spin. As the hybridization mixes states of integer and half-integer spin, it is itself a spinor and this "hastatic" (hasta: [Latin] spear) order parameter therefore breaks both time-reversal and double time-reversal symmetries. A microscopic theory of hastatic order naturally unites a number of disparate experimental results, and provides a number of experimental predictions. Moreover, this new spinorial order parameter provides a window into a number of new heavy fermion phases.

Argonne Physics Division Colloquium Schedule