A New Broken Symmetry: Hidden (hastatic) Order in URu2Si2
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 twenty five years – while there is clear mean-field 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 recent observation of Ising 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 timereversal symmetries. A microscopic theory of hastatic order naturally unites a number of disparate experimental results from the large entropy of condensation to the spin rotational symmetry breaking seen in torque magnetometry. Hastatic order also has a number of experimental consequences, most notably a tiny transverse magnetic moment in the conduction electrons.