Mitchell Institute for Fundamental Physics & Astronomy
College Station, Texas 77843
The development of collective long-range order occurs by the spontaneous breaking of fundamental symmetries, but the broken symmetry that develops below 17.5K in the heavy fermion material **URu_2Si_2** has eluded identification for over twenty five years – while there is clear mean-field-like 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 heavy Ising quasiparticles in the hidden order phase provides the missing puzzle piece. To form Ising quasiparticles, the conduction electrons must hybridize with a local Ising moment - a **5f_2** 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 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.