Progress In Realizing Quantum Spin Liquids and Quantum Spin Ice

Spin Ice is a problem in magnetism in which the anisotropy of dipole magnetic moments, and their interactions in certain solids, conspire with their local geometry to maintain a disordered state to low temperatures.  It is analogous to the problem of proton disorder in water Ice - hence the name.  The quantum version of this problem, Quantum Spin Ice (QSI), has been much studied for almost 20 years, as it would realize a quantum entangled ground state described by an emergent quantum electrodynamics.  I'll describe the (mostly) experimental effort to identify such a QSI state in real solids, which features neutron spectroscopy covering over 4 orders of magnitude in energy.  Interestingly, the most promising candidates are Ce-based pyrochlore magnets, which have S=1/2 degrees of freedom with both dipolar and octupolar character.  Three such Ce-pyrochlores (Ce2Zr2O7, Ce2Sn2O7 and Ce2Hf2O7) have been synthesized but the study of Ce2Zr2O7 is currently most mature due to the availability of high-quality single crystals.  These studies show Ce2Zr2O7’s ground state to be a pi-flux QSI, near the border between a dipolar QSI and an octupolar QSI.  I will also review experimental efforts to directly observe octupolar correlations via diffuse neutron scattering at high Q in these systems, which have recently concluded that such scattering is too weak for such a direct observation, at least in the case of Ce2Zr2O7.