Special QM Seminar Speaker, Chunxiao Liu (UCSB) Friday November 20 at 2:00 pm

Time/Venue Friday November 20 at 2:00 pm Pacific Time via this Zoom link
Host Ehud Altman/Joel Moore
Title Quantum spin liquids on the pyrochlore lattice: symmetry classification, proximate orders, and gauge theories
Abstract Quantum spin liquids are zero-temperature phases of interacting spin systems which possess intrinsic long-range entanglement and support nonlocal excitations carrying fractionalized quantum numbers. The geometrically frustrated pyrochlore lattice has long been predicted to host a quantum spin ice state, a special type of U(1) spin liquid in three dimensions whose only low energy excitations are emergent photons of Maxwell type. Existing pyrochlore experiments, on the other hand, have discovered several weakly ordered states and a tendency of close competition amongst them. Motivated by these facts, we give a complete classification of spin-orbit-coupled Z2 and U(1) spin-liquid states on the pyrochlore lattice by using the projective symmetry group (PSG) approach for bosonic and fermionic partons. The bosonic PSG allows us to map out phase diagrams that link magnetic orders to specific spin liquids. We find that seemingly unrelated magnetic orders are intertwined with each other and that the conventional spin orders seen in the experiments are accompanied by more exotic hidden orders. The fermionic PSG leads to the discovery of novel classes of U(1) spin liquids that possess an unusual gapless multi-nodal line structure in the spinon bands protected by the projective symmetry of the pyrochlore space group. Through a toy model, we study the effect of gauge fluctuations for such a nodal structure and specify the leading contributions to the low temperature specific heat. Our study provides a clear map and various new types of pyrochlore phases for future experiments and variational Monte Carlo studies in pyrochlore materials.

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