Harvard Condensed Matter Theory Seminars

Roger Melko , Oak Ridge National Laboratories

Monte Carlo Simulations of Quantum Spin Models on Frustrated 2D Lattices
The search for exotic quantum phenomena has long focussed on frustrated antiferromagnetic (AFM) spin models. Unfortunately, quantum Monte Carlo simulations of isotropic AFM exchange suffer from the debilitating sign problem, making them useless for obtaining ground state results for such systems. Recently however, algorithmic advances in QMC have opened up the possibility of studying certain S=1/2 XXZ models on frustrated lattices. With AFM diagonal (Jz) and ferromagnetic off-diagonal exchange interactions, these models (equivalent to hard-core boson models) can be solved exactly using QMC without the sign problem, with a variety of intriguing results. We discuss simulations of the triangular lattice model where an order-by-disorder supersolid phase has been discovered, as well as the kagome lattice model where the existence of an exotic valence-bond solid phase has recently been suggested. We also outline current work on next-nearest neighbor exchange models, searching for examples of non Landau-Ginzburg-Wilson quantum criticality predicted by recent analytical theories.

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