RQMP (CPM) Seminar

Abundance of Ground States in a Frustrated Quantum Antiferromagnet

Owen Benton

MPI PKS

Many body systems are considered frustrated when they exhibit mutually incompatible interactions. At a classical level of description, this often leads to degenerate manifolds of ground states, with large entropy. Generally, however, such degeneracies do not survive in the full quantum description of the system; since the system can tunnel between classical minima, leading to a unique ground state made from the superposition of classical states. This leads to the formation of exotic ground states, including those with fractionalized excitations.

In this talk, I will discuss recent progress on one of the archetypal frustrated systems: the S=1/2 antiferromagnet on the pyrochlore lattice [1] - a network of corner sharing tetrahedra. Remarkably, we find that quantum fluctuations seem to fail in stabilising a unique ground state. Instead, we find a family of valence bond crystal like states, exponentially numerous in linear system size, which are degenerate up to an extremely close precision and have an energy within numerical error bars of the best known variational energy for the model. We explain these results by introducing a new perspective on frustration. Instead of focussing on the frustrated building blocks (tetrahedra); one can consider the lattice as built from unfrustrated hexagons, with highly suppressed coupling between them.

Our results open up a new window on one of the oldest problems in frustrated magnetism; and suggest a fruitful approach to other highly frustrated models.

[1] R. Schäfer, B. Placke, O. Benton and R. Moessner, arXiv:2210.07235