Dr Andrew Ho (Department of Physics, Royal Holloway, University of London)

A new state of quantum matter with intertwined superfluid and density wave order in a system of two dimensional bosons subject to a triangular lattice potential has been suggested to explain the anomalous recent experiments at RHUL [1] . Using a torsional oscillator, superfluid response of the second atomic layer of 4He adsorbed on the surface of graphite have been measured over a wide temperature range down to 2mK. Superfluidity is observed over a narrow range of film densities, emerging suddenly and subsequently collapsing towards a quantum critical point. The unusual temperature dependence of the superfluid density in the limit of zero temperature and the absence of a clear superfluid onset temperature are explained, self-consistently, by an ansatz for the excitation spectrum, reflecting density wave order, and a non-Abelian condensate wavefunction breaking both gauge and translational symmetry. In a separate model that can potentially be realised in cold atom experiments, we have studied a system of p-orbital BEC with interactions that vary slowly over the lattice spacing. [2] Using the same variational condensate wavefunction as for the helium system, we find that the order parameter space is isomorphic to S5. We show that the enlarged symmetry leads to the loss of topologically stable vortices, as well as two extra gapless modes with quadratic dispersion. The former feature implies that this non-Abelian condensate is a “failed superfluid” that does not undergo a Berezinskii-Kosterlitz-Thouless (BKT) transition.

[1] Jan Nyéki, Anastasia Phillis, Andrew Ho, Derek Lee, Piers Coleman, Jeevak Parpia, Brian Cowan and John Saunders, Nature Physics 13, 455-459 (2017)
[2] Simon Lieu, Andrew F. Ho, Derek K. K. Lee, and Piers Coleman, PHYSICAL REVIEW B 99, 014504 (2019)

Location : Shilling Lecture Theatre