Superfluid-Insulator Transitions on the Triangular Lattice

TL;DR

This paper develops a dual vortex field theory to study superfluid-insulator transitions on the triangular lattice, revealing emergent symmetries and explaining the properties of supersolid phases and quantum critical points.

Contribution

It introduces a novel dual vortex framework for arbitrary rational fillings, uncovering emergent nonabelian symmetries and explaining supersolid phases on the triangular lattice.

Findings

At f=1/3, a deconfined quantum critical point similar to the square lattice case.

At f=1/2, an emergent SU(2)×U(1) symmetry appears near the Mott transition.

The supersolid phase is interpreted as a partially melted Mott insulator with skyrmion-like excitations.

Abstract

We report on a phenomenological study of superfluid to Mott insulator transitions of bosons on the triangular lattice, focusing primarily on the interplay between Mott localization and geometrical charge frustration at 1/2-filling. A general dual vortex field theory is developed for arbitrary rational filling factors f, based on the appropriate projective symmetry group. At the simple non-frustrated density f=1/3, we uncover an example of a deconfined quantum critical point very similar to that found on the half-filled square lattice. Turning to f=1/2, the behavior is quite different. Here, we find that the low-energy action describing the Mott transition has an emergent nonabelian SU(2)\times U(1) symmetry, not present at the microscopic level. This large nonabelian symmetry is directly related to the frustration-induced quasi-degeneracy between many charge-ordered states not related by microscopic symmetries. Through this ``pseudospin'' SU(2)symmetry, the charged excitations in the insulator close to the Mott transition develop a skyrmion-like character. This leads to an understanding of the recently discovered supersolid phase of the triangular lattice XXZ model (cond-mat/0505258, cond-mat/0505257, cond-mat/0505298) as a ``partially melted'' Mott insulator. The latter picture naturally explains a number of puzzling numerical observations of the properties of this supersolid. Moreover, we predict that the nearby quantum phase transition from this supersolid to the Mott insulator is in the recently-discovered non-compact CP^1 critical universality class (PRB 70, 075104 (2004)). A description of a broad range of other Mott and supersolid states, and a diverse set of quantum critical points between them, is also provided.