Striped phase in a quantum XY-model with ring exchange

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Abstract
We present quantum Monte Carlo results for a square-lattice S=1/2 XY-model with a standard nearest-neighbor coupling J and a 4-spin ring exchange term K. As a function of increasing ring exchange K/J, we find that the ground state spin-stiffness (or superfluid density in a boson representation) vanishes at a critical point at which a spin gap opens and a striped bond-plaquette order emerges. At still higher K/J, this phase becomes unstable and the system develops a staggered magnetization (charge-density-wave). We discuss the quantum phase transitions between the different phases and point out non-monotonicities in the finite-size scaling behavior when the striped and staggered phases compete.

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