Ground state phase diagram of ultrasoft bosons

TL;DR

This paper develops a variational mean-field method to accurately map the ground-state phase diagram of 2D ultrasoft bosons, revealing coexistence regimes and suggesting markers for supersolidity.

Contribution

The authors introduce a fixed particle number per cluster mean-field approach to detail the phase diagram of ultrasoft bosons, aligning well with simulations.

Findings

Good agreement with computational phase diagrams

Identification of coexistence regimes of crystals and superfluids

Solid-superfluid coexistence as a marker for supersolidity

Abstract

In 2D bosonic systems ultra-soft interactions develop an interesting phenomenology that ultimately leads to the appearance of supersolid phases in free space conditions. While suggested in early theoretical works and despite many further analytical efforts, the appearance of these exotic phases as well as the detailed shape of the ground-state phase diagrams have not been established yet. Here we develop a variational mean-field calculation for a generic quantum system with cluster-forming interactions. We show that by including the restriction of a fixed integer number of particles per cluster the ground-state phase diagram can be obtained in great detail. The latter includes the determination of coexistence regimes of crystals of different occupancy as well as crystals with super-fluid phases. To illustrate the application of the method we consider an ultrasoft potential for which the computational phase diagram is known. Our results show very good quantitative agreement with the simulations and suggest that the solid-superfluid coexistence could be a reliable marker to locate supersolidity.