Emergent Superfluidity of Hard-Core Excitons in Single-Layer Breathing-Kagome Nb$_3$Te$_x$Cl$_{8-x}$

TL;DR

This paper develops a microscopic theory of superfluidity for hard-core excitons on a triangular lattice, revealing a BKT transition and connecting lattice exciton dynamics to continuum critical behavior.

Contribution

It introduces a novel mapping of the Bose-Hubbard model to an XXZ spin model and derives a continuum Landau-Ginzburg functional for exciton superfluidity.

Findings

Identification of superfluid phase between Mott insulators at fillings 0 and 1

Derivation of a Landau-Ginzburg functional for the exciton superfluid

Superfluid-normal transition governed by BKT mechanism

Abstract

We develop a microscopic theory of superfluidity for hard-core dark excitons on the triangular lattice by mapping the large-$U$ Bose--Hubbard model to an effective XXZ spin-$\frac{1}{2}$ Hamiltonian including virtual hopping processes. Within this framework, we identify the superfluid phase that emerges between the two Mott-insulating endpoints at fillings 0 and 1, and derive its mean-field structure via a canted-spin solution. We then construct the corresponding continuum Landau-Ginzburg (LG) functional and analyze phase fluctuations and vortex dynamics. In two dimensions, the superfluid--normal transition is shown to be governed by a Berezinskii--Kosterlitz--Thouless (BKT) mechanism with a stiffness determined by microscopic parameters. Our results provide a unified description connecting lattice-scale exciton dynamics to continuum critical behavior in triangular geometries.