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arXiv:2311.01572 · Physical review. B./Physical review. B

Sign-problem-free effective models of triangular lattice quantum antiferromagnets

Henry Shackleton, Subir Sachdev A

Published Feb 3, 2025• 2 citations
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Abstract

The triangular lattice antiferromagnet with $S=1/2$ spins and nearest-neighbor interactions is known to have long-range antiferromagnetic order, with nearest-neighbor spins at an angle of $120{}^{\ensuremath{\circ}}$. Numerical studies of quantum phases proximate to this state have been limited to small systems because of the sign problem in Monte Carlo simulations in imaginary time. We propose an effective lattice model for quantum fluctuations of the antiferromagnetic order and a sign-problem-free Monte Carlo algorithm enabling studies in large systems sizes. The model is a ${\mathbb{Z}}_{2}$ gauge theory coupled to gauge-charged scalars which have a relativistic dispersion in the continuum limit. Crucially, the gauge theory is odd, i.e., there is a static, background ${\mathbb{Z}}_{2}$ gauge charge on each site, accounting for the Berry phases of the half-odd-integer spins on each site. We present results of simulations on lattices of sizes up to $36\ifmmode\times\else\texttimes\fi{}36\ifmmode\times\else\texttimes\fi{}36$. Along with the antiferromagnetically ordered phase, our phase diagram has a valence bond solid state with a $\sqrt{12}\ifmmode\times\else\texttimes\fi{}\sqrt{12}$ unit cell and a gapped ${\mathbb{Z}}_{2}$ spin liquid. Deconfined critical points or phases in intermediate regions are not ruled out by our present simulations.

DOI
10.1103/physrevb.111.075101

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A chiral spin liquid (CSL) phase has been recently reported in the Hubbard model on a triangular lattice at half-filling. It emerges in an intermediate coupling regime, which is sandwiched between a 120∘120^\circ120∘ antiferromagnetic (AFM) phase and a metallic phase as a function of on-site repulsion UUU. In this work, we examine how the charge fluctuation may cause the CSL and complex phase diagram. We first identify an exact sign structure of the model at arbitrary temperature, sample size, and doping, which is reduced from the original Fermi exchange signs of the electrons by finite UUU. In particular, the spin and charge degrees of freedom are generally entangled via the phase-string in the sign structure. By precisely switching off such a phase-string, the CSL phase is shown to disappear, with only the 120∘120^\circ120∘ AFM order remaining down to weak UUU by a density matrix renormalization group calculation. Here the charge fluctuation is effectively decoupled from the AFM background, which is a 120∘120^\circ120∘ AFM order due to a geometric Berry phase in the sign structure of the triangular lattice. General implications for the Mott physics will also be discussed.
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