Competition between Neel, Haldane nematic, plaquette valence bond solid, and $(\pi,\pi)$ valence bond solid phases in SU(N) analogs of $S=1$ square-lattice antiferromagnets

TL;DR

This study uses quantum Monte Carlo simulations to explore phase transitions and competing orders in SU(N) antiferromagnets on a square lattice, revealing a rich phase diagram with first-order transitions and novel intermediate states.

Contribution

It introduces sign-free Hamiltonians for SU(N) spin models and characterizes their phase diagram, including the discovery of an intermediate $(\pi,\pi)$ state and critical behavior in the melting of valence bond solid order.

Findings

Large-Q/J favors plaquette valence bond solid for all N>3.

First-order transition from Néel to p-VBS for 3<N≤9.

Existence of an intermediate $(\pi,\pi)$ state for N≥10.

Abstract

We use stochastic series expansion (SSE) quantum Monte Carlo (QMC) methods to study the phases and transitions displayed by a class of sign-free designer Hamiltonians for SU($N$) analogs of spin $S=1$ quantum antiferromagnets on the square lattice. The SU($N$) spins are generators of the single-row two-column representation (complex conjugate of single-row two-column representation) on $A$ ($B$) sublattices of the square lattice, and the Hamiltonian is designed to explore the competition between the nearest neighbour antiferromagnetic exchange couplings $J$ and four-spin interactions $Q$ that favor a plaquette-ordered valence bond solid (p-VBS) ground state. We find that this state is indeed established at large $Q/J$ for all $N > 3$. For $3< N \leq 9$, the ground state exhibits a direct first order quantum phase transition from a small-$Q/J$ N\'eel ordered antiferromagnetic state to this large-$Q/J$ p-VBS state. The ground state at $Q/J=0$ for $N \geq 10$ has been previously reported to be a valence bond nematic state, dubbed the Haldane nematic in recent literature. For small nonzero $Q/J$ and $N \geq 10$, we additionally find an unusual intermediate state in which the bond energy has a Bragg peak at wavevector $(\pi,\pi)$ with no accompanying Bragg peaks at wavevectors $(\pi,0)$ and $(0, \pi)$. This $(\pi, \pi)$ state also appears to be metastable at $Q/J =0$, as evidenced by low temperature histograms of the Haldane nematic and $(\pi, \pi)$ order parameters. Deep in the p-VBS phase of the ground state phase diagram, we find the temperature-driven melting of the p-VBS order is in the Ashkin-Teller universality class. In this regime, we identify an interesting signature of Ashkin-Teller criticality in bond correlations at wavevector $(\pi, \pi)$; this is in addition to the expected critical fluctuations of the conventional p-VBS order parameter at wavevectors $(\pi,0)$ and $(0,\pi)$.