Spin vestigial orders in extended Heisenberg-Kitaev models near hidden SU(2) points: Application to Na$_2$Co$_2$TeO$_6$

Abstract: The honeycomb magnet Na$_2$Co$_2$TeO$_6$ has recently been argued to realize an approximate hidden SU(2) symmetry that can be understood by means of a duality transformation. Using large-scale classical Monte Carlo simulations, we study the finite-temperature phase diagram of the pertinent Heisenberg-Kitaev-$\Gamma$-$\Gamma'$ model near the hidden-SU(2)-symmetric point, in the presence of a six-spin ring exchange perturbation. At low temperatures, the model features collinear single-$\mathbf{q}$ zigzag and noncollinear triple-$\mathbf{q}$ ground states, depending on the sign of the ring exchange coupling. We show that in the vicinity of the hidden-SU(2)-symmetric point, the magnetic long-range orders melt in two stages. The corresponding finite-temperature transitions are continuous and fall into 2D Ising and 2D Potts universality classes, respectively. The two fluctuation-induced phases at intermediate temperatures spontaneously break spin rotational and lattice translational symmetries, respectively, but both leave time reversal symmetry intact. They are characterized by finite expectation values of a real, symmetric, traceless, second-rank tensor, and are naturally understood as vestigial orders of the underlying magnetic states. We identify these vestigial orders as $\mathbb{Z}_3$ spin nematic and $\mathbb{Z}_4$ spin current density wave phases, respectively. For increasing ring exchange perturbations, the width of the vestigial phases decreases, eventually giving rise to a direct first-order transition from the magnetically-ordered phase to the disordered paramagnet. We propose the $\mathbb{Z}_4$ spin current density wave phase, which is the vestigial phase of the primary triple-$\mathbf q$ magnetic order, as a natural candidate for the paramagnetic 2D long-range-ordered state observed in Na$_2$Co$_2$TeO$_6$ in a small window above the antiferromagnetic ordering temperature.