Direct determination of the magnetic ground state in the square lattice S = 1/2 antiferromagnet Li2VOSiO4

Structure, Spin Correlations, and Magnetism of the S = 1/2 Square-Lattice Antiferromagnet Sr2CuTe1-xWxO6 (0 ≤ x ≤ 1)

Quantum spin liquids are highly entangled magnetic states with exotic properties. The S = 1/2 square-lattice Heisenberg model is one of the foundational models in frustrated magnetism with a predicted, but never observed, quantum spin liquid state. Isostructural double perovskites Sr2CuTeO6 and Sr2CuWO6 are physical realizations of this model but have distinctly different types of magnetic order and interactions due to a d10/d0 effect. Long-range magnetic order is suppressed in the solid solution Sr2CuTe1-xWxO6 in a wide region of x = 0.05-0.6, where the ground state has been proposed to be a disorder-induced spin liquid. Here, we present a comprehensive neutron scattering study of this system. We show using polarized neutron scattering that the spin liquid-like x = 0.2 and x = 0.5 samples have distinctly different local spin correlations, which suggests that they have different ground states. Low-temperature neutron diffraction measurements of the magnetically ordered W-rich samples reveal magnetic phase separation, which suggests that the previously ignored interlayer coupling between the square planes plays a role in the suppression of magnetic order at x ≈ 0.6. These results highlight the complex magnetism of Sr2CuTe1-xWxO6 and hint at a new quantum critical point between 0.2 < x < 0.4.

Effects of the interplay of neighboring couplings on the possible phase transition of a two-dimensional antiferromagnetic system

We investigate the phase transition of the quantum spin-1 anisotropic antiferromagnet on a square lattice. The model is described by the Heisenberg Hamiltonian with the nearest-neighbor coupling strengths J_{1a} and J_{1b} along the x and y directions, respectively, and next-nearest-neighbor coupling J_{2}. This model allows Néel state (AF1) and collinear state (AF2). The effects of the spatial and single-ion anisotropy on phase transformation between these two states are explored. Our results show that the two states can exist and have the same critical temperature at D>0 as long as J_{2}=J_{1b}/2. Under such parameters, a first-order phase transformation between these two states below the Néel point can occur when J_{1b} value is not very small and D value is within a narrow range. For J_{2}≠J_{1b}/2, although both states may exist, their Néel temperatures differ. If the Néel point of the AF1 (AF2) state is larger, then at very low temperature, the AF1 (AF2) state is more stable. Thus, in an intermediate temperature, a first-order phase transition between these two states may occur.

Spin-wave energy dispersion of a frustrated spin-½ Heisenberg antiferromagnet on a stacked square lattice

The effects of interlayer coupling and spatial anisotropy on the spin-wave excitation spectra of a three-dimensional spatially anisotropic, frustrated spin-½ Heisenberg antiferromagnet (HAFM) are investigated for the two ordered phases using second-order spin-wave expansion. We show that the second-order corrections to the spin-wave energies are significant and find that the energy spectra of the three-dimensional HAFM have similar qualitative features to the energy spectra of the two-dimensional HAFM on a square lattice. We also discuss the features that can provide experimental measures for the strength of the interlayer coupling, spatial anisotropy parameter, and magnetic frustration.

Magnetic phase diagram of a spatially anisotropic, frustrated spin-¹/₂ Heisenberg antiferromagnet on a stacked square lattice

The magnetic phase diagram of a spatially anisotropic, frustrated spin-[Formula: see text] Heisenberg antiferromagnet on a stacked square lattice is investigated using a second-order spin-wave expansion. The effects of interlayer coupling and the spatial anisotropy on the magnetic ordering of two ordered ground states are explicitly studied. It is shown that with increase in next nearest neighbor frustration the second-order corrections play a significant role in stabilizing the magnetization. We obtain two ordered magnetic phases (Néel and stripe) separated by a paramagnetic disordered phase. Within the second-order spin-wave expansion we find that the width of the disordered phase diminishes with increase in the interlayer coupling or with decrease in spatial anisotropy but it does not disappear. Our obtained phase diagram differs significantly from the phase diagram obtained using linear spin-wave theory.