Two-dimensional quantum universality in the spin-1/2 triangular-lattice quantum antiferromagnet Na2BaCo(PO4)2.

An interplay of geometrical frustration and strong quantum fluctuations in a spin-1/2 triangular-lattice antiferromagnet (TAF) can lead to exotic quantum states. Here, we report the neutron-scattering, magnetization, specific heat, and magnetocaloric studies of the recently discovered spin-1/2 TAF Na2BaCo(PO4)2, which can be described by a spin-1/2 easy axis XXZ model. The zero-field neutron diffraction experiment reveals an incommensurate antiferromagnetic ground state with a significantly reduced ordered moment of about 0.54(2) µB/Co. Different magnetic phase diagrams with magnetic fields in the ab plane and along the easy c-axis were extracted based on the magnetic susceptibility, specific heat, and elastic neutron-scattering results. In addition, two-dimensional (2D) spin dispersion in the triangular plane was observed in the high-field polarized state, and microscopic exchange parameters of the spin Hamiltonian have been determined through the linear spin wave theory. Consistently, quantum critical behaviors with the universality class of d = 2 and νz = 1 were established in the vicinity of the saturation field, where a Bose-Einstein condensation (BEC) of diluted magnons occurs. The newly discovered quantum criticality and fractional magnetization phase in this ideal spin-1/2 TAF present exciting opportunities for exploring exotic quantum phenomena.

Fidelity susceptibility of the anisotropic XY model: The exact solution.

We derive several closed-form expressions for the fidelity susceptibility (FS) of the anisotropic XY model in the transverse field. The basic idea lies in a partial fraction expansion of the expression so that all the terms are related to a simple fraction or its derivative. The critical points of the model are reiterated by the FS, demonstrating its validity for characterizing the phase transitions. Moreover, the critical exponents ν associated with the correlation length in both critical regions are successfully extracted by the standard finite-size scaling analysis.

Tunable spin states in the two-dimensional magnet CrI3.

The recent discovery of ferromagnetic single-layer CrI3 creates ample opportunities for studying the fundamental properties and the spintronic applications of atomically thin magnets. Through first-principles calculations and model Hamiltonian simulations, here we build for the first time a substantial magnetic phase diagram under lateral strain and charge doping, the two factors that are easily modulated in single-layer CrI3via substrate and gating controls. We demonstrate that both lateral strain and charge doping efficiently change the coupling between the local spins and thus have unexpected effects on the magnetic properties of CrI3. In particular, the strain tunes the magnetic order and anisotropy: a compressive strain leads to a phase transition from a ferromagnetic insulator to an antiferromagnetic insulator, while a tensile strain can flip the magnetic orientation from off-plane to in-plane. Furthermore, we find that the phase transition under compressive strain is insensitive to charge doping, whereas the phase transition under tensile strain is modulated by electron doping significantly. Our predicted magnetic phase diagram and rationalized analysis indicate the single-layer CrI3 to be an ideal system to harness both basic magnetic physics and building blocks for magnetoelastic applications.

Symmetry-Protected Topological Phase in a One-Dimensional Correlated Bosonic Model with a Synthetic Spin-Orbit Coupling.

By performing large-scale density-matrix renormalization group simulations, we investigate a one-dimensional correlated bosonic lattice model with a synthetic spin-orbit coupling realized in recent experiments. In the insulating regime, this model exhibits a symmetry-protected topological phase. This symmetry-protected topological phase is stabilized by time-reversal symmetry and it is identified as a Haldane phase. We confirm our conclusions further by analyzing the entanglement spectrum. In addition, we find four conventional phases: a Mott insulating phase with no long range order, a ferromagnetic superfluid phase, a ferromagnetic insulating phase, and a density-wave phase.

Midgap states in antiferromagnetic Heisenberg chains with a staggered field.

We study low-energy excitations in antiferromagnetic Heisenberg chains with a staggered field which splits the spectrum into a longitudinal and a transverse branch. Bound states are found to exist inside the field induced gap in both branches. They originate from the edge effects and are inherent to spin-chain materials. The sine-Gordon scaling h(2/3)(s)[log(h(s)](1/6) (h(s), the staggered field) provides an accurate description for the gap and midgap energies in the transverse branch for S=1/2 and the midgap energies in both branches for S=3/2 over a wide range of magnetic field; however, it can fit other low-energy excitations only at much lower field. Moreover, the integer-spin S=1 chain displays scaling behavior that does not fit this scaling law. These results reveal intriguing features of magnetic excitations in spin-chain materials that deserve further investigation.