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.

Crystal Growth, Structure, and Diverse Magnetic Behaviors in Frustrated Triangular Lattice REBO3 (RE = Tb-Yb).

Triangular lattice (TL) materials are a rich playground for investigating exotic quantum spin states and related applications in quantum computing and quantum information. Millimeter-level single crystals of REBO3 (RE = Tb-Yb) with a nearly perfect RE-based TL have been successfully grown via a high-temperature flux method and structurally characterized via single-crystal X-ray diffraction. These 113-type materials crystallize in a monoclinic crystal system with a C2/c space group. Anisotropic magnetism and dominant antiferromagnetic interactions are found for the above materials based on DC magnetic susceptibility measurements. The comprehensive low-temperature specific heat data of REBO3 (RE = Tb-Tm) are characterized on single crystals for the first time, which exhibit diverse magnetic behaviors. Specifically, two weak-field-induced transitions could be found in the case of DyBO3 based on the specific heat measurements. Our results suggest that REBO3 (RE = Tb-Yb) is a TL magnetic system for investigating potential quantum magnetism.

Topological Crystalline Insulator Candidate ErAsS with Hourglass Fermion and Magnetic-Tuned Topological Phase Transition.

Topological crystalline insulators (TCIs) with hourglass fermion surface state have attracted a lot of attention and are further enriched by crystalline symmetries and magnetic order. Here, the emergence of hourglass fermion surface state and exotic phases in the newly discovered, air-stable ErAsS single crystals are shown. In the paramagnetic phase, ErAsS is expected to be a TCI with hourglass fermion surface state protected by the nonsymmorphic symmetry. Dirac-cone-like bands and nearly linear dispersions in large energy range are experimentally observed, consistent well with theoretical calculations. Below TN  ≈ 3.27 K, ErAsS enters a collinear antiferromagnetic state, which is a trivial insulator breaking the time-reversal symmetry. An intermediate incommensurate magnetic state appears in a narrow temperature range (3.27-3.65 K), exhibiting an abrupt change in magnetic coupling. The results reveal that ErAsS is an experimentally available TCI candidate and provide a unique platform to understand the formation of hourglass fermion surface state and explore magnetic-tuned topological phase transitions.

Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6.

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO's quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (⊥ b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.

The influence of the structural transition on magnetic fluctuations in NaFeAs.

NaFeAs belongs to a class of Fe-based superconductors which have parent compounds that show separated structural and magnetic transitions. Effects of the structural transition on spin dynamics therefore can be investigated separately from the magnetic transition. A plateau in dynamic spin response is observed in a critical region around the structural transition temperature T S. It is interpreted as being due to the stiffening of spin fluctuations along the in-plane magnetic hard axis due to the d xz and d yz orbital ordering. The appearance of anisotropic spin dynamics in the critical region above the T S at T (*) offers a dynamic magnetic scattering mechanism for anisotropic electronic properties in the commonly referred 'nematic phase'.