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We report the crystal structure and magnetic behavior of the spin- silicophosphate studied by high-resolution synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements, and ab initio band-structure calculations. Our data revise the crystallographic symmetry of this compound and establish its rhombohedral space group () along with the geometrically perfect honeycomb lattice of the ions residing in disconnected octahedra. Long-range antiferromagnetic order with the propagation vector observed below K is a combined effect of the nearest-neighbor in-plane exchange coupling K, easy-plane single-ion anisotropy K, and a weak interlayer coupling K. The 12% reduction in the ordered magnetic moment of the ions and the magnon gap of K induced by the single-ion anisotropy further illustrate the impact of spin-orbit coupling on the magnetism. Our analysis puts forward single-ion anisotropy as an important ingredient of honeycomb antiferromagnets despite their nominally quenched orbital moment. ©2021 American Physical Society

The silver ruthenium oxide AgRuO3 consists of honeycomb Ru25+O62- layers and can be considered an analogue of SrRu2O6 with a different intercalation. We present measurements of magnetic susceptibility and specific heat on AgRuO3 single crystals, which reveal a sharp antiferromagnetic transition at 342(3) K. The electrical transport in single crystals of AgRuO3 is determined by a combination of activated conduction over an intrinsic semiconducting gap of ≈100 meV and carriers trapped and thermally released from defects. From powder neutron diffraction data a Néel-type antiferromagnetic structure with the Ru moments along the c axis is derived. Raman spectroscopy on AgRuO3 single crystals and muon spin rotation spectroscopy on powder samples indicate a further weak phase transition or a crossover in the temperature range 125-200 K. The transition does not show up in the magnetic susceptibility, and its origin is argued to be related to defects but cannot be fully clarified. The experimental findings are complemented by density-functional-theory-based electronic structure calculations. It is found that the magnetism in AgRuO3 is similar to that in SrRu2O6, however, with stronger intralayer and weaker interlayer magnetic exchange interactions. © 2021 authors. Published by the American Physical Society.

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN4. A triclinic phase of beryllium tetranitride tr-BeN4 was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN4 layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN4 layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN4 layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions. © 2021 American Physical Society.

We measured the temperature dependence of low-frequency Raman spectra in Li2RuO3 and observed multipeak quasielastic scattering in the Ru honeycomb polarizations below and above the magnetostructural transition temperature. We attribute this scattering to the fluctuations of the energy density in the spin system. High-frequency electronic light scattering was observed at 2150cm-1. Its intensity increased significantly below the transition temperature, confirming substantial modification of the electronic structure due to removal of degeneracy in the t2g manifold of Ru4+ ions. © 2020 American Physical Society.

Fractionalized excitations are of considerable interest in recent condensed-matter physics. Fractionalization of the spin degrees of freedom into localized and itinerant Majorana fermions is predicted for the Kitaev spin liquid, an exactly solvable model with bond-dependent interactions on a two-dimensional honeycomb lattice. As a function of temperature, theory predicts a characteristic two-peak structure of the heat capacity as a fingerprint of these excitations. Here we report on detailed heat-capacity experiments as a function of temperature and magnetic field in high-quality single crystals of α-RuCl3. We undertook considerable efforts to determine the exact phonon background. We measured single-crystalline RhCl3 as a nonmagnetic reference and performed ab initio calculations of the phonon density of states for both compounds. These ab initio calculations document that the intrinsic phonon contribution to the heat capacity cannot be obtained by a simple rescaling of the nonmagnetic reference using differences in the atomic masses. Sizable renormalization is required even for nonmagnetic RhCl3 with its minute difference from the title compound. In α-RuCl3 in zero magnetic field, excess heat capacity exists at temperatures well above the onset of magnetic order. In external magnetic fields far beyond quantum criticality, when long-range magnetic order is fully suppressed, the excess heat capacity exhibits the characteristic two-peak structure. In zero field, the lower peak just appears at temperatures around the onset of magnetic order and seems to be connected with canonical spin degrees of freedom. At higher fields, beyond the critical field, this peak is shifted to 10 K. The high-temperature peak located around 70 K is hardly influenced by external magnetic fields, carries the predicted amount of entropy R/2ln2, and may resemble remnants of Kitaev physics. © 2019 American Physical Society.