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In our study, we conduct magnetization and heat capacity measurements to investigate field induced magnetic phase transitions within the newly synthesized compound K2Ni2(SeO3)3, a spin-1 dimer system arranged on a triangular lattice. From our first-principles simulations, we determine that the spin system in K2Ni2(SeO3)3 can be represented as a two-dimensional triangular-lattice spin-1 dimer model, including an intra-dimer exchange of J1 = 0.32 meV, an inter-dimer exchange of J2 = 0.79 meV, and an easy-axis anisotropy of D = 0.14 meV. The presence of easy-axis magnetic anisotropy explains the distinct magnetic phase diagrams observed under c-axis directional and in-plane magnetic fields. Notably, our investigation unveils a two-step phase transition with the magnetic field aligned with the c direction. Our findings yield valuable insights into the magnetic phase transitions inherent to geometrically frustrated magnetic systems featuring dimer structures., Comment: 11 pages, 11 figures

The charge-density-wave (CDW) order with macroscopically occupied electrons distorts the underlying lattice and usually causes the softening of the associated phonon mode. However, previous studies demonstrated that the spin-Peierls transition does not always induce an associated phonon softening, but the central-peak scenario applied in the quasi-one-dimensional compound CuGeO$_3$. We generalize the lattice-dynamics studies on the two-dimensional CDW state at van-Hove-singularity (VHS) filling and find that the CDW ordering could develop a central peak at zero frequency while the associated phonon undergoes hardening. The particle-hole scatterings between VHS points give rise to a low-energy increased charge-density susceptibility, and their coupling to the lattice dynamics induces two poles in the Green function for the CDW-associated phonon mode. The zero-frequency pole corresponds to the collective charge-density and phonon coupling mode. The high-frequency one is related to the high-temperature phonon mode that hardens as reaching the CDW transition. Our result may have the potential implication for the recently discovered Kagome metal $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) in which no soft phonon is observed during the CDW transition.

Electronic flat bands serve as a unique platform to achieve strongly-correlated phases. The emergence of a flat band around the Fermi level in 1T-TaS$_2$ in accompany with the development of a $\sqrt{13}\times\sqrt{13}$ charge density wave (CDW) superlattice has long been noticed experimentally, but a transparent theoretical understanding remains elusive. We show that without CDW, the primary feature of the $1\times1$ bands can be fitted by a simple trigonometric function, and physically understood by choosing a rotated $\tilde{t}_{2g}$ basis with the principle axes aligning to the tilted TaS$_6$ octahedron. Using this basis, we trace the band evolution in the $\sqrt{13}\times\sqrt{13}$ superlattice by progressively including different CDW effects. We point out that CDW strongly rehybridizes the three $\tilde{t}_{2g}$ orbitals, which leads to the formation of a well-localized molecular orbital and spawns the flat band.

Mermin-Wagner-Coleman theorem predicts no long-range magnetic order at finite temperature in the two-dimensional (2D) isotropic systems, but a quasi-long-range order with a divergent correlation length at the Kosterlitz-Thouless (KT) transition for planar magnets. As a representative of two-dimensional planar antiferromagnets, single-layer CoPS3 carries the promise of monolayer antiferromagnetic platforms for the ultimately thin spintronics. Here, with the aid of magnetostriction which is sensitive to the local magnetic order, we observe the signatured phonon mode splitting of below TKT in monolayer CoPS3, revealing the presence of quasi-long-range ordering in XY-type antiferromagnet. Moreover, the ratio (J'/J) between the interlayer and intralayer interactions, which characterizes the 2D behaviors, is evaluated to be around 0.03 for the first time. Our results provide an efficient method to detect the quasi-long-range antiferromagnetic ordering in the two-dimensional magnets down to monolayer limit.

Research on topological physics of phonons has attracted enormous interest but demands appropriate model materials. Our {\it ab initio} calculations identify silicon as an ideal candidate material containing extraordinarily rich topological phonon states. In silicon, we identify various topological nodal lines protected by glide mirror or mirror symmetries and characterized by quantized Berry phase $\pi$, which gives drumhead surface states observable from any surface orientations. Remarkably, a novel type of topological nexus phonon is discovered, which is featured by double Fermi-arc-like surface states and distinguished from Weyl phonons by requiring neither inversion nor time-reversal symmetry breaking. Versatile topological states can be created from the nexus phonons, such as Hopf nodal link by strain. Furthermore, we generalize the symmetry analysis to other centrosymmetric systems and find numerous candidate materials, demonstrating the ubiquitous existence of topological phonons in solids. These findings open up new opportunities for studying topological phonons in realistic materials and their influence on surface physics.