Tunable Van Hove Singularity without Structural Instability in Kagome Metal CsTi_{3}Bi_{5}

In kagome metal CsV_{3}Sb_{5}, multiple intertwined orders are accompanied by both electronic and structural instabilities. These exotic orders have attracted much recent attention, but their origins remain elusive. The newly discovered CsTi_{3}Bi_{5} is a Ti-based kagome metal to parallel CsV_{3}Sb_{5}. Here, we report angle-resolved photoemission experiments and first-principles calculations on pristine and Cs-doped CsTi_{3}Bi_{5} samples. Our results reveal that the van Hove singularity (vHS) in CsTi_{3}Bi_{5} can be tuned in a large energy range without structural instability, different from that in CsV_{3}Sb_{5}. As such, CsTi_{3}Bi_{5} provides a complementary platform to disentangle and investigate the electronic instability with a tunable vHS in kagome metals.

Straight and twisted open nodal-line phonon states in the CaI2 family of materials

Based on first-principles calculations and symmetry analysis, we propose that trigonal CaI₂ with the space group P3̄m1 possesses straight and twisted open nodal-line phonon states with linear dispersion. The symmetry analysis indicates that joint symmetry PT and rotational symmetry C_3z protect the straight nodal lines along Γ-A and K-H while PT and mirror symmetry M₀₁₀ (M₁₁₀) maintain the twisted nodal lines that traverse Γ-M (Γ-K) and A-L (A-H). The calculated π Berry phase suggests that all the nodal lines are nontrivial and the corresponding drumhead-like surface states are clearly visible in the observation window, which is less than 6 THz, suggesting a significant chance for them to be measured using meV-resolution inelastic X-ray scattering. The distribution of the nodal lines in the Brillouin zone is also confirmed by the phononic tight-binding model. Furthermore, the isostructural compounds MgBr₂ and MgI₂ show similar phonon spectra and topological nontrivial surface states. This work provides promising candidates for investigating straight and twisted open nodal-line phonon states in a single material, which will facilitate future experimental observation.

Investigations on the electron paramagnetic resonance parameters for Mn2+ in the ABF3 fluoroperovskites

The electron paramagnetic resonance (EPR) parameters (g factor, the hyperfine structure constant A and the superhyperfine parameters A' and B') for Mn(2+) in the fluoroperovskites ABF(3) (A=K and Cs; B=Zn, Mg, Cd and Ca) are theoretically investigated from the perturbation formulas of these parameters for a 3d(5) ion under ideal octahedra. In the above treatments, not only the crystal-field mechanism but also the charge transfer mechanism is considered uniformly on the basis of the cluster approach. The theoretical EPR parameters are in good agreement with the experimental data. The charge transfer contribution to the g-shift Δg (≈g-g(s), where g(s)≈2.0023 is the spin-only value) is opposite (positive) in sign and comparable in magnitude to the crystal-field one. Nevertheless, the charge transfer contribution to the hyperfine structure constant shows the same sign and about 10% that of the crystal-field one. So, the conventional argument that the charge transfer contributions to the zero-field splittings are negligible for 3d(5) ions under low symmetrically distorted fluorine octahedra is proved no longer valid for the Δg analysis of ABF(3):Mn(2+) in view of the dominant second-order charge transfer perturbation terms. The unpaired spin densities of the fluorine 2s, 2p σ and 2p π orbitals are determined from the quantitative dependences upon the related molecular orbital coefficients, rather than obtained by fitting the observed superhyperfine parameters in the previous works.