No abstract

The tetrachalcogenide TaTe4 is known as an excellent example of a charge-density wave (CDW) system that has a commensurately modulated structure at room temperature. Using density function perturbation theory, we find that the unmodulated phase of TaTe4 has a giant Kohn anomaly at room temperature, which manifests itself as softened phonon modes at the CDW vector (1/2a*,1/2b*,1/3c*). Interestingly, after the application of 8 GPa hydrostatic pressure, this CDW instability can be effectively suppressed and disappears at room temperature. By studying the topology of the Fermi surface and the phonon linewidth, we show that the Kohn anomaly in TaTe4 is driven by a large electron–phonon coupling coefficient at the CDW vector and not by Fermi surface nesting.

We investigate the Kondo effect of a spin-1/2 magnetic impurity in a topological nodal loop semimetal, in which band touchings form a nodal loop. The Fermi surface of a nodal loop semimetal is a torus or a drum-like structure, which is determined by chemical potential. When the chemical potential μ lies at the nodal loop ([Formula: see text]), the magnetic impurity and the conduction electrons form bound states only if their coupling exceeds a critical value. As the chemical potential is tuned away from the nodal loop, the Fermi surface becomes a torus or drum-like structure and the impurity and the host material always favor a bound state due to the finite density of state. Due to the anisotropic dispersion relationship in the energy band, the spatial spin-spin correlations [Formula: see text]([Formula: see text]) are of power-law decay with the decay rates proportional to [Formula: see text] and [Formula: see text] in different directions, respectively. The product [Formula: see text] and [Formula: see text] oscillates in coordinate space and the period is enhanced gradually as the Fermi surface evolves from a torus surface into a drum-like structure.