Comparative Raman study of Weyl semimetals TaAs, NbAs, TaP and NbP

TL;DR

This study compares the Raman spectra of Weyl semimetals TaAs, NbAs, TaP, and NbP, revealing how atomic vibrations evolve with composition and pressure, and confirming theoretical predictions about phonon mode atomic involvement.

Contribution

It provides experimental Raman data on phonon modes in Weyl semimetals, clarifying atomic contributions and their evolution with chemical pressure, which was previously predicted by first-principles calculations.

Findings

Certain phonon modes involve specific atoms as predicted.

Most modes become harder with chemical pressure, except A1 mode.

A1 mode energy decreases under chemical pressure, contrasting with external pressure effects.

Abstract

We report a comparative polarized Raman study of Weyl semimetals TaAs, NbAs, TaP and NbP. The evolution of the phonon frequencies with the sample composition allows us to determine experimentally which atoms are mainly involved for each vibration mode. Our results confirm previous first-principles calculations indicating that the A$_1$, B$_1(2)$, E$(2)$ and E$(3)$ modes involve mainly the As(P) atoms, the B$_1(1)$ mode is mainly related to Ta(Nb) atoms, and the E$(1)$ mode involves both kinds of atoms. By comparing the energy of the different modes, we establish that the B$_1(1)$, B$_1(2)$, E$(2)$ and E$(3)$ become harder with chemical pressure increasing. This behavior differs from our observation on the A$_1$ mode, which decreases in energy, in contrast to its behavior under external pressure.