2, Kavli Institute of Theoretical Sciences, University of the Chinese Academy of Sciences, Beijing, , China
3, Materials Department, UC Santa Barbara, Santa Barbara, California, United States
We make detailed polarized micro-Raman spectroscopy measurements of single nanoflake of the layered ternary compound NbIrTe4. Thin 10 to 100 nm thick layers were exfoliated from single crystals and dispersed onto a silicon substrate. The a and b crystalline axes lay in the plane of the nanoflake with the c-axis perpendicular to the layer. Micro-Raman spectra were taken in the backscattered geometry using both 633 nm and 514 nm laser light with the incoming and outgoing light parallel to the c-axis with the scattered light polarization selected to be both parallel and perpendicular to the incoming laser polarization. Our results indicate strongly anisotropic Raman peaks which are consistent with the broken inversion symmetry of the crystal, which is an essential enabling condition for a Weyl semimetal. The large orthorhombic unit cell of this material has 24 atoms and belongs to the space group Pmn21 (point group c2v) which predicts 69 active Raman modes with A1,2 and B1,2 irreducible representations. Since our excitation is parallel to the c-axis we can only detect the A1,2 modes. Density Functional Theory (DFT) calculations for the A1,2 modes show close correspondence with both the frequency and symmetries of the modes detected in our measurement. The normal mode vibrations for each mode are also determined. All A1 modes have vibrations in the bc-plane of the crystal structure, so they include out-of-plane vibrations, while the A2 modes show atoms which vibrate only along the a-axis of the lattice structure, with in-plane vibrations. Fits to the rotational symmetry of each mode allow us to extract the relative electron-phonon constants for each mode. Some evidence is seen that the electron-phonon interaction is modified for the different energy excitation conditions.
We acknowledge the financial support of the NSF through grants DMR 1507844, DMR 1531373, DMR 1505549 and ECCS 1509706.