Anomalous phonon-mode dependence in polarized Raman spectroscopy of the topological Weyl semimetal TaP

Abstract

Topological Weyl semimetals (WSMs) have attracted widespread interests due to the chiral Weyl fermions and surface Fermi arcs that enable unique optical and transport phenomena. In this work, we present angle-resolved Raman spectroscopy of TaP, a prototypical noncentrosymmetric WSM, for five excitation wavelengths ranging from 364 to 785 nm. The Raman active modes, A 1 , B 1 1 , and B 1 2 modes, exhibit two main unique features beyond the conventional Raman theory. First, the relative intensities of Raman active modes change as a function of the excitation wavelength. Second, angle-resolved polarized Raman spectra show systematic deviation from the Raman tensor theory. In particular, the B 1 1 mode is absent for 633 nm excitation, whereas the B 1 2 modeshows an unusual two-fold symmetry instead of a four-fold symmetry for 488, 532, and 633 nm excitations. These unconventional phenomena are attributed to the interference effect in the Raman process owing to the existence of multiple carrier pockets with almost the same energy but different symmetries.

Anisotropic Fano resonance in the Weyl semimetal candidate LaAlSi

“…On the other hand, the three higher-frequency Raman modes, B 2 1 , B 3 1 , and A 2 1 , have higher intensities for the 364-, 488-, and 235162-2 532-nm excitations. The excitation-wavelength dependence of Raman intensity suggests that the scattering-resonant effect is phonon-mode dependent, which is analogous to the resonant Raman spectra of the WSM TaP [23]. In the present case of LaAlSi, the 532-nm excitation is unique in the sense that all the Raman peaks are observed with relatively strong intensities.…”

“…This is also consistent with the experimental results that the background spectrum is relatively strong for the 532-nm excitation. Since the optical absorption for 532 nm occurs at particular points in the Brillouin zone, the k value of the photoexcited electrons depends on the polarization direction of the electric field in the xy plane [23,40,41], which is a possible reason for the polarization-dependent background spectrum and the Fano effect.…”

“…In contrast, the asymmetric Fano resonance of the B 1 Raman-active phonon in NbAs stemmed from phonon self-energy correction is observed to be temperature independent [8]. In TaP, on the other hand, the anisotropy of the B 1 phonon is strongly affected by electron-phonon coupling as a result of nondegenerate energy bands [23].…”

A Raman probe of phonons and electron–phonon interactions in the Weyl semimetal NbIrTe4

“…Changes in the phonon dispersion, optical reflectivity and Raman scattering are expected 29 , 30 , 32 , 33 . Evidence for the strong coupling between phonons and Weyl Fermions is seen by Fano resonances observed in temperature tuned TaAs (IR reflectivity) 34 , temperature-dependent Raman measurements in NbAs, TaAs and WP2 35 , 36 , and also in Resonant Raman Scattering in TaP 37 . The chiral anomaly in NbAs was seen in phonon measurements using magnetic field-dependent IR reflectivity measurements 38 .…”

First-principles calculations of double resonance Raman spectra for monolayer MoTe$_2$

“…Raman spectroscopy as a versatile probe tool has been widely used for characterizing a broad range of physical properties including superconductive [1], topological [2], ferroelectric [3] properties, magnetic ordering [4] and phase transition [5], electronic interference effect [6], phonon helicity [7]. With the rise of two-dimensional (2D) van der Waals materials, Raman spectroscopy plays † Corresponding to: Yangteng@imr.ac.cn, hhguo@alum.imr.ac.cn an essential role in supplying information on the heterostructure and intrinsically topological properties [8][9][10][11]. In particular, the second-order Raman spectra, which have been widely observed in transition metal dichalcogenides (TMDCs) and other semiconducting 2D materials [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], host non zone-centered two phonons, which are strongly dependent on the laser excitation energy.…”