[Paper Review] Raman Signatures of Broken Inversion Symmetry and In-plane Anisotropy in Type-II Weyl Semimetal Candidate TaIrTe4

Yinan Liu, Qiangqiang Gu+10 more|arXiv (Cornell University)|Apr 3, 2018

Inorganic Chemistry and Materials4 citations

TL;DR

This study uses angle-resolved polarized Raman spectroscopy to unambiguously confirm broken inversion symmetry in TaIrTe4, a candidate type-II Weyl semimetal, and reveals a strong in-plane electrical anisotropy of up to 200% at 10 K. The method enables nondestructive crystallographic orientation determination, offering a scalable approach for identifying and characterizing other type-II Weyl semimetal candidates.

ABSTRACT

The layered ternary compound TaIrTe4 is an important candidate to host the recently predicted type-II Weyl Fermions. However, a direct and definitive proof of the absence of inversion symmetry in this material, a prerequisite for the existence of Weyl Fermions, has so far remained evasive. Herein, an unambiguous identification of the broken inversion symmetry in TaIrTe4 is established using angle-resolved polarized Raman spectroscopy. Combining with high-resolution transmission electron microscopy, we demonstrate an efficient and nondestructive recipe to determine the exact crystallographic orientation of TaIrTe4 crystals. Such technique could be extended to the fast identification and characterization of other type-II Weyl Fermions candidates. A surprisingly strong in-plane electrical anisotropy in TaIrTe4 thin flakes is also revealed, up to 200% at 10K, which is the strongest known electrical anisotropy for materials with comparable carrier density, notably in such good metals as copper and silver.

Motivation & Objective

• To provide direct experimental evidence for broken inversion symmetry in TaIrTe4, a prerequisite for hosting type-II Weyl fermions.
• To develop a nondestructive method for determining the crystallographic orientation of TaIrTe4 flakes using Raman spectroscopy.
• To investigate the electrical anisotropy in TaIrTe4 thin flakes and quantify its magnitude at low temperatures.
• To establish a generalizable technique applicable to other type-II Weyl semimetal candidates for rapid identification and characterization.

Proposed method

• Angle-resolved polarized Raman spectroscopy is employed to probe the symmetry properties of TaIrTe4 crystals.
• High-resolution transmission electron microscopy (HRTEM) is used to validate the crystal structure and orientation.
• Raman spectra are collected under varying polarization configurations to identify symmetry-breaking features.
• The polarization dependence of Raman modes is analyzed to confirm the absence of inversion symmetry.
• Electrical transport measurements are performed on thin flakes to quantify in-plane anisotropy.
• A correlation between Raman signatures and crystallographic orientation is established to enable nondestructive orientation assignment.

Experimental results

Research questions

• RQ1 Does TaIrTe4 exhibit broken inversion symmetry, as required for type-II Weyl fermions?
• RQ2 Can angle-resolved polarized Raman spectroscopy be used to determine the crystallographic orientation of TaIrTe4 flakes without destructive methods?
• RQ3 What is the magnitude of in-plane electrical anisotropy in TaIrTe4 at low temperatures?
• RQ4 How does the observed anisotropy compare to other good metals with similar carrier densities?

Key findings

• Broken inversion symmetry in TaIrTe4 is unambiguously confirmed through the polarization-dependent Raman response, which shows selection rules inconsistent with centrosymmetric structures.
• The combination of Raman spectroscopy and HRTEM enables nondestructive and accurate determination of crystallographic orientation in TaIrTe4 flakes.
• A strong in-plane electrical anisotropy of up to 200% is observed in TaIrTe4 thin flakes at 10 K, the highest reported for materials with comparable carrier density.
• This anisotropy is significantly larger than in conventional metals like copper and silver, indicating strong electron-near-fs symmetry effects.
• The Raman-based orientation mapping technique is robust and transferable to other layered type-II Weyl semimetal candidates.
• The study establishes a fast, nondestructive protocol for identifying and characterizing Weyl semimetal candidates using optical and structural probes.