Raman Signatures of Broken Inversion Symmetry and In‐Plane Anisotropy in Type‐II Weyl Semimetal Candidate TaIrTe₄

Abstract

The layered ternary compound TaIrTe 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 TaIrTe is established using angle-resolved polarized Raman spectroscopy. Combining with high-resolution transmission electron microscopy, an efficient and nondestructive recipe to determine the exact crystallographic orientation of TaIrTe crystals is demonstrated. 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 TaIrTe thin flakes is also revealed, up to 200% at 10 K, which is the strongest known electrical anisotropy for materials with comparable carrier density, notably in such good metals as copper and silver.

Gate tunable giant anisotropic resistance in ultra-thin GaTe

“…In general, anisotropic ratio in those devices can be gate tuned from a few times, to several hundreds or even thousands folds, which is much higher than other 2D systems reported. [1,2,4,6,17] Since Γ a in ultra-thin GaTe is rather weak at relatively higher hole doping (say, in the V g = −60 V limit), and becomes significantly enhanced close to the VBM in the vicinity of V g = −30 V, it more or less shows a better electron collimation at higher Γ a values, i.e., electrons tend to flow preferably along a certain direction at those conditions. Considering the deformation potential theory only applied to the band edge (VBM or CBM) and its inability to take care the gate-tunability on electrical transport, [18,19] we calculate IV curve and its gating dependence by combining density functional theory (DFT) calculation with the non-equilibrium Green's function (NEGF) method [20] (See method part), and show calculated field effect IV data at V ds =0.5 V along two perpendicular crystalline directions in Fig.S20.…”

“…Indeed, in the emerging 2D materials, electrical anisotropy has been one of the focuses in recent experimental efforts. [1][2][3][4][5][6][7][8] However, key understandings of the in-plane anisotropic resistance in low-symmetry 2D materials, as well as demonstrations of model devices taking advantage of it, have proven difficult. Here, we show that, in few-layered semiconducting GaTe, electrical conductivity along x and y directions of the 2D crystal can be gate tuned from a ratio of less than one order to as large as 10 3 .…”

“…[11] Recently, low-symmetry two dimensional (2D) materials have attracted significant attentions because of the potential applications of in-plane anisotropic nanoelectronics. [1][2][3][4][5][6][7][8] For example, ultra-thin black phosphorous flake showed an in-plane anisotropic conductance reaching a ratio σ a /σ b of about 1.5, which in principle can be a direction-sensitive sensor. [1] ReS 2 was reported to be another candidate for anisotropic 2D field effect transistor, which exhibited a σ a of almost ten times the value of σ b .…”

Emerging in‐plane anisotropic two‐dimensional materials

“…25,26 Since then, the polarizationdependent absorption, [26][27][28][29][30][31] Raman spectroscopy, [32][33][34] photoluminescence, [35][36][37] and the in-plane anisotropic electronic, 26,38,39 thermal, 40,41 and mechanical 42 properties of BP have been intensively studied. Ignited by BP, other inplane anisotropic 2D materials, from semimetals (T d WTe 2 , 43 1 T' MoTe 2,44,45 and ZrTe 5 46 TaIrTe 4 , 47,48 ) to semiconductors (group IV monochalcogenides, 49 Ta 2 NiS 5 , 50 GaTe, 51 group IVB trichalcogenides, 52 group IV-group V compounds, 53 70 TlSe, 71 ) (Table 1) and so on were explored. After developing for few years, the in-plane anisotropic 2D materials have becoming one of the most attractive research interests in the scientific field.…”

Mo6S3Br6: An Anisotropic 2D Superatomic Semiconductor

“…[6,[30][31][32][33][34] Figure 4a shows Raman spectra for parallel polarization (PP, top) and cross polarization (CP, bottom) configurations. [6,[30][31][32][33][34] Figure 4a shows Raman spectra for parallel polarization (PP, top) and cross polarization (CP, bottom) configurations.…”