Raman signatures of inversion symmetry breaking and structural phase transition in type-II Weyl semimetal MoTe2

Zhang, Kenan; Bao, Changhua; Gu, Qiangqiang; Ren, Xiao; Zhang, Haoxiong; Deng, Ke; Wu, Yang; Li, Yuan; Feng, Ji; Zhou, Shuyun

doi:10.1038/ncomms13552

Cited by 130 publications

(170 citation statements)

References 36 publications

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“…According to the previous works3334, the evolution of the Raman mode at near 130 cm −1 is a direct verification of the structural phase transition of MoTe 2 from high temperature β to low temperature Td phase. In addition, there are different Raman signals for β- and Td-phase at the range of 150~300 cm −1 .…”

Section: Resultsmentioning

confidence: 70%

Composition and temperature-dependent phase transition in miscible Mo1−xWxTe2 single crystals

Cao

Xiao

et al. 2017

Sci Rep

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Transition metal dichalcogenides (TMDs) WTe2 and MoTe2 with orthorhombic Td phase, being potential candidates as type-II Weyl semimetals, are attracted much attention recently. Here we synthesized a series of miscible Mo1−xWxTe2 single crystals by bromine vapor transport method. Composition-dependent X-ray diffraction and Raman spectroscopy, as well as composition and temperature-dependent resistivity prove that the tunable crystal structure (from hexagonal (2H), monoclinic (β) to orthorhombic (Td) phase) can be realized by increasing W content in Mo1−xWxTe2. Simultaneously the electrical property gradually evolves from semiconductor to semimetal behavior. Temperature-dependent Raman spectroscopy proves that temperature also can induce the structural phase transition from β to Td phase in Mo1−xWxTe2 crystals. Based on aforementioned characterizations, we map out the temperature and composition dependent phase diagram of Mo1−xWxTe2 system. In addition, a series of electrical parameters, such as carrier type, carrier concentration and mobility, have also been presented. This work offers a scheme to accurately control structural phase in Mo1−xWxTe2 system, which can be used to explore type-II Weyl semimetal, as well as temperature/composition controlled topological phase transition therein.

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Section: Resultsmentioning

confidence: 70%

Composition and temperature-dependent phase transition in miscible Mo1−xWxTe2 single crystals

Cao

Xiao

et al. 2017

Sci Rep

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“…For cooling below 300 K, the intensity of peak E grows gradually with decreasing temperature, while the intensities of A and D slightly decrease. None of these modes, however, display large, abrupt changes characteristic of a first-order − phase transition as in the bulk crystal at ~250 K [31,32]. For heating close to 400 K, the intensities of all three peaks decrease, which could indicate the beginning of a transition into another phase different from both the and phases.…”

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confidence: 99%

“…We have followed the convention used by Zhang et al and labeled the phase peaks as A, D, and E in order of increasing energy [32]. The lower energy A peak has been assigned to a shear mode, while the activated as a consequence of inversion symmetry breaking in the orthorhombic state [31,32]. has been observed in other TMDC materials and could be due to a reduced interlayer force constant in few layer systems [33][34][35].…”

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confidence: 99%

“…In order to discriminate between the two, we have performed temperature dependent Raman spectroscopy, which has been demonstrated to clearly distinguish between the and phases of bulk MoTe2 [30][31][32]. In our optical microscopy setup, we focused a linearly polarized, 532 nm wavelength laser through either a 50 (for samples in the cryostat) or a 100 (for samples outside cryostat) objective, yielding a ~2 µm or ~1 µm spot size, respectively.…”

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confidence: 99%

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Dimensionality-driven orthorhombic MoTe2 at room temperature

Zhong

Kim

et al. 2018

Phys. Rev. B

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Abstract:We use a combination of Raman spectroscopy and transport measurements to study thin flakes of the type-II Weyl semimetal candidate MoTe2 protected from oxidation. In contrast to bulk crystals, which undergo a phase transition from monoclinic to the inversion symmetry breaking, orthorhombic phase below ~250 K, we find that in moderately thin samples below ~12 nm, a single orthorhombic phase exists up to and beyond room temperature. This could be due to the effect of c-axis confinement, which lowers the energy of an out-of-plane hole band and stabilizes the orthorhombic structure. Our results suggest that Weyl nodes, predicated upon inversion symmetry breaking, may be observed in thin MoTe2 at room temperature. Main text:

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“…1 (b)). Above 250 K the 1T' structure transforms to the centrosymmetric monoclinic (space group P12/m1) 1T" structure [16] Raman spectroscopy and resistivity measurements indicate a structural phase transition at T* ∼ 257.5 K [18]. Details about the crystals growth and their characterization can be found in the Supplementary Information [19].…”

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confidence: 99%

Enhanced ultrafast relaxation rate in the Weyl semimetal phase of MoTe2 measured by time- and angle-resolved photoelectron spectroscopy

et al. 2017

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MoTe2 has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time-and angle-resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points in the electron dynamics above the Fermi level EF, by comparing the ultrafast response of MoTe2 in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound WTe2 is slower and temperature-independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of MoTe2.The recent discovery of Weyl fermions as low-energy quasiparticles in TaAs [1][2][3] and other related compounds [4,5] has boosted the interest in topological semimetals (TSMs) [6]. Type-II Weyl semimetals (WSMs) are a novel class of materials that host fermions violating Lorentz invariance [7]. These quasiparticles are realized in the strongly tilted cones that form in momentum space around special Weyl points (WPs) where the valence and conduction bands touch. WTe 2 [7] and MoTe 2 [8-10] have been proposed as possible type-II WSMs. While the topological phase of WTe 2 is still under debate, the existence of the WSM phase in the low temperature non-centrosymmetric structure of MoTe 2 is supported by the observation of surface Fermi arcs [11][12][13][14][15]. The Weyl points, however, are located above the Fermi level E F and this hinders a direct observation by conventional angle-resolved photoelectron spectroscopy (ARPES). To circumvent this difficulty, one would need a probe that is sensitive to the presence/absence of small energy band gaps in the unoccupied density of states.In this Letter we show that time-and angle-resolved photoelectron spectroscopy (tr-ARPES) can provide such information. We report a shortening of the relaxation time in the gapless type-II Weyl phase of MoTe 2 , which reflects the enhanced interband scattering from the conduction band (CB) to the valence band (VB) mediated by electron -electron scattering along the Weyl cone. These scattering processes are active only when the band gap is locally closed. This conclusion is supported by the observation of a slower, temperatureindependent dynamics in WTe 2 , indicative of a local direct band gap, which acts as an effective bottleneck for the electron relaxation.MoTe 2 is a layered material. It can be cleaved to expose large flat (001) terraces, ideal for ARPES studies. Figure 1 illustrates the WSM phase, which is only realized in the lowtemperature orthorhombic (space group Pmn2 1 ) structurehereafter referred to as the 1T' phase [17] -where inversion symmetry is broken. The crystal structure is sketched in Fig. 1 (a), along wi...

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Raman signatures of inversion symmetry breaking and structural phase transition in type-II Weyl semimetal MoTe2

Cited by 130 publications

References 36 publications

Composition and temperature-dependent phase transition in miscible Mo1−xWxTe2 single crystals

Composition and temperature-dependent phase transition in miscible Mo1−xWxTe2 single crystals

Dimensionality-driven orthorhombic MoTe2 at room temperature

Enhanced ultrafast relaxation rate in the Weyl semimetal phase of MoTe2 measured by time- and angle-resolved photoelectron spectroscopy

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