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Transition metal dichalcogenide MoTe2 is an important candidate for realizing the newly predicted type-II Weyl fermions, for which the breaking of the inversion symmetry is a prerequisite. Here we present direct spectroscopic evidence for the inversion symmetry breaking in the low-temperature phase of MoTe2 by systematic Raman experiments and first-principles calculations. We identify five lattice vibrational modes that are Raman-active only in the low-temperature noncentrosymmetric structure. A hysteresis is also observed in the peak intensity of inversion symmetry-activated Raman modes, confirming a temperature-induced structural phase transition with a concomitant change in the inversion symmetry. Our results provide definitive evidence for the low-temperature noncentrosymmetric Td phase from vibrational spectroscopy, and suggest MoTe2 as an ideal candidate for investigating the temperature-induced topological phase transition.

Inducing or enhancing superconductivity in topological materials is an important route toward topological superconductivity. Reducing the thickness of transition metal dichalcogenides (e.g. WTe 2 and MoTe 2 ) has provided an important pathway to engineer superconductivity in topological matters; for instance, emergent superconductivity with T c ∼ 0.82 K was observed in monolayer WTe 2 1, 2 which also hosts intriguing quantum spin Hall effect 3 , although the bulk crystal is nonsuperconducting. However, such monolayer sample is difficult to obtain, unstable in air, and with extremely low T c , which could pose a grand challenge for practical applications. Here we report an experimentally convenient approach to control the interlayer coupling to achieve tailored topological properties, enhanced superconductiv-1 arXiv:1911.02228v1 [cond-mat.mtrl-sci]

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We report the electronic structure of CuTe with a high charge density wave (CDW) transition temperature Tc = 335 K by angle-resolved photoemission spectroscopy (ARPES). An anisotropic charge density wave gap with a maximum value of 190 meV is observed in the quasi-one-dimensional band formed by Te px orbitals. The CDW gap can be filled by increasing temperature or electron doping through in situ potassium deposition. Combining the experimental results with calculated electron scattering susceptibility and phonon dispersion, we suggest that both Fermi surface nesting and electron-phonon coupling play important roles in the emergence of the CDW. Low dimensional materials have the tendency to form new orderings with novel physics. Charge density wave (CDW) [1] is one of the most fundamental phenomena and has been discovered in various transition metal dichalcogenides (TMDCs) [2, 3] (e.g. TaSe 2 [4], TaS 2 [5] and NbSe 2 [6]) and rare-earth tritellurides RTe 3 [7, 8](R = rare earth elements, e.g. CeTe 3 [9] and SmTe 3 [10]).In the CDW state [11,12], an instability of the metallic Fermi surface involving electron-phonon interaction [13] or electron-electron scattering [10,14] leads to a modulation of the lattice coupled to changes in the conduction electron density in the real space with a period λ c . Such modulation induces an energy gap at the Fermi wave vector k F =π/λ c , thereby lowering the electronic energy of the occupied electronic states in the CDW phase.Copper-based chalcogenides form a large family with diverse properties and potential applications in solidstate devices such as thermoelectrics, batteries, and photovoltaics [15][16][17]. Among these materials, the stoichiometric compound CuTe exhibits a modulation of Te atoms, forming dimers and trimers at low temperature, suggesting a CDW transition [18]. Different from the quasi-2D TMDCs and RTe 3 , Te atoms in CuTe form quasi-1D chains, and therefore, CuTe can be viewed as a quasi-1D CDW system with Peierls-like distortion [19]. So far, the electronic structure of CuTe, in particular the electronic structure signature of the CDW (e.g. CDW gap, nesting vector etc.) as well as the underlying mechanism for CDW, has remained missing.Here we provide direct experimental evidence for the CDW in CuTe by angle-resolved photoemission spectroscopy (ARPES) measurements. An anisotropic CDW gap with a maximum value of 190 meV is observed in the quasi-1D band formed by Te p x orbitals through a nesting wave vector q x = 0.4 a * which matches well with previous diffraction results [18]. By increasing the tem-perature or electron doping through in situ potassium deposition, the CDW gap is gradually filled and eventually disappears. These experimental results, combined with analysis from the calculated electron scattering susceptibility and phonon spectrum, suggest that both electronphonon coupling and electron-electron interaction play important roles in the CDW formation in CuTe.High quality CuTe single crystals were grown by selfflux method. Surface-sensitive ARPES me...

SnSe, a group IV-VI monochalcogenide with layered crystal structure similar to black phosphorus, has recently attracted extensive interests due to its excellent thermoelectric properties and potential device applications. Experimental electronic structure of both the valence and conduction bands is critical for understanding the effects of hole versus electron doping on the thermoelectric properties, and to further reveal possible change of the band gap upon doping. Here, we report the multivalley valence bands with a large effective mass on semiconducting SnSe crystals and reveal single-valley conduction bands through electron doping to provide a complete picture of the thermoelectric physics. Moreover, by electron doping through potassium deposition, the band gap of SnSe can be widely tuned from 1.2 eV to 0.4 eV, providing new opportunities for tunable electronic and optoelectronic devices.

Manipulating the strength of the interlayer coupling is an effective strategy to induce intriguing properties in layered materials. Recently, enhanced superconductivity has been reported in Weyl semimetal MoTe2 and WTe2 via ionic liquid cation intercalation. However, how the superconductivity enhancement depends on the interlayer interaction still remains elusive. Here by inserting ionic liquid cations with different sizes into MoTe2 through this strategy, we are able to tune the interlayer spacing of the intercalated MoTe2 samples and reveal the dependence of superconducting transition temperature T c on the interlayer spacing. Our results show that T c increases with the interlayer spacing, suggesting that the weakened interlayer coupling plays an important role in the superconductivity. Interestingly, the intercalation induced superconductivity shows a high Ginzburg-Landau anisotropy, which suggests a quasi-two-dimensional nature of the superconductivity where the adjacent superconducting layers are coupled through Josephson tunneling.

The weak van der Waals interaction between adjacent layers of quasi-two-dimensional materials provides opportunities for inserting intercalants to induce novel properties distinct from the host materials. Here we report induced superconductivity in an intercalated SnSe2 crystal by using a new type of intercalants—organic cations from ionic liquids, [C2MIm]+ and [DEMB]+. The intercalation of both cations increases the interlayer spacing and leads to induced superconductivity with T c of 7.1 and 6.9 K and a large superconducting anisotropy. Angle-resolved photoemission spectroscopy and Hall measurements reveal the importance of electron doping by the cations in the induced superconductivity, and the interlayer expansion and electric polarization of the cations in the large anisotropy. Our work reports induced superconductivity in an intercalated material with new intercalants which contribute both charge carriers and interlayer expansion, and provides a new pathway to the manipulation of superconductivity in layered materials.