>> Type-II Weyl and Dirac semimetals  [Back To Top]

>> Experimental realization of type-II Weyl semimetal, topological phase transition and superconductivity  [Back To Top]

  Three dimensional Weyl and Dirac semimetals host the condensed matter physics counterpart of relativistic Weyl or Dirac fermions. In condensed matter physics, the low energy excitations are quasiparticles and new types of Weyl or Dirac fermions that do not obey the Lorentz invariance can also exist. We reported the first experimental observation of type-II Weyl fermions in MoTe₂. The topological surface states are observed by ARPES and quasiparticle interference (Nat. Phys. 12, 1105-1110 (2016).). Moreover, a phase transition from 1T’ to T_d phase is revealed by Raman spectroscopy, suggesting a topological phase transition (Nat. Commun. 7, 13552 (2016).).

  Recently, we have succeeded in intercalating the cations from ionic liquid into MoTe₂ and WTe₂ to form inorganic-organic hybrid materials (Sci. Bull. 65, 188 (2020)). The intercalation leads to an expanding interlayer spacing with decoupled MoTe₂ and WTe₂ layers, and the electronic structure is expected to change from a three-dimensional Weyl semimetal to many layers of two-dimensional quantum spin Hall insulators. More importantly, such intercalation can also induce superconductivity with a transition temperature of 7.0 K for the intercalated MoTe₂ and 2.3 K for the intercalated WTe₂.

For more details, see references:
1. “Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2”, Nat. Phys. 12, 1105-1110 (2016).
2. “Raman signatures of inversion symmetry breaking and structural phase transition in type-II Weyl semimetal MoTe2”, Nat. Commun. 7, 13552 (2016).
3. “Enhancement of superconductivity in organic-inorganic hybrid topological materials”, Sci. Bull. 65, 188 (2020).

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>> Type-II Dirac semimetals  [Back To Top]

• Type-II Dirac fermions in PtTe₂ and PtSe₂

  We extend the concept of type-II semimetal to include Dirac fermions. Moreover, we found that PtTe₂ and PtSe₂ host type-II Dirac fermions with highly tilted Dirac cones along the out-of-plane direction, and the Dirac point is protected by the crystal symmetry. This is confirmed by ARPES measurements of PtTe₂ (Nat. Commun. 8, 257 (2017).) and PtSe₂ (Phys. Rev. B 96, 125102 (2017).) as well as first principles calculations (Phys. Rev. B 94, 121117 (2016).).

For more details, see references:
1. “Lorentz-violating type-II Dirac fermions in transition metal dichalcogenide PtTe₂”, Nat. Commun. 8, 257 (2017).
2. “Experimental evidence of type-II Dirac fermions in PtSe₂”, Phys. Rev. B 96, 125102 (2017).
3. "Type-II Dirac Fermions in the Transition Metal Dichalcogenide PtSe₂ Class", Phys. Rev. B 94, 121117 (2016).

 [To overview]

• Atomically thin PtSe₂, PtTe₂ films and spin-layer locking by local Rashba effect

  In the past few years, we have applied a few methods for growing atomically thin PtSe₂ and PtTe₂ films, including direct selenization (Nano Lett. 15, 4013 (2015).), MBE (2D Mater. 4, 045015 (2017)., Sci. Bull. 34, 1044 (2019).), and a modified selenization method for large scale films and phase control (Nano Res. 14, 1663 (2021).). Moreover, the evolution of electronic structure with sample thickness is revealed by ARPES measurements, and a transition of electronic structures from bulk topological semimetal to a 2D semiconductor (2D Mater. 4, 045015 (2017)., Sci. Bull. 34, 1044 (2019).).

  Atomically thin PtSe₂ and PtTe₂ films exhibit interesting spin textures. For example, in monolayer PtSe₂ film which is centrosymmetric, our Spin-ARPES measurements still reveal the interesting helical spin texture, which is induced by the new mechanism called local Rashba (R-2) effect. Different from the conventional Rashba effect where different spin states are split in energy, here the spins are degenerate in energy, while locally attached to the top and bottom layer, namely, there is spin-layer locking (Nat. Commun. 8, 14216 (2017).). This makes it potentially useful for applications in electric-field tunable spintronics.

For more details, see references:
1. “Monolayer PtSe₂, a new semiconducting transition-metal-dichalcogenide, epitaxially grown by direct selenization of Pt”, Nano Lett. 15, 4013 (2015).
2. “High quality atomically thin PtSe₂ films grown by molecular beam epitaxy”, 2D Mater. 4, 045015 (2017).
3. “Crossover from 2D metal to 3D Dirac semimetal in metallic PtTe₂ films with local Rashba effect”,Sci. Bull. 34, 1044 (2019).
4. “Growth of PtTe, PtTe₂ and PtSe₂ thin films on a wide range of substrates”, Nano Res. 14, 1663 (2021).
5.“Direct observation of spin-layer locking by local Rashba effect in monolayer semiconducting PtSe₂”, Nat. Commun. 8, 14216 (2017).

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