Electronic structures of intercalation complexes of the layered compound 2H-TaS2

Computational Methods for Charge Density Waves in 2D Materials

Two-dimensional (2D) materials that exhibit charge density waves (CDWs)-spontaneous reorganization of their electrons into a periodic modulation-have generated many research endeavors in the hopes of employing their exotic properties for various quantum-based technologies. Early investigations surrounding CDWs were mostly focused on bulk materials. However, applications for quantum devices require few-layer materials to fully utilize the emergent phenomena. The CDW field has greatly expanded over the decades, warranting a focus on the computational efforts surrounding them specifically in 2D materials. In this review, we cover ground in the following relevant theory-driven subtopics for TaS2 and TaSe2: summary of general computational techniques and methods, resulting atomic structures, the effect of electron-phonon interaction of the Raman scattering modes, the effects of confinement and dimensionality on the CDW, and we end with a future outlook. Through understanding how the computational methods have enabled incredible advancements in quantum materials, one may anticipate the ever-expanding directions available for continued pursuit as the field brings us through the 21st century.

Surface-Limited Superconducting Phase Transition on 1 T-TaS2

Controlling superconducting phase transition on a two-dimensional (2D) material is of great fundamental and technological interest from the viewpoint of making 2D resistance-free electronic circuits. Here, we demonstrate that a 1 T-to-2 H phase transition can be induced on the topmost monolayer of bulk (<100 nm thick) 1 T-TaS₂ by thermal annealing. The monolayer 2 H-TaS₂ on bulk 1 T-TaS₂ exhibits a superconducting transition temperature ( T_c) of 2.1 K, which is significantly enhanced compared to that of bulk 2 H-TaS₂. Scanning tunneling microscopy measurements reveal a 3 × 3 charge density wave (CDW) in the phase-switched monolayer at 4.5 K. The enhanced T_c is explained by the suppressed 3 × 3 CDW and a charge-transfer doping from the 1 T substrate. We further show that the monolayer 2 H-TaS₂ could be switched back to 1 T phase by applying a voltage pulse. The observed surface-limited superconducting phase transition offers a convenient way to prepare robust 2D superconductivity on bulk 1 T-TaS₂ crystal, thereby bypassing the need to exfoliate monolayer samples.

Lattice-matched transition metal disulfide intergrowths: the metallic conductors Ag2Te(MS2)3 (M = V, Nb)

We present new chalcogenide compounds, Ag2Te(MS2)3 (M = V, Nb), built up of alternating planes of [MS2] and [Ag2Te]. The Ag and Te atoms are linearly coordinated by S atoms in the [MS2] layers and held in place by covalent interactions. Structural polymorphism was found by single crystal X-ray diffraction studies, where long-range ordering or disorder of the Ag and Te atoms within the hexagonal planar [Ag2Te] layer yielded two distinct crystal forms. When the Ag and Te atoms are ordered, the two isostructural compounds crystallize in the non-centrosymmetric P62m space group, with a = 5.5347(8) Å, c = 8.0248(16) Å, and V = 212.89(6) Å(3) for α-Ag2Te(VS2)3 and a = 5.7195(8) Å, c = 8.2230(16) Å, and V = 232.96(6) Å(3) for α-Ag2Te(NbS2)3. For the occupationally disordered Ag/Te arrangement, a subcell of the ordered phase that crystallizes in the non-centrosymmetric P6m2 space group, with a = 3.2956(6) Å (=a(a)/(3)(1/2)), c = 8.220(2) Å, and V = 77.31(3) Å(3) for β-Ag2Te(VS2)3, was identified. Furthermore, pair distribution function analysis revealed local distortions in the [Ag2Te] layer. Band structure calculations at the density functional theory level were carried out to investigate the electronic structure of Ag2Te(MS2)3. Electronic transport measurements on Ag2Te(MS2)3 show that they exhibit p-type metallic behavior. Thermal analyses and temperature-dependent powder X-ray diffraction studies were focused on the stability and transformation/decomposition of the Ag2Te(MS2)3 phases. Magnetic susceptibility data are also reported. The new intercalated Ag2Te(MS2)3 system features a unique hypervalent Te with a three-center, four-electron bonding environment isoelectronic to that found in I3(-).

Probing the chemical and electronic properties of the core-shell architecture of transition metal trisulfide nanoribbons

Ultraviolet and X-ray photoelectron spectroscopies are used to probe the chemical and electronic structure of an amorphous, 2-20 nm-thick shell that encases the crystalline core in core-shell nanoribbons of TaS(3). The shell is chemically heterogeneous, containing elemental sulfur and a with a notable (S(2))(2-) deficiency over the crystalline TaS(3) core. We find nanoribbon stability to be substrate-dependent; whilst the ribbons are stable on the native oxide of a silicon surface, mass transport of sulfur species between the amorphous shell and a gold substrate leads to a significant change in the electronic properties of the nanomaterials. Our observations may have general implications for the incorporation of nanostructured transition metal chalcogenides into electronic devices.

Anisotropic intermediate coupling superconductivity in Cu(0.03)TaS(2)

The anisotropic superconducting state properties in Cu(0.03)TaS(2) have been investigated by magnetization, magnetoresistance and specific heat measurements. They clearly show that Cu(0.03)TaS(2) undergoes a superconducting transition at T(C) = 4.03 K. The obtained superconducting parameters demonstrate that Cu(0.03)TaS(2) is an anisotropic type-II superconductor. Combining specific heat jump ΔC/γ(n)T(C) = 1.6(4), gap ratio 2Δ/k(B)T(C) = 4.0(9) and the estimated electron-phonon coupling constant λ∼0.68, the superconductivity in Cu(0.03)TaS(2) is explained within the intermediate coupling BCS scenario. First-principles electronic structure calculations suggest that copper intercalation of 2H-TaS(2) causes a considerable increase of the Fermi surface volume and the carrier density, which suppresses the CDW fluctuation and favors the raise of T(C).

Novel mechanism of a charge density wave in a transition metal dichalcogenide

The charge density wave (CDW) is usually associated with Fermi surfaces nesting. We here report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide, where the two essential ingredients of the CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure. Namely, the CDW gap is only partially open, and charge density wave vector match is fulfilled through participation of states of the large Fermi patch, while the straight Fermi surface sections have secondary or negligible contributions.