The role of charge density waves in structural transformations of 1T TaS₂

Atomistic origin of an ordered superstructure induced superconductivity in layered chalcogenides

“…Since intensity of an atomic column in a HAADF imaging mode is proportional to Z 1.7 (Z is atomic number) 32 , the brighter spots represent the Ta columns (larger circles), whereas the darker ones represent the S/Se columns (smaller circles). Evidently, Ta atoms (marked by pink solid circles) are almost located at identical sites in all cases, forming the star-of-David clusters, where stars are interlocked by forming a triangular superlattice with a O13 Â O13 periodicity 7,8,13 , in line with the crystal structure (Fig. 1a,c).…”

“…Current understanding of 1T-TaS 2 and its associated phases, which is based mainly on electronic states, can be summarized in three aspects: (1) the CCDW phase of 1T-TaS 2 can be attributed structurally to the fact that Ta atoms are displaced to form star-of-David clusters, wherein the 12 in-plane Ta atoms point towards a central Ta atom, interlocking each cluster to form a triangular superlattice 7,8,13 . However, the 12 electrons occupy the states below the distortion-induced gap and the thirteenth one dominates the states above the gap, resulting in a Mott insulating state that accounts for the high resistivity of the CCDW phase [14][15][16] , (2) the neighbouring NCCDW phase contains the star-of-David clusters as well albeit that they are less homogeneously arranged 7 , (3) with the rise of temperature, the Mott phase melts into the NCCDW phase with an extremely fast charge response and a sudden drop in resistivity, where several tens of stars organize into roughly hexagonal domains 10,17 .…”

Coexistence of superconductivity and charge-density-wave domain in 1T-FexTa1−xSSe

“…4(a)). [13,26] In the NCCDW phase of 1T -TaSSe, the CDWs within the domain is commensurate, which allows us to adopt such a CCDW superstructure as 1T -TaSSe initial structure model. The initial structure was fully relaxed by taking into the lattice parameter and atom position optimizations, and the optimized structure was used for property calculations.…”

“…Below T = 180 K, it forms a commensurate CDW (CCDW) order with √ 13 × √ 13 superstructure. [11,12,13,14] With increasing temperature, it melts into a nearly-commensurate CDW (NCCDW), which forms roughly hexagonal CDW domains. [15] The electronic phase diagrams in pressurized 1T -TaS 2 , [16,17] 1T -Fe x Ta 1−x S 2 (0 ≤ x ≤ 0.05), [18,19] or 1T -TaS 2−x Se x (0 ≤ x ≤ 2), [20,21] uniformly show the SC coexists with the NC-CDW phase.…”

“…We use a √ 13× √ 13 superlattice in Ta-plane to model the CCDW superstructure within domains. [13,26] Brillouin zone (BZ) sampling was performed on a MonkhorstPack (MP) k-point mesh [27] of 6 × 6 × 12 for the structural relaxation, and an 8 × 8 × 16 MP grid was chosen for the electronic structure calculations. The Fermi-surface was calculated on a dense k-point mesh grid of 16 × 16 × 24.…”