Coexistence of Elastic Modulations in the Charge Density Wave State of 2 H-NbSe 2

• Published in: Nano letters 2019-05, Vol.19 (5), p.3027-3032
• Main Authors: Guster, Bogdan, Rubio-Verdú, Carmen, Robles, Roberto, Zaldívar, Javier, Dreher, Paul, Pruneda, Miguel, Silva-Guillén, José Ángel, Choi, Deung-Jang, Pascual, José I, Ugeda, Miguel M, Ordejón, Pablo, Canadell, Enric
• Format: Article
• Language: English
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/acs.nanolett.9b00268

Bulk and single-layer 2 H-NbSe exhibit identical charge density wave order (CDW) with a quasi-commensurate 3 × 3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2 H-NbSe with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations for single-layers reveal that six different atomic structures are compatible with the 3 × 3 CDW distortion, although all of them lie on a very narrow energy range of at most 3 meV per formula unit, suggesting the coexistence of such structures. Our atomically resolved STM images of bulk 2 H-NbSe unambiguously confirm this by identifying two of these structures. Remarkably, these structures differ from the X-ray crystal structure reported for the bulk 3 × 3 CDW which in fact is also one of the six DFT structures located for the single-layer. Our calculations also show that due to the minute energy difference between the different phases, the ground state of the 3 × 3 CDW could be extremely sensitive to doping, external strain or internal pressure within the crystal. The presence of multiphase CDW order in 2 H-NbSe may provide further understanding of its low temperature state and the competition between different instabilities.