Coexistence of Elastic Modulations in the Charge Density Wave State of 2H-NbSe2

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Abstract
Bulk and single-layer NbSe2 exhibit identical charge density wave order (CDW) with a quasi-commensurate 3x3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2H-NbSe2 with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations 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 2H-NbSe2 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 multi-phase CDW order in 2H-NbSe2 provides further understanding of its low temperature state and the competition between different instabilities.
Funding Information
  • Generalitat de Catalunya (2017SGR1506)
  • Horizon 2020 Framework Programme (824143)
  • H2020 European Research Council (758558)
  • Ministerio de Economía y Competitividad (MAT2017-82074-ERC, SEV-2015-0496, FIS2015-64886-C5-3-P, FIS2015-64886-C5-4-P, SEV-2017-0706)

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