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
Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST , Campus UAB, Bellaterra , 08193 Barcelona , Spain.
CIC nanoGUNE , 20018 San Sebastián , Spain.
Nano Lett. 2019 May 8;19(5):3027-3032. doi: 10.1021/acs.nanolett.9b00268. Epub 2019 Apr 22.
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.
块状和单层的2H-NbSe呈现出相同的电荷密度波序（CDW），具有准共格的3×3超晶格周期性。在这里，我们将2H-NbSe在T = 1K时的扫描隧道显微镜（STM）成像与第一性原理密度泛函理论（DFT）计算相结合，以研究这种CDW相的结构原子重排。我们对单层的计算表明，六种不同的原子结构与3×3 CDW畸变兼容，尽管它们都位于每个化学式单位最多3 meV的非常窄的能量范围内，这表明这些结构共存。我们对块状2H-NbSe的原子分辨STM图像通过识别其中两种结构明确证实了这一点。值得注意的是，这些结构与报道的块状3×3 CDW的X射线晶体结构不同，而该X射线晶体结构实际上也是单层中定位的六种DFT结构之一。我们的计算还表明，由于不同相之间的能量差很小，3×3 CDW的基态可能对晶体中的掺杂、外部应变或内部压力极其敏感。2H-NbSe中多相CDW序的存在可能有助于进一步理解其低温状态以及不同不稳定性之间的竞争。
