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

When a material undergoes a phenomenon called charge density wave both its atomic lattice and electronic density distorts coherently below a certain temperature. In this work, we demonstrate that these distortions may appear and coexist in different forms as long as they follow the same spatial periodicity. In particular, we have studied NbSe₂, a material that shows different surface distortions that we identify with atomic resolution by means of a scanning tunneling microscope

In condensed matter systems, exotic collective electronic phases usually emerge at low temperatures. One of them is the charge density wave order, which is an ideal phenomenon for investigating the interplay between strongly interacting electrons and the atomic lattice. When the material is in this ground state, below a certain critical temperature (<33K for NbSe₂), the atomic lattice as well as the electron density rearrange themselves to form an energetically more stable structure. The genuine origin of the charge density wave state in NbSe₂ is still a matter of debate. Our work provides new insight regarding the mutual interaction between the lattice and the electronic structure of this prototypical transition metal dichalcogenide (TMD).

Figure: Unoccupied [+50 mV] (a) and occupied [−50 mV] (b) STM images (14 nm×14 nm) of the same region of bulk 2H-NbSe2 showing the coexistence of two structural phases. The black dashed line represents the boundary between them. (c−f) Zoom-in STM images from panels a and b of the two phases for unoccupied (c,d) and occupied (e,f) states.(g−j) Simulated STM images for both phases and bias polarities (VS=±50 mV).

Here we study bulk NbSe₂ by means of scanning tunnelling microscopy imaging (STM) at temperatures of 1K combined with density functional theory calculations (DFT). Our DFT calculations reveal six different, iso-stable atomic structures compatible with a periodic 3×3 lattice distortion. Each structure is expected to trigger a different electronic density rearrangement that, therefore, can be detected by measuring the conductance of NbSe₂ at the atomic level. Our atomically resolved STM images (Figure 1) can unambiguously confirm this by identifying two of these structures (blue and yellow distortions in fig.1) among the theoretical phases. The comparison between the experimental and simulated STM images of these two 3×3 phases is shown in figs. 1c-j. Lastly, these two phases are seen to coexist in the same NbSe₂ regions. Given the equal energetic stabilities of the six phases, we expect this ground state to be extremely sensitive to doping, external strain or internal pressure within the crystal. Further measurements on NbSe₂ at extremely low temperatures and subject to strong external perturbations will be therefore carried out at the CFM to unveil the energetic landscape of this correlated TMD material.