Defect-Selective Charge-Density-Wave Condensation in $2H\text{−}{\mathrm{NbSe}}_2$

Defects have been known to substantially affect quantum states of materials including charge density wave (CDW). However, the microscopic mechanism of the influence of defects is often elusive due partly to the lack of atomic scale characterization of defects themselves. We investigate native defects of a prototypical CDW material $2H\text{−}{\mathrm{NbSe}}_2$ and their microscopic interaction with CDW. Three prevailing types of atomic scale defects are classified by scanning tunneling microscope, and their atomic structures are identified by density functional theory calculations as Se vacancies and Nb intercalants. Above the transition temperature, two distinct CDW structures are found to be induced selectively by different types of defects. This intriguing phenomenon is explained by competing CDW ground states and local lattice strain fields induced by defects, providing a clear microscopic mechanism of the defect-CDW interaction.

Figure 1

Empty state STM topographic images of $2H\text{−}{\mathrm{NbSe}}_2$ at 4.3 K. (a) Large area topographic image (bias $-100\mathrm{mV}$, tunneling current 1 nA) with major types of defects ($A$, $B$, and $C$) denoted. Zoomed-in images for two different CDW structures coexisting. (b) Hollow-centered (HC) CDW [blue squared area in (a)] and (c) anion-centered (AC) CDW [180° rotated image of green square area in (a)]. (d),(e) Schematic models for HC and AC CDW. $3\times 3$ CDW unit cells are indicated.

Figure 2

Schematic models (side and top views), STM images ($+100$ and $-100\mathrm{mV}$ bias), and corresponding DFT simulations of defect type $A$ (an Se vacancy in the top layer, top row), $B$ (an Nb intercalant below a top layer Se atom, center row), and $C$ (an Nb intercalant below the hollow site of the top layer, bottom row).

Figure 3

(a) Large area STM topographic image of $2H\text{−}{\mathrm{NbSe}}_2$ at 40 K (bias $+100\mathrm{mV}$, tunneling current 1 nA). Local CDW modulation areas around defects $A$, $B$, and $C$ are encircled by white dashed lines. Yellow and blue colored area within dashed lines indicate different, AC and HC, respectively, CDW structures. (b)–(d) Zoomed-in ($6.15\times 6.15{\mathrm{nm}}^2$) images around defect $A$, $B$, and $C$ indicated by white arrows in (a). (e)–(g) Fast-Fourier-transformation filtered topographic images for (b)–(d) to enhance the CDW modulations by eliminating pristine lattice structure ($1Q/2\le q\le 3Q/2$, $Q=Q_{\text{Bragg}}/3$). (h) Line profiles along the atomic wave vector in the two-dimensional FFT of zoomed-in images of (b)–(d) for defects $A$, $B$, and $C$. A similar line profile for a pristine area ($P$) is given for comparison. Offsets are given for clarity. The FFT intensity is normalized by the peak intensity at $Q_{\text{Bragg}}$

Figure 4

(a),(b) DFT-simulated atomic structures around defects $A$ and $C$. Solid lines represent contracted $\mathrm{Nb}―\mathrm{Nb}$ bonds ($<3.475\AA$). Yellow (blue) colored areas indicate to Nb trimers/hexamers of AC (HC) CDW structures without defects. Green arrows indicate the displacements (enlarged by 50 times) at each Nb atom from the pristine structure. Red circles indicate defect positions. (c),(d) The same DFT-simulated atomic structures where the green arrows indicate the difference of the Nb distortions from the AC (c) or HC (d) CDW structures.