Atomic-scale visualization of chiral charge density wave superlattices and their reversible switching

Chirality is essential for various phenomena in life and matter. However, chirality and its switching in electronic superlattices, such as charge density wave (CDW) superlattices, remain elusive. In this study, we characterize the chirality switching with atom-resolution imaging in a single-layer NbSe2 CDW superlattice by the technique of scanning tunneling microscopy. The atomic arrangement of the CDW superlattice is found continuous and intact although its chirality is switched. Several intermediate states are tracked by time-resolved imaging, revealing the fast and dynamic chirality transition. Importantly, the switching is reversibly realized with an external electric field. Our findings unveil the delicate switching process of chiral CDW superlattice in a two-dimensional (2D) crystal down to the atomic scale.


Determination of the distance between the tip and the substrate
For the determination of the initial tip-sample distance (VB = -1.5 V, It = 20 pA), the tunneling gap is set and the STM tip (scanner) retraction or extension is recorded.
Subsequently, the feedback loop is turned off and the tip is moved toward the substrate surface gently until a sudden increase in the tunneling current to a saturation of 10 nA is reached. The distance traveled during this process was 0.1 nm, which was used as the initial distance between the tip and the substrate. domains are rotated by about ±14° relative to the close-packed direction of the Se atomic lattice, as shown in the green, blue and black arrows in (d) The neighboring close-packed direction of the SOD superlattice has a rotation of 60° angle, due to the six-fold symmetry. So, the angle between two different chiral domains can also be defined as either 28° or 88°, that is, 14°-(-14°) S4 = 28° and 14°-(-14°) + 60° = 88°. The rotation of the two chiral CDW domains is close to 90°, and this observation can be found in Fig. 3  The DB movement is controlled by scanning direction and current setpoint. S5 We managed to control the movement direction of the DB by using different current setpoint values for the upward and downward scan. When the upward scan is performed with 2 nA, the DB moves up (from Fig. S2a to Fig. S2b); while the downward scan is performed with a current less than 1.5 nA, the DB remains unchanged in Fig. S2c. When the downward scan is performed with a current larger than 2 nA, the DB move downward (from Fig. S2c to Fig. S2d). This is like a broom-wiping process, and it may be related to a mechanical effect in combination with the electron tunneling effect. Thus, these measurements indicate that the DB movement can be triggered by a larger tunneling current (also demonstrated in Fig. 2), and the DB movement direction can be controlled with the tip scanning direction and current setpoint. The lateral contraction distance is approximately 20 pm 1 . It also induces a vertical shift of the Se atoms, which can be imaged by STM (triangular SOD) as the tunneling current is exponentially dependent on the tip-sample distance.
To reveal the contraction of the top-layer Se lattice before and after the SOD CDW formation, we overlay the model for the Se atomic lattice without considering the SOD contraction onto the STM image (Fig. S5). The construction of the Se atomic lattice consists of the following steps.
In the first step, the center position of the SOD cluster is marked (the green hexagram star) in the atomic-resolution STM image, which is the position of the central Nb atom in each SOD. It is well known that the other 12 Nb atoms contract toward the central atom, so that the center remains immobile.
In the second step, the center points of four neighboring SOD clusters are S8 connected to form a rhombus, which represents the unit cell of the SOD CDW. The positions of the Nb atoms are marked with blue balls within the rhombus according to the (√13 × √13)R13.9° superstructure relationship.
In the third step, the positions of the Se atoms are marked with orange balls, each of which is at the center of three adjacent Nb atoms. The resulting orange ball lattice represents the Se lattice without considering SOD contraction. The position difference between the centers of the Se atom (the white round protrusion) in the STM image and orange balls represents the SOD contraction.
In general, each orange ball well matches the center of the Se atom. The contraction is on the order of the diameter of the orange ball (~30 pm), which is at the lower limit of the STM resolution. This tiny distortion is challenging to detect and is not considered as a lattice defect in our discussion throughout this paper. S9 Supplementary Fig. 6. Additional Bragg spots from BLG by including wide area. pA; c, -1.5 V, 100 pA. S10 Supplementary Fig. 7. The energy barrier model of the L and R chirality.
As for the concern why not a larger disorder area at the DB or one chirality outcompeting the other, we think there are two possible reasons: One is the similar energy barrier of both L and R chiral domains, as shown by the "energy barrier model"

Movie S1.
Dynamic example of the DB movement of the chiral CDW superlattice.