Creation and manipulation of many-body quantum states are crucial for developing radically new technologies in quantum computation, communications, security, and sensing1-4. Individual atomic-scale defects in a solid material provides one of the ideal candidates for generating localized quantum states due to the introduction of symmetry breaking, degeneracy lifting and scattering sources in the vicinity of the defects5-11. The atomically-precise engineering of defect-localized bound states has been realized in a few material platforms such as insulating film12-14, diamond15, graphene16, h-BN17 and two-dimensional transition-metal dichalcogenides18, which is appealing for practical quantum applications. However, so far, the engineering of defect bound states is limited to a few material platforms due to the challenges in modifying the atomic defects of complex lattice.
Topological quantum materials have recently attracted considerable attention due to their fascinating symmetry-protected band structures and cooperative effects involving the interplay of multiple degrees of freedom (charge, spin, orbital, lattice)19,20. The interactions of multiple degrees of freedom in quantum materials are dynamically intertwined with each other, which results in exotic quantum states21,22. In recent years, the transition-metal kagome lattice materials which host Dirac points and nearly flat bands that naturally promote topological and correlation effects23,24 are discovered, providing exciting opportunities for exploring frustrated, correlated, and topological quantum states of matter25-33. Remarkably, defects-localized quantum states including the single-vacancy-localized magnetic polarons have been discovered in magnetic transition-metal kagome shandites, which provides a promising way to engineer quantum bound states for dilute magnetic topological materials and kagome-lattice-based devices34,35. On the other hand, the difficulty of reliability of engineering defects leaves it largely unexplored.
Herein, we report the atomically precise engineering of spin-orbit polarons (SOPs) localized around the S vacancy antidots in a kagome Weyl semimetal Co3Sn2S2 by using low-temperature scanning tunneling microscopy (STM). The vacancy quantum antidots with well-organized geometry are precisely constructed though tip-assisted repairing method (Fig. 1a). Spin-polarized STM and magnetic-field dependent measurements demonstrate the SOP nature of the bound states localized at the S vacancy quantum antidots. When the geometric size increases, the bound states of quantum antidots shift towards to the Fermi energy. In addition, the energy shift of bound states depends on the geometric shape of vacancies, which agree with the theoretical simulations based on the anti-bonding interaction between adjacent quantum antidots. Interestingly, as vacancy size increases, the magnetic moments extend from localized magnetic moment to the flat band negative magnetism from Co3Sn kagome layer.
Atomically-precise construction of Sulfur quantum antidots
The crystal structure of Co3Sn3S2 consist of the rhombohedral lattice where the kagome Co3Sn layer are sandwiched between two triangular S layers, which are further encapsulated by two separated triangular Sn layers 32. Cleavage in vacuum typically results in Sn and S terminated surfaces with kagome Co3Sn surfaces rarely obtained 29.
We start with the S terminated surface, which has been identified by STM and atomic force microscopy (AFM) in previous works29,35,36. The vacancies, which appear as hole-like features in STM images, are randomly distributed at S-terminated surface. The absence of single atom surrounding the hole-like feature in the STM image is further confirmed by the non-contact AFM (Fig. S1). The vacancies consist of single vacancies and vacancy aggregates with various shapes (Fig. S2).
We then achieve the atomically-precise repairing of S vacancy through applying a voltage pulse from an STM tip. Figures 1b and 1c depict the experimental demonstration of repairing a single S vacancy. Briefly, to repair a single S vacancy, we position the STM tip close to the vacancy center (red arrow in Fig. 1b), followed by applying a tip pulse with a small voltage (approximately range from 0.5 V to 1.0 V). It is evident that the single S vacancy is filled with an additional S atom (highlighted by the blue hexagon in Fig. 1c). The filling of a S atom is also confirmed by the non-contact AFM (Fig. S3).
Prior to the repairing, the dI/dV spectrum obtained at a single vacancy exhibit a series of approximately equal-spaced spectral peaks, emerging just above the valence band inside the region of suppressed density of states. These peaks arise from a localized spin-orbit polaron around the single vacancy 35. However, after applying the tip pulse, the dI/dV spectrum obtained at the same position exhibits features identical to those of vacancy-free region, providing additional evidence that the S single vacancy is repaired by the tip pulse (Fig. S3).
The origin of additional S atom to fill the single vacancy is illustrated in Figure 1d. As there are no topographic changes between the relatively-large scale STM images before and after the tip pulse except for the repair of single S vacancy (Fig. S4), it suggests that the filling S atom originates from the underlying layers rather than the surface layer. Considering that the crystal structure of Co3Sn2S2 is composed of stacked…–Sn-[S-(Co3-Sn)-S]–…layers, we define that top S layer of the sandwich structure corresponding to the as-cleaved S surface is the Sup layer (brown triangle in Fig. 1d) while the underlying S layer of sandwich structure is Sdown layer (light brown triangle in Fig. 1d). During the tip pulse, one of S atoms in the Sdown layer transfers to the vacancy site of the Sup layer, repairing the vacancy at the Sup layer. In addition, the calculations indicate that the energy barrier for the S atom transfer from Sdown layer to Sup layer is approximately 0.73 eV. The relatively low energy barrier means that it is possible to overcome it the with a small voltage pulse applied at sufficiently close tip-sample distances (Fig. S5).
The capabilities in controlled repairing of specific single vacancies provides a pathway for the controlled fabrication of physically interesting vacancy “quantum antidots” (QADs) at the atomic scale. Motivated by this, we apply a step-to-step manipulation method to transform naturally-formed vacancy aggregates into QADs with well-defined shapes and sizes. For instance, we can manipulate the length of a one-dimensional S vacancy chain by gradually filling S atoms into specific sites (Fig. 1e). Similarly, filling one S atom at a specific site of a cross-shaped vacancy consisting of four S absences leads to the formation of a triangular vacancy (Fig. 1f). In more complex case, we are able to create quasi-regularly-shaped QADs such as quasi-triangular and quasi-hexagonal vacancy by filling specific sites of vacancy aggregates with irregular polygon shapes (Fig. S6).
Interacting bound states with controlled spacings of two neighboring S vacancies
The atomically-precise construction of well-defined QADs immediately provides an excellent opportunity to systematically investigate the evolution of the localized bound states with the geometric size. We firstly study the simplest case of two spatially-separated single S vacancies with decreasing spacings (Fig. 2a). The dI/dV spectra obtained around one single vacancy show that a series of approximately equal-spaced spectral peaks at −322, −300, and −283 mV gradually become suppressed as another vacancy approaches closer. Upon the formation of dimer vacancy, the series of bound states vanishes and a new series of sharp peaks at -292, -277, -265 and -254 meV appear (Fig. 2b).
To further study the evolution of density of states, we simultaneously collect the dI/dV maps at the energy corresponding to bound states (Fig. 2a). Prior to the merging of two vacancies, the bound states in each vacancy exhibit localized flower-petal shaped patterns with three-fold rotation symmetry. After merging, the spatial distribution of the new four distinguishable dI/dV peaks show two-fold rotation-symmetry patterns, with the shared S atoms connecting two single vacancies exhibiting the highest density of states (Fig. 2a and Fig. S7). The peak located at−254 mV (Fig. S5) is the sharpest and the most localized one, which is referred to the primary bound state.
Since the bound states around the single vacancy is emergent from the exotic SOP35, it is natural to investigate the bound states around the dimer vacancy. Thus, we further study the magnetic properties of the primary bound states around the dimer vacancy through spin-polarized STM. The dI/dV spectra using a magnetic Ni tip demonstrate that both the primary bound states localized at single vacancy and dimer vacancy are magnetic with a spin-down majority (Fig. 2c). The polarization of bound states for the dimer vacancy is larger than the one for single vacancy. In addition, by applying a magnetic field perpendicular to the surface (Bz) and a non-magnetic STM tip, we observe anomalous Zeeman effect that primary bound states shift linearly toward the higher energy side independent of the direction of the magnetic field (Fig. 2d). According to the above evidences, we conclude that the bound states observed around the dimer vacancy are originated from SOP with a larger magnetic moment than that of single vacancy35. We also study the lattice distortion around the S vacancies by applying a geometric phase analysis method37,38 based on the Lawler–Fujita drift-correction algorithm39 (Fig. 2e). The antisymmetric strain map U(r) and symmetric strain map S(r) show that the strain is mainly localized around the S vacancies, further supporting the polaron nature of the bound states around the dimer vacancies and other vacancies.
Tunable bound states of SOPs with custom-designed geometry
In spired by the exotic bound states around dimer vacancy, we further study the evolution of the bound states with the length of single-chain vacancies. We construct a series of linear vacancy with atomic length N using tip-assisted controlled repairing method (Fig. 3a) and collect the dI/dV spectra on the vacancies (N=1, 2, 3 …) respectively. All linear vacancies exhibit series of several peaks with equally spacing energy that emergent from the localized bound states. To facilitate comparison, we defined the sharpest one of the peaks with highest energy position to be the primary bound states of each vacancy (P(N)). We find that the P(N) shift to the Fermi energy with increasing N, and eventually reaches a critical energy position at about -240 meV at N>4, as highlighted by the black arrows (Fig. 3b and Fig. S8).
In addition to the single-chain vacancy, the tunability of the bound states extends to the vacancies with more elaborate shapes, including double-chains, equilateral-triangle and equilateral-hexagon vacancies (Figs. 3c-g). All vacancies exhibit series of several peaks with equally spacing energy and the sharpest peak with highest energy position is similarly assigned as the primary bound states P(N). As summarized in Figure 3I, the evolution of P(N) for each symmetric shape follows an exponential function, with all P(N) shifting exponentially towards a critical energy value near the Fermi level as the size increases. The critical energy level of P(N) depends on the vacancy shape (highlighted by different color in Fig. 3), with higher symmetry shapes possessing higher critical energy levels (Fig. 3i). For instance, the critical energy of P(N) of single chain vacancies is about -240 meV while one of the hexagonal vacancies is almost at Fermi levels.
The geometry-dependent bound states suggest the strong interactions between adjacent single vacancies. Furthermore, the spatial distributions of the bound states across the single chain vacancies (Fig. S9) exhibit quasi one-dimensional band behaviors40-42, indicating the existences of vacancy-vacancy interactions. To gain insight into the shape-dependent energy shift behaviors of P(N), we develop a simple model (see method in Supplementary Materials and Fig. S10) to simulate the bound states around vacancies. We simulate vacancies with a simple tight-binding model with a nearest neighbor hopping t. We construct four types of vacancy patterns with different number of S vacancies, which consist of single chain, double chain, triangle and hexagon. In each vacancy configuration, the highest energy level is extracted as the anti-bonding state. We find that the anti-bonding state undergoes a similar exponential shift towards higher energy (Fig. 3j), which is consistent with experimental observations in Figure 3I. This suggests that the anti-bonding interaction between the single-vacancy-localized quantum states plays a crucial role in shaping the bound states of vacancy antidots.
Engineering the magnetic moment of SOPs
In addition to the energy position, the magnetic moment of the bound states is also tuned by the geometric size of vacancy antidots. We focus on the magnetic moment of triangular vacancy antidots due to their high yields and relatively-large lattice distortions (Fig. S11). The bound states of triangular vacancy antidots present the anomalous Zeeman effect with external magnetic field (Figs. 4a,b). Fitting the energy position as a function of the magnetic field, we obtained the effective magnetic moment value |µ(N)|. For example, |µ(N=3)| = 0.09meV/T =1.55 µB (Fig. 4a) and |µ(N=10)| = 0.17 meV/T = 2.93 µB (Fig. 4b). These results indicate that the magnetic moment of bound states localized at vacancy is directly related to the size of vacancy.
The Co3Sn terrace, confined by the step edges of adjacent S terraces (Fig. 4c), is considered as a naturally occurring vacancy antidot with an enormous size (N=∞). The spatially-averaged dI/dV spectrum obtained at the Co3Sn terrace shows a sharp peak in the vicinity of Fermi level, which is consistent with previous STM results on the Co3Sn surface29,36. The magnetic dependent dI/dV curves show similar anomalous Zeeman effect with an effective magnetic moment of µ(N=∞) = -0.19 meV/T = -3.28 µB (Fig. 4d). The negative orbital magnetic moment results from the spin-orbital coupling in the kagome flat band considering the non-trivial Berry phase of the flat band 43. Evolution of the magnetic moment with the atomic number of vacancies shows that the magnetic moments extend from localized magnetic moment around vacancies to the flat band negative magnetism from Co3Sn kagome layer (Fig. 4e).
The atomically-precise manipulation of atomic vacancies opens a new playground for investigating fundamental physical properties of vacancy nanostructures with custom-designed geometries and their coupling with physical parameters such as magnetic moment, orbital and charges. In addition, the controlled integration of individual functional vacancies of topological quantum materials into extended, scalable atomic circuits, which is promising for practical applications such as atomic memory 13 and quantum qubit 44. Through precise engineering of vacancies, the artificial vacancy lattice can be achieved, which is essential for realizing designer quantum materials with tailored properties 14. The antibonding interactions among the vacancies in Co3Sn2S2 could improve the understanding of polaron-like bound states in coupled quantum systems 42, to explore artificial coupled quantum systems with great control.