Nanoscale Modulation of Flat Bands via Controllable Charge-Density-Waves Defects in 4Hb-TaS2

Electron correlation is a main driver of exotic quantum phases and their interplay. The 4Hb-TaS2 system, possessing intrinsic heterostructure of 1T- and 1H-TaS2 monolayers, offers a unique opportunity to control electron correlation by distorting the atomic lattice or tuning interlayer coupling. Here, we investigated intrinsically deformed charge-density-waves (CDW) in the 1T layer of 4Hb-TaS2 to elucidate and control their effects on flat bands using scanning tunneling microscopy and spectroscopy (STM/S) combined with first-principles calculations. We identified two types of CDW defects: Type 1 has structural distortion and locally suppressed flat bands, while Type 2 features an increased flat band filling factor of intact CDW structure. Density functional theory calculations indicate that a sulfur vacancy in the 1T layer distorts the CDW structure and gives rise to a Type 1, whereas a sulfur vacancy in the 1H layer reduces the interlayer charge transfer and lead to a Type 2. Furthermore, we demonstrated creating and erasing individual CDW defects via STM manipulation. Our findings provide a pathway to not only tune flat bands but also selectively manipulate the interaction between CDW, the atomic lattice, and interlayer coupling in strongly correlated systems with atomic precision.

💡 Research Summary

This work investigates how charge‑density‑wave (CDW) defects in the 1T layer of the intrinsic heterostructure 4Hb‑TaS₂ can be used to modulate the flat‑band electronic structure that underlies strong electron correlations. Using low‑temperature scanning tunneling microscopy and spectroscopy (STM/STS) together with density‑functional theory (DFT), the authors identify two distinct types of native CDW defects. Type 1 defects appear as “dark holes” in STM topographs, are associated with a sulfur vacancy in the 1T layer (V_S₁T), and cause a local distortion of the Star‑of‑David (SoD) CDW cluster. Spectroscopically, the characteristic flat‑band peak near +40 meV is suppressed at the defect core and shifts to higher energy nearby, while new in‑gap states emerge. DFT simulations confirm that V_S₁T dramatically reduces the d_z² projected density of states (PDOS) at the Fermi level, reproducing the experimental suppression of the flat band.

Type 2 defects, in contrast, preserve the SoD geometry but display a reduced apparent height in STM images. They are linked to a sulfur vacancy in the adjacent 1H layer (V_S₁H). This vacancy reduces interlayer charge transfer from the 1T to the 1H sheet, effectively electron‑doping the 1T layer. The flat‑band peak splits into three features identified as lower Hubbard band (LHB), upper Hubbard band (UHB), and an intermediate resonance, and a zero‑bias Fano‑shaped peak reminiscent of a Kondo‑like resonance appears. DFT shows that V_S₁H splits the d_z² PDOS into two symmetric peaks straddling the Fermi level, consistent with the observed Hubbard‑band separation of ~50 meV, which is much smaller than the 250–400 meV gap of pristine 1T‑TaS₂. This reduced on‑site Coulomb energy reflects the fractional occupancy of the flat band caused by the altered charge balance.

Beyond characterization, the authors demonstrate deterministic creation and erasure of individual defects using the STM tip. Applying a 2.7 V bias pulse at room temperature creates a V_S₁T, turning a pristine SoD cluster into a Type 1 defect. Subsequent increase of the tunneling current to ~1.4 nA while bringing the tip close to the surface restores the missing sulfur, likely by diffusion from the underlying 1H layer, thereby erasing the defect. At 4.7 K, a similar bias pulse can generate a Type 2 defect, but this defect spontaneously relaxes back to the pristine state, indicating that Type 2 configurations are metastable under the experimental conditions.

The study establishes that CDW defects act as a powerful local knob for tuning flat‑band occupancy, bandwidth, and Hubbard interaction strength. By controlling whether a defect suppresses, splits, or leaves intact the flat band, one can locally modulate electron correlation strength, carrier type (hole versus electron doping), and even induce Kondo‑like many‑body resonances. Because 4Hb‑TaS₂ also exhibits enhanced superconductivity (T_c ≈ 2.7 K), broken time‑reversal symmetry, and non‑trivial topology, the ability to pattern CDW defects with atomic precision opens a route to engineer superconducting, magnetic, or topological phases in a spatially resolved manner.

In summary, the paper provides (i) a clear microscopic identification of two CDW defect families linked to specific sulfur vacancies, (ii) a detailed correlation between defect‑induced structural changes, interlayer charge transfer, and flat‑band electronic structure, (iii) a demonstration of reversible, atom‑scale defect engineering via STM, and (iv) a conceptual framework for using CDW defects as a design element to manipulate strong correlations, superconductivity, and topology in layered transition‑metal dichalcogenides. This work thus advances both fundamental understanding of CDW‑flat‑band interplay and practical strategies for nanoscale quantum‑material engineering.