Intercalation-Induced\nMonolayer Behavior in Bulk NbSe<sub>2</sub>

Superconductivity at the 2D limit is significant for\nadvancing\nfundamental physics, leading to extensive research on monolayer two-dimensional\nmaterials. In particular, monolayer transition metal dichalcogenides\nsuch as NbSe<sub>2</sub> exhibit Ising superconductivity due to broken\nin-plane inversion symmetry and strong spin–orbit coupling,\nwhich has garnered significant attention. In this letter, we adopted\nan organic cation intercalation technique to modulate the interlayer\ninteraction of NbSe<sub>2</sub> by expanding the interlayer distance,\nthereby making intercalated NbSe<sub>2</sub> behave similarly to monolayer\nNbSe<sub>2</sub>. The interlayer distances of NbSe<sub>2</sub> intercalated\nwith THA<sup>+</sup>, CTA<sup>+</sup>, and TDA<sup>+</sup> cations\nare almost double or triple that of pristine NbSe<sub>2</sub>. The\nsuperconducting transition temperature (<i>T</i><sub>c</sub>) of THA-NbSe<sub>2</sub> is comparable to that of pristine NbSe<sub>2</sub>, while the charge density wave (CDW) transition temperature\nis higher. The <i>T</i><sub>c</sub> of intercalated NbSe<sub>2</sub> decreases with a reduced hole concentration, and the enhanced\nCDW is ascribed to the dimensional reduction. Notably, the in-plane\nupper critical field of intercalated NbSe<sub>2</sub> significantly\nexceeds the Pauli paramagnetic limit, which is similar to the Ising\nsuperconductivity observed in monolayer NbSe<sub>2</sub>. Our work\ndemonstrates that the organic cations intercalated two-dimensional\nmaterials exhibit behavior similar to their monolayer counterparts,\nproviding a convenient platform for exploring and modulating physical\nphenomena at the two-dimensional limit.