Orbital-driven competition: Ferromagnetism and superconductivity in Li-intercalated transition metal dichalcogenides

Symmetry breaking is a fundamental concept in condensed matter physics, driving various quantum phenomena. Layered transition metal dichalcogenides (TMDs) serve as an excellent platform for investigating electronic instabilities and emergent phases. Using first-principles calculations, we reveal a striking transformation in ${\mathrm{TiSe}}₂$ and ${\mathrm{ZrSe}}₂$ induced by Li intercalation. Specifically, ${\mathrm{TiSe}}₂$ transitions from a charge density wave state to a ferromagnetic phase, while ${\mathrm{ZrSe}}₂$ evolves from a semiconductor to a superconducting state. In ${\mathrm{LiTiSe}}₂$, the localization of Ti $3d$ orbitals creates overlapping van Hove singularities near the Fermi level, stabilizing a Stoner-type ferromagnetic phase via exchange interactions that break spin-rotational symmetry. In contrast, superconductivity in ${\mathrm{LiZrSe}}₂$ arises from enhanced electron-phonon coupling, facilitated by delocalized Zr $4d$ orbitals and Zr-Zr bond-stretching phonon modes, leading to Cooper pair condensation and the breaking of U(1) gauge symmetry. These findings highlight how variations in $d$-orbital localization and interatomic interactions govern distinct quantum phases, demonstrating the transformative potential of intercalation for tuning electronic properties and accessing unique quantum states in layered TMDs.