Finite-momentum Cooper pairing driven by chiral charge-density-wave quantum fluctuation in TiSe₂

Recently, multi-orbital van der Waals materials have been intensively discussed in the context of unconventional symmetry-breaking ordering. In these systems, the multi-orbital flavor symmetry is intertwined with the space group symmetry, and the resulting particle-hole composite order parameter, with its multi-orbital character, leads to further symmetry breaking compared to the conventional ordering patterns seen in single-orbital (s-wave) systems. An intriguing mechanism for the chiral charge density wave (CDW) order in TiSe₂ was proposed by Kim et al. [Nat. Phys. (2024)], where symmetry 'frustration' between charge ordering and phonons with incompatible irreducible representations is resolved through further symmetry breaking driven by electronic quantum fluctuations. This process results in the 'complete' breaking of symmetries, giving rise to the chiral CDW order.

Here, we propose Bose-Einstein condensation (BEC)-type superconductivity (SC) in TiSe₂ under pressure, where the pairing glue arises from chiral CDW fluctuations near the CDW quantum criticality. The BEC-type SC is attributed to the presence of small Fermi surfaces. Based on the effective interaction potential in the random phase approximation, we explicitly demonstrate that electrons in the p and d orbitals form inter-orbital Cooper pairs with finite center-of-mass momentum, reminiscent of the Fulde-Ferrell and Larkin-Ovchinnikov state or a pair-density wave state without magnetic fields. A group-theoretical approach to the inter-orbital pairing suggests that this material's most likely pairing symmetry is an orbital-selective s-wave symmetry. We discuss the multi-orbital composite order parameter that appears in chiral CDW and SC from the perspective of symmetry breaking.