Orbital-Selective Instabilities and Spin Fluctuations at the Verge of Superconductivity in Interlayer-Expanded Iron...

Understanding electron correlation-driven instabilities and their coupling to structural phases is essential for deciphering multiorbital pairing in unconventional superconductors. We investigate Li _x (C₅H₅N) _y Fe₂Se₂ (x ∼ 0.6; y ∼ 0.7-0.9), a tetragonal β-FeSe intercalate with a superconducting transition temperature (T _c = 39 K) closely tied to an expanded Fe-layer spacing (∼11.4 Å). High-resolution synchrotron X-ray diffraction and core-level absorption spectroscopy reveal subtle lattice distortions on cooling without a symmetry-breaking transition. Instead, the material exhibits negative thermal expansion (NTE) in the two-dimensional Fe network below T _S ∼ 70 K, and stiffening of local Se-Fe-Se bond dynamics near T _c. The spatially incoherent rearrangement of FeSe₄ tetrahedra and the site-local fluctuations, signal reduced electron correlations compared to those of parent β-FeSe (T _c = 8 K). Complementary X-ray emission spectroscopy, a fast local probe of Fe 3d valence states, detects persistent local Fe spin moments below T _S, unlike quenching in related systems. These findings indicate that decoupling of Fe planes leads to an electronically driven lattice instability. The latter emerges as NTE induced from weak, orbital-selective localization of in-plane Fe 3d states rather than conventional transverse vibrations. Governed by Hund's coupling, this selectivity permits coexistence of local spin fluctuations with itinerant d-electronscritical for enhancing T _c. These results suggest that intercalation-driven d-orbital differentiation moderates electron correlations, providing a pathway to optimize the superconductivity in low-dimensional quantum materials.