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

## TL;DR

This study explores how electron interactions and structural changes in a modified iron selenide material influence its superconducting properties.

## Contribution

The paper reveals orbital-selective instabilities and spin fluctuations that enhance superconductivity in interlayer-expanded iron selenide.

## Key findings

- Negative thermal expansion occurs in the Fe network below 70 K, indicating lattice instability.
- Persistent local Fe spin moments are observed below 70 K, unlike in related systems.
- Orbital-selective localization of Fe 3d states, governed by Hund’s coupling, supports coexistence of spin fluctuations and itinerant electrons.

## Abstract

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.