1. Local structure and phonon states mediated by intercalation-driven doping in superconducting $Li_{1.0}(C_5H_5N)_yFe_{2-z}Se_2$
- Author
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Deltsidis, Alexandros, Kaitatzi, Myrsini, Simonelli, Laura, Stock, Chris, Voneshen, David, and Lappas, Alexandros
- Subjects
Condensed Matter - Superconductivity ,Condensed Matter - Materials Science ,Condensed Matter - Strongly Correlated Electrons - Abstract
Intercalation of two-dimensional (2D) iron chalcogenides with molecular species requires disentangling electronic and structural contributions to understand the puzzling limit to superconducting transition temperature ($T_c$) at the frontier of long interlayer separations. Here, synchrotron X-ray absorption spectroscopy (XAS) at the Se K-edge sheds light on the impact of carrier-doping on the local structure of the high-$T_c$ (~39 K) $Li_{1.0}(C_5H_5N)_yFe_{2-z}Se_2$ phase. This material is derived by annealing the structurally related as-made derivative ($T_c$~ 44 K), with layers being primed apart by [alkali-molecule] guests. Metrics, such as, a reduced filling of Se $4p$ orbitals and shorter Fe-Se bonds in the annealed phase, corroborate to a lower electron doping level with respect to the as-made one. Analysis of the metal-ligand thermal motion, based on the correlated Debye model, further relates the higher $T_c$ intercalates with the softening of the local Fe-Se bond. Beyond electronic effects, intercalation brings forth host-guest interactions that mediate the dynamics of the bulk crystal structure. For this, neutron time-of-flight spectroscopy on the annealed derivative, corroborates to the Se-Fe-Se layer being sensitive to chemical pressure effects imposed by the confined organic guests. This reflects in the phonon density of states, where harder low-energy transverse acoustic matrix phonons and molecular vibrations are witnessed, with respect to the pristine inorganic ($\beta$-FeSe) and organic ($C_5D_5N$) counterparts. On cooling through $T_c$, these excitations arrive without a collective magnetic-resonance mode - essential in unconventional, spin-mediated mechanisms - enquiring about deviations from optimal doping. The work highlights that when the Fe-square planes are tuned far apart, carrier-doping leveraged by intercalation plays a key role in the $T_c$ parametrization., Comment: MAIN text: 19 pages, 5 figures; Supplemental Material: 7 pages, 3 figures
- Published
- 2024