Evolution of spin excitations from bulk to monolayer FeSe

Jonathan Pelliciari,Abhishek Nag,Qisi Wang,Steven Johnston,Tianlun Yu,Mirian García-Fernández,Jun Zhao,Rui Peng,G Ghiringhelli ,Thomas Maier ,Kejin Zhou ,X Chen ,Mario E Rossi ,Qianqian Song ,J Li ,A C Walters ,D L Feng ,Seher Karakuzu,Riccardo Arpaia,Riccardo Comin

doi:10.1038/s41467-021-23317-3

Abstract

In ultrathin films of FeSe grown on SrTiO3 (FeSe/STO), the superconducting transition temperature Tc is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to its bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems revealing a remarkable reconfiguration of spin excitations in FeSe/STO, essential to understand the role of spin fluctuations in the pairing mechanism.

Highlights

In ultrathin films of FeSe grown on SrTiO3 (FeSe/STO), the superconducting transition temperature Tc is increased by almost an order of magnitude, raising questions on the pairing mechanism
The X-ray Absorption Spectroscopy (XAS) of FeSe/STO has an additional peak at higher energy, which could originate from new interfacial valence states induced by hybridization with orbitals of the STO substrate
Any quantitative model for the spin fluctuation contribution to pairing must account for the observed evolution of the spin dynamics

Summary

Results

These changes are a direct consequence of the fact that intraband scattering is strongly suppressed at low-energies once the hole pocket is shifted below the Fermi level. This hardening and flattening of the electronic excitations is clearly observed in the experimental data for FeSe/STO as corroborated by the white diamonds overlaid with the color plot (Fig. 3). Ttohethineteirnbtaranbdanχd0s0ðqχ;0s0ωðqÞ ;aωlsÞo, has the and is, expected to dominate the RIXS signal when neglecting cross-section effects

Discussion

Methods

Code availability