Spin-excitation anisotropy in the nematic state of detwinned FeSe

Abstract

The origin of the electronic nematicity in FeSe is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at ${\mathbf{Q}}_1=(1,0)$ and ${\mathbf{Q}}_2=(0,1)$ of twin-free FeSe are expected to provide decisive criteria for clarifying this issue. Although a spin-fluctuation anisotropy below 10 meV between ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2$ has been observed by inelastic neutron scattering around $T_c\sim 9$ K ($<<T_s\sim 90$ K), it remains unclear whether such an anisotropy also persists at higher energies and associates with the nematic transition $T_{\mathrm{s}}$. Here we use resonant inelastic x-ray scattering (RIXS) to probe the high-energy magnetic excitations of uniaxial-strain detwinned FeSe and {\BFA}. A prominent anisotropy between the magnetic excitations along the $H$ and $K$ directions is found to persist to $\sim 200$ meV in FeSe, which is even more pronounced than the anisotropy of spin waves in {\BFA}. This anisotropy decreases gradually with increasing temperature and finally vanishes at a temperature around the nematic transition temperature $T_{\mathrm{s}}$. Our results reveal an unprecedented strong spin-excitation anisotropy with a large energy scale well above the $d_{xz}/d_{yz}$ orbital splitting, suggesting that the nematic phase transition is primarily spin-driven. Moreover, the measured high-energy spin excitations are dispersive and underdamped, which can be understood from a local-moment perspective. Our findings provide the much-needed understanding of the mechanism for the nematicity of FeSe and points to a unified description of the correlation physics across seemingly distinct classes of Fe-based superconductors.

15 pages, 5 figures

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• Uniaxial Pressure Induced Stripe Order Rotation in La${}_{1.88}$Sr${}_{0.12}$CuO${}_4$
• Spontaneous orbital polarization in the nematic phase of FeSe
• Spin Excitations and Flat Electronic Bands in a Cr-based Kagome Superconductor
• Systematic Mapping of Altermagnetic Magnons by Resonant Inelastic X-Ray Circular Dichroism
• Nematic spin correlations pervading the phase diagram of FeSe${}_{1-x}$S${}_x$
• Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors
• Nematic spectral signatures of the Hund's metal
• Theory of spin-excitation anisotropy in the nematic phase of FeSe obtained from RIXS measurements
• Coexistence of insulator-like paramagnon and metallic spin-orbit exciton modes in SrIrO${}_3$
• Ubiquity of rotational symmetry breaking in superconducting films, from Fe(Te,Se)/Bi${}_2$Te${}_3$ to Nb, and the effect of measurement geometry
• Signatures of three-state Potts nematicity in spin excitations of the van der Waals antiferromagnet FePSe${}_3$