X-ray free-electron lasers (XFELs) enable the exploration of matter with atomic-scale spatiotemporal resolution. Generating radiation wavelengths below one angstrom is critical for applications such as crystallography and Mössbauer studies. Until now, achieving the subangstrom regime has required electron beam energies ≥ 8 GeV and undulator K parameters ≥ 1.5 . In this Letter, we demonstrate the generation of high-power subangstrom FEL radiation using a 6.2 GeV electron beam and a K parameter around unity and below. We show that exceptional electron beam brilliance—characterized by high peak current and low slice emittance—compensates for the reduced undulator coupling, enabling robust lasing. The demonstration was carried out at SwissFEL, where the subangstrom regime has already been successfully employed in user experiments. Our Letter paves the way for more compact and cost-effective accelerator designs through the optimization of electron beam quality.
We investigate lattice dynamics in LaCoO 3 using inelastic neutron and x-ray scattering over T = 2 − 6 5 0 K , spanning the spin-state crossover at T 1 ≈ 1 0 0 K and the insulator-metal transition at T 2 ≈ 5 5 0 K . Comparison with quasiharmonic lattice-dynamical calculations helps reveal anomalous softening of a ≈ 1 0 − meV oxygen phonon, confined to the temperature interval T 1 ≤ T ≤ T 2 and localized in momentum space at q SSO = ( ½ , ½ , ½ ) c . This wave vector is characteristic of the spin-state-ordering pattern originally proposed by Goodenough [An interpretation of the magnetic properties of the perovskite-type mixed crystals La 1 − x Sr x CoO 3 − λ , ]. Our results provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co 3 + states characterized by q SSO = ( ½ , ½ , ½ ) c , demonstrating that phonons are sensitive probes of fluctuating spin-state correlations in LaCoO 3 .
First-principles predictions of excitation energies and cross sections in inelastic tunneling spectroscopy are challenging, making it difficult to assign experimental features to specific excitations. There is, however, a need for a reliable identification of conductance features. We investigated the spin- 1 / 2 molecule cobaltocene on superconducting Pb(110) and employ Yu-Shiba-Rusinov states combined with a superconducting tip to detect a large number of sharp spectral features. From an analysis of the peak heights at both bias voltage polarities along with model calculations and density-functional theory results we identify spin, vibrational, and orbital excitations that coexist within a single conductance spectrum. The peak height asymmetry turns out to be a useful indicator to determine the type of excitation.
