We present a detailed study of the valence and conduction bands of VO2 across the metal-insulator transition using bulk-sensitive photoelectron and O K x-ray absorption spectroscopies. We observe a giant transfer of spectral weight with distinct features that require an explanation which goes beyond the Peierls transition model as well as the standard single-band Hubbard model. Analysis of the symmetry and energies of the bands reveals the decisive role of the V 3d orbital degrees of freedom. Comparison to recent realistic many body calculations shows that much of the k dependence of the self-energy correction can be cast within a dimer model.

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible2, 3, 4, 5, 6, 7, 8. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics10

Resonant Inelastic X-ray Scattering (RIXS) or X-ray Emission Spectroscopy (XES) access the energy and dispersion of elemental vibronic, charge, spin and orbital excitations for samples down to micron size [1][2][3][4][5][6][7] . Those low energy excitations govern functionality in matter, where small external triggers lead to drastic changes of materials properties. The achilles heel of RIXS has been the needed high photon densities on the sample to compensate for sub-percent fluorescence yields for soft X-rays 8 . Thus sample damage from the dominant non-radiative decays poses limits to materials and the spectral resolution. Means to improve the yield are thus highly desirable. In this work, we report on stimulated X-ray emission for crystalline silicon at photon densities easily achievable with free-electron lasers (FELs) 9 . The 1 stimulated radiative decay of core excited species at the expense of non-radiative processes reduces sample damage and allows for the narrow bandwidth detection in the directed beam of stimulated radiation. We deduce how stimulated X-ray emission can be enhanced by several orders of magnitude to provide with high yield and reduced sample damage a superior probe for low energy excitations and their dispersion in matter. This is the first step to bring non-linear X-ray physics in the condensed phase from theory [10][11][12][13][14][15][16] to application.In the soft X-ray region, the use of non-linear techniques to enhance the signal levels has so far been rendered impossible due to small cross-sections and the short life-time of core-excited states in the regime of some femtoseconds. Only in the last few years, FELs have become available, producing ultrashort, intense soft X-ray pulses 9,[17][18][19][20] . Recently, the stimulation of emission from a single fluorescence line in a rare gas 21 and hard X-ray / optical sum frequency generation 22 have been demonstrated.We present here stimulated X-ray emission from a solid state sample recorded at the freeelectron laser in Hamburg (FLASH) for non-resonant silicon L-edge excitation at 115 eV photon energy. With FEL radiation we produce regions with high 2p core excitation densities. The spontaneously emitted radiation from recombination of the 2p core holes (85 to nearly 100 eV photon energy) seeds the stimulated emission of soft X-ray photons. The emitted spectrum is determined by the spontaneous emission as observed in a typical RIXS or XES experiment and thus conserves all the information and specificity of these methods. By carefully choosing the geometry, we can significantly enhance the weak fluorescence signal at the expense of Auger decays. Fewer elec-2 trons are emitted and electronic damage to the sample is minimized. By properly shaping the FEL beam footprint on the sample the detected signal can be enhanced by orders of magnitude since the usually isotropic emission can get directed towards the detector. This opens a perspective for nonlinear spectroscopy in the X-ray region adopting concepts from non-linear optics, merged wi...

The thickness dependence of the helical antiferromagnetic ordering temperature T(N) was studied for thin Ho metal films by resonant magnetic soft x-ray and neutron diffraction. In contrast with the Curie temperature of ferromagnets, T(N) was found to decrease with film thickness d according to [T(N)(infinity)-T(N)(d)]/T(N)(d) proportional variant (d-d(0))(-lambda(')), where lambda(') is a phenomenological exponent and d(0) is of the order of the bulk magnetic period L(b). These observations are reproduced by mean-field calculations that suggest a linear relationship between d(0) and L(b) in long-period antiferromagnets.