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X-ray free-electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths.

Using high resolution resonant inelastic x-ray scattering measurements, we have observed that the orbital excitations of the quasi-1D spin chain compound CuGeO3 has nontrivial and noticeable orbital mixing effects from 3d valence spin-orbit coupling. In particular, the SOC leads to a significant correction of dz2 state, which has a direct interplay with the low energy physics of cuprates. Guided by atomic multiplet based modeling, our results strongly support a 3d spin-orbit mixing scenario and explore in detail the nature of these excitations.

In the past few years, we have been witnessing an increased interest for studying materials properties under non-equilibrium conditions. Several well established spectroscopies for experiments in the energy domain have been successfully adapted to the time domain with sub-picosecond time resolution. Here we show the realization of high resolution resonant inelastic X-ray scattering (RIXS) with a stable ultrashort X-ray source such as an externally seeded free electron laser (FEL). We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M2,3-edge at FERMI FEL (Elettra-Sincrotrone Trieste, Italy). The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). We established experimental protocols for performing time resolved RIXS experiments at a FEL source to avoid X ray-induced sample damage, while retaining comparable acquisition time to the synchrotron based measurements. Finally, we measured and modelled the influence of the FEL mixed electromagnetic modes, also present in externally seeded FELs, and the beam transport with ~120 meV experimental resolution achieved in the presented RIXS setup.

In the past few years, we have been witnessing an increased interest for studying materials properties under non-equilibrium conditions. Several well established spectroscopies for experiments in the energy domain have been successfully adapted to the time domain with sub-picosecond time resolution. Here we show the realization of high resolution resonant inelastic X-ray scattering (RIXS) with a stable ultrashort X-ray source such as an externally seeded free electron laser (FEL). We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M2,3-edge at FERMI FEL (Elettra-Sincrotrone Trieste, Italy). The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). We established experimental protocols for performing time resolved RIXS experiments at a FEL source to avoid X ray-induced sample damage, while retaining comparable acquisition time to the synchrotron based measurements. Finally, we measured and modelled the influence of the FEL mixed electromagnetic modes, also present in externally seeded FELs, and the beam transport with ~120 meV experimental resolution achieved in the presented RIXS setup.

Synchrotron radiation photoemission and transmission electron microscopy are used to study the growth of [Lu.sub.2][O.sub.3] on Si(001). The results are discussed in terms of the suitability of [Lu.sub.2][O.sub.3] as a high-[kappa] dielectric and also in terms of theoretical predictions of thermodynamic stability versus the formation of silicon oxide, silicates and silicides and of the band discontinuity problem.

Rare-earth oxides are among the materials which are presently studied as possible replacements of amorphous silicon dioxide as gate insulators in nanometric Si devices; in fact, they generally exhibit high values of the dielectric constant (“high κ”), a necessary requirement to obtain a high capacitance with layer thickness greater than the value below which tunneling currents become unacceptably high. Lu2O3 is one of the rare-earth oxides which may have the required properties in view of its quite high values of κ and forbidden band gap. Since the envisaged dielectric layers are only a few nanometers thick, a description and a physical understanding of the atomic and electronic structure of the interface are of great importance. In this paper, we report a study by synchrotron radiation photoemission and transmission electron microscopy of the growth of Lu2O3 on Si(001). Thanks to the high spectral and spatial resolution, we provide clear evidence of a rather complex structure in which all silicon suboxides and SiO2 are present at the same time, along with a silicatelike phase and Lu2O3 itself; moreover, some grains and both crystalline and amorphous portions are present. In the photoemission experiment, the contribution of the Si surface to the core level line shapes has been taken into account; in the electron microscopy measurements, we present line scans on the nanometer scale of O, Si, and Lu concentrations and a Fourier transform discussion of the structure of the crystalline portions of the overlayer. The valence band discontinuity, which is measured in situ and is relative to the structurally well characterized interface, is found to be 3.16±0.16 eV. These findings are discussed in relation to the suitability of Lu2O3 as a high-κ dielectric and in the context of available theoretical predictions of thermodynamic stability versus the formation of silicon oxide, silicates, and silicides and of the band discontinuity problem. [ABSTRACT FROM AUTHOR]