Electronic screening-enhanced hole pairing in two-leg spin ladders studied by high-resolution resonant inelastic x-ray scattering at Cu M edges

The TRIXS end-station for femtosecond time-resolved resonant inelastic x-ray scattering experiments at the soft x-ray free-electron laser FLASH

We present the experimental end-station TRIXS dedicated to time-resolved soft x-ray resonant inelastic x-ray scattering (RIXS) experiments on solid samples at the free-electron laser FLASH. Using monochromatized ultrashort femtosecond XUV/soft x-ray photon pulses in combination with a synchronized optical laser in a pump-probe scheme, the TRIXS setup allows measuring sub-picosecond time-resolved high-resolution RIXS spectra in the energy range from 35 eV to 210 eV, thus spanning the M-edge (M₁ and M_2,3) absorption resonances of 3d transition metals and N_4,5-edges of rare earth elements. A Kirkpatrick-Baez refocusing mirror system at the first branch of the plane grating monochromator beamline (PG1) provides a focus of (6 × 6) μm² (FWHM) at the sample. The RIXS spectrometer reaches an energy resolution of 35-160 meV over the entire spectral range. The optical laser system based on a chirped pulse optical parametric amplifier provides approximately 100 fs (FWHM) long photon pulses at the fundamental wavelength of 800 nm and a fluence of 120 mJ/cm² at a sample for optical pump-XUV probe measurements. Furthermore, optical frequency conversion enables experiments at 400 nm or 267 nm with a fluence of 80 and 30 mJ/cm², respectively. Some of the first (pump-probe) RIXS spectra measured with this setup are shown. The measured time resolution for time-resolved RIXS measurements has been characterized as 287 fs (FWHM) for the used energy resolution.

Ultrafast dynamics of spin and orbital correlations in quantum materials: an energy- and momentum-resolved perspective

Many remarkable properties of quantum materials emerge from states with intricate coupling between the charge, spin and orbital degrees of freedom. Ultrafast photo-excitation of these materials holds great promise for understanding and controlling the properties of these states. Here, we introduce time-resolved resonant inelastic X-ray scattering (tr-RIXS) as a means of measuring the charge, spin and orbital excitations out of equilibrium. These excitations encode the correlations and interactions that determine the detailed properties of the states generated. After outlining the basic principles and instrumentations of tr-RIXS, we review our first observations of transient antiferromagnetic correlations in quasi two dimensions in a photo-excited Mott insulator and present possible future routes of this fast-developing technique. The increasing number of X-ray free electron laser facilities not only enables tackling long-standing fundamental scientific problems, but also promises to unleash novel inelastic X-ray scattering spectroscopies. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Alignment of the aberration-free XUV Raman spectrometer at FLASH

An extreme-ultraviolet (XUV) double-stage Raman spectrometer is permanently installed as an experimental end-station at the PG1 beamline of the soft X-ray/XUV free-electron laser in Hamburg, FLASH. The monochromator stages are designed according to the Czerny-Turner optical scheme, adapted for the XUV photon energy range, with optical elements installed at grazing-incidence angles. Such an optical scheme along with the usage of off-axis parabolic mirrors for light collimation and focusing allows for aberration-free spectral imaging on the optical axis. Combining the two monochromators in additive dispersion mode allows for reaching high resolution and superior stray light rejection, but puts high demands on the quality of the optical alignment. In order to align the instrument with the highest precision and to quantitatively characterize the instrument performance and thus the quality of the alignment, optical laser interferometry, Hartmann-Shack wavefront-sensing measurements as well as off-line soft X-ray measurements and extensive optical simulations were conducted. In this paper the concept of the alignment scheme and the procedure of the internal optical alignment are presented. Furthermore, results on the imaging quality and resolution of the first monochromator stage are shown.

Interface Engineering and Emergent Phenomena in Oxide Heterostructures

Complex oxide interfaces have mesmerized the scientific community in the last decade due to the possibility of creating tunable novel multifunctionalities, which are possible owing to the strong interaction among charge, spin, orbital, and structural degrees of freedom. Artificial interfacial modifications, which include defects, formal polarization, structural symmetry breaking, and interlayer interaction, have led to novel properties in various complex oxide heterostructures. These emergent phenomena not only serve as a platform for investigating strong electronic correlations in low-dimensional systems but also provide potentials for exploring next-generation electronic devices with high functionality. Herein, some recently developed strategies in engineering functional oxide interfaces and their emergent properties are reviewed.

Large Enhancement of 2D Electron Gases Mobility Induced by Interfacial Localized Electron Screening Effect

The interactions between delocalized and localized charges play important roles in correlated electron systems. Here, using a combination of transport measurements, spectroscopic ellipsometry (SE), and X-ray absorption spectroscopy (XAS) supported by theoretical calculations, we reveal the important role of interfacial localized charges and their screening effects in determining the mobility of (La_0.3 Sr_0.7 )(Al_0.65 Ta_0.35 )O₃ /SrTiO₃ (LSAT/SrTiO₃ ) interfaces. When the LSAT layer thickness reaches the critical value of 5 uc, the insulating interface abruptly becomes conducting, accompanied by the appearance of a new midgap state. This midgap state emerges at ≈1 eV below the Ti t_2g band and shows a strong character of Ti 3d_xy - O 2p hybridization. Increasing the LSAT layer from 5 to 18 uc, the number of localized charges increases, resulting in an enhanced screening effect and higher mobile electron mobility. This observation contradicts the traditional semiconductor interface where the localized charges always suppress the carrier mobility. These results demonstrate a new strategy to probe localized charges and mobile electrons in correlated electronic systems and highlight the important role of screening effects from localized charges in improving the mobile electron mobility at complex oxide interfaces.

Diffraction gratings metrology and ray-tracing results for an XUV Raman spectrometer at FLASH

The extreme-ultraviolet double-stage imaging Raman spectrometer is a permanent experimental endstation at the plane-grating monochromator beamline branch PG1 at FLASH at DESY in Hamburg, Germany. This unique instrument covers the photon energy range from 20 to 200 eV with high energy resolution of about 2 to 20 meV (design values) featuring an efficient elastic line suppression as well as effective stray light rejection. Such a design enables studies of low-energy excitations like, for example, phonons in solids close to the vicinity of the elastic line. The Raman spectrometer effectively operates with four reflective off-axial parabolic mirrors and two plane-grating units. The optics quality and their precise alignment are crucial to guarantee best performance of the instrument. Here, results on a comprehensive investigation of the quality of the spectrometer diffraction gratings are presented. The gratings have been characterized by ex situ metrology at the BESSY-II Optics Laboratory, employing slope measuring deflectometry and interferometry as well as atomic force microscopy studies. The efficiency of these key optical elements has been measured at the at-wavelength metrology laboratory using the reflectometer at the BESSY-II Optics beamline. Also, the metrology results are discussed with respect to the expected resolving power of the instrument by including them in ray-tracing studies of the instrument.

Extreme ultraviolet resonant inelastic X-ray scattering (RIXS) at a seeded free-electron laser

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 M_2,3-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.

Microfocusing at the PG1 beamline at FLASH

The Kirkpatrick-Baez (KB) refocusing mirror system installed at the PG1 branch of the plane-grating monochromator beamline at the soft X-ray/XUV free-electron laser in Hamburg (FLASH) is designed to provide tight aberration-free focusing down to 4 µm × 6 µm full width at half-maximum (FWHM) on the sample. Such a focal spot size is mandatory to achieve ultimate resolution and to guarantee best performance of the vacuum-ultraviolet (VUV) off-axis parabolic double-monochromator Raman spectrometer permanently installed at the PG1 beamline as an experimental end-station. The vertical beam size on the sample of the Raman spectrometer, which operates without entrance slit, defines and limits the energy resolution of the instrument which has an unprecedented design value of 2 meV for photon energies below 70 eV and about 15 meV for higher energies up to 200 eV. In order to reach the designed focal spot size of 4 µm FWHM (vertically) and to hold the highest spectrometer resolution, special fully motorized in-vacuum manipulators for the KB mirror holders have been developed and the optics have been aligned employing wavefront-sensing techniques as well as ablative imprints analysis. Aberrations like astigmatism were minimized. In this article the design and layout of the KB mirror manipulators, the alignment procedure as well as microfocus optimization results are presented.