The displacive phase transition in SrTiO3 was investigated by means of x-ray diffraction. We used 4.5 keV photons thus probing only a very thin region near the surface. In the low temperature phase the lattice parameters evolve substantially different than in bulk material. We also investigated the phase transition under the influence of an epitaxial coating with YBaCu2O7 and found the nature of the phase transition changed. The near-surface region behaves like an epitaxial thin SrTiO3 film.

Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Sub-luminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of 57 Fe Mössbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data.Strong nonlinear interaction of light with matter is a key requirement for fundamental and applied quantum optical technologies alike. Since conventional materials typically exhibit weak nonlinearities, the ultimate quest for strong nonlinear interactions of individual quanta has led to the development of a number of methods to significantly enhance nonlinear light-matter interactions. Among the most prominent ones are coherently prepared media based on electromagnetically induced transparency, sub-luminal light and related effects [1,2], as well as cavity-enhanced light matter interactions [3].Recently, nuclear quantum optics featuring the interaction of x-ray light with Mössbauer nuclei in the few keV transition energy range has gained considerable momentum, both theoretically [4-9] and experimentally [10][11][12][13][14][15][16][17][18][19]. Interestingly, these experiments operate with less than one resonant x-ray photon per pulse on average due to restrictions in the available x-ray light sources. This raises the question, whether coherent or cavity-based enhancement techniques could be utilized to realize nonlinear light-matter interactions in nuclear quantum optics despite the low number of resonant photons.Here, we report a first step towards this goal, and demonstrate group velocity control of spectrally narrow x-ray pulses (SNXP). Sub-luminal light propagation is achieved by inducing a steep positive linear material dispersion, and verified by direct measurements of the temporal delay imposed on the SNXP. For this, we suitably manipulate the optical response of the ω 0 = 14.4 keV Mössbauer resonance (single nucleus linewidth γ = 4.7 neV) of a large ensemble of 57 Fe nuclei embedded in a thin film planar x-ray cavity. Our approach thereby combines coherent control, as well as cooperative and cavity enhancements of light-matter interaction in a single setup. To enable the direct detection of the temporal pulse delay, we further propose and implement a flexible scheme to generate frequency-tunable SNXP from broadband synchrotron radiation for applications in xray quantum optics. Our theoretical model is in good agreement with the experimental data.Sub-luminal light was first demonstrated in the visible frequency range [20][21][22], and by now has been implemented in a number of platforms [2], particularly also in cavity settings [23,24]. Manipulation of light propagation has also been reported in the x-ray regime. In Ref.[12], a...

The properties of two strongly bent Highly Annealed Pyrolytic Graphite (HAPG) crystals with different thicknesses of 40 µm and 100 µm are studied at all possible reflection orders using x-rays at 4.5 keV and 8 keV photon energies. Typical reflecting areas within 50% reflectivity drop boundaries have sizes of about ≤ 1 mm. These domains are mis-oriented by ≤ 1 minutes of arc to each other. The mosaicity was measured to be ∼ 0.06 • on a 1 × 1 mm 2 scale, whereas it amounts to ∼ 0.14 • when the probed area becomes > 2 × 1 mm 2 . We find that the integrated reflectivity of the reflection (004) is in good agreement with the kinematical diffraction theory, while a maximum value of 2.3 mrad is achieved for 8 keV and reflection (002). The highest spectral resolution is obtained with an x-ray source of ≤ 50 µm size and a 40 µm thin graphite coating, which amounts to E/∆E ≥ 1000 for 4.5 keV and 8 keV. In the case of 8 keV and reflection (008), the resolving power exceeds E/∆E = 2000. In von-Hámos geometry, it was found that > 60% of the reflected photons are confined in a central 500 µm wide profile where high spectral resolution is pertained. Ray tracing simulations reveal that in order to pertain a certain resolution, a larger mosaicity would result in less contributing photons. Thus the efficiency of the crystal drops significantly when the mosaicity is increased and could not be increased by large crystal opening angles.