The Heisenberg-RIXS instrument at the European XFEL.

Resonant inelastic X-ray scattering (RIXS) is an ideal X-ray spectroscopy method to push the combination of energy and time resolutions to the Fourier transform ultimate limit, because it is unaffected by the core-hole lifetime energy broadening. Also, in pump-probe experiments the interaction time is made very short by the same core-hole lifetime. RIXS is very photon hungry so it takes great advantage from high-repetition-rate pulsed X-ray sources like the European XFEL.

Collective Nature of Orbital Excitations in Layered Cuprates in the Absence of Apical Oxygens.

We have investigated the 3d orbital excitations in CaCuO_{2} (CCO), Nd_{2}CuO_{4} (NCO), and La_{2}CuO_{4} (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the d_{xy} orbital clearly disperses, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen.

Signature of quantum criticality in cuprates by charge density fluctuations.

The universality of the strange metal phase in many quantum materials is often attributed to the presence of a quantum critical point (QCP), a zero-temperature phase transition ruled by quantum fluctuations. In cuprates, where superconductivity hinders direct QCP observation, indirect evidence comes from the identification of fluctuations compatible with the strange metal phase. Here we show that the recently discovered charge density fluctuations (CDF) possess the right properties to be associated to a quantum phase transition.

Restored strange metal phase through suppression of charge density waves in underdoped YBaCuO.

The normal state of optimally doped cuprates is dominated by the “strange metal” phase that shows a linear temperature () dependence of the resistivity persisting down to the lowest For underdoped cuprates, this behavior is lost below the pseudogap temperature *, where charge density waves (CDWs), together with other intertwined local orders, characterize the ground state. We found that the -linear resistivity of highly strained, ultrathin, underdoped YBaCuO films is restored when the CDW amplitude, detected by resonant inelastic x-ray scattering, is suppressed. This observation suggests an intimate connection between the onset of CDWs and the departure from -linear resistivity in underdoped cuprates.

Evolution of spin excitations from bulk to monolayer FeSe.

In ultrathin films of FeSe grown on SrTiO (FeSe/STO), the superconducting transition temperature T is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations.

Large Polarons as Key Quasiparticles in SrTiO_{3} and SrTiO_{3}-Based Heterostructures.

Despite its simple structure and low degree of electronic correlation, SrTiO_{3} (STO) features collective phenomena linked to charge transport and, ultimately, superconductivity, that are not yet fully explained. Thus, a better insight into the nature of the quasiparticles shaping the electronic and conduction properties of STO is needed. We studied the low-energy excitations of bulk STO and of the LaAlO_{3}/SrTiO_{3} two-dimensional electron gas (2DEG) by Ti L_{3} edge resonant inelastic x-ray scattering.

Structural, Electronic and Magnetic Properties of a Few Nanometer-Thick Superconducting NdBaCuO Films.

Epitaxial films of high critical temperature ( T c ) cuprate superconductors preserve their transport properties even when their thickness is reduced to a few nanometers. However, when approaching the single crystalline unit cell (u.c.

Experimental Determination of Momentum-Resolved Electron-Phonon Coupling.

We provide a novel experimental method to quantitatively estimate the electron-phonon coupling and its momentum dependence from resonant inelastic x-ray scattering (RIXS) spectra based on the detuning of the incident photon energy away from an absorption resonance. We apply it to the cuprate parent compound NdBa_{2}Cu_{3}O_{6} and find that the electronic coupling to the oxygen half-breathing phonon branch is strongest at the Brillouin zone boundary, where it amounts to ∼0.17  eV, in agreement with previous studies.

Theoretical demonstration of how the dispersion of magnetic excitations in cuprate compounds can be determined using resonant inelastic X-ray scattering.

We show that in resonant inelastic x-ray scattering (RIXS) at the copper L and M edge direct spin-flip scattering is in principle allowed. We demonstrate how this possibility can be exploited to probe the dispersion of magnetic excitations, for instance magnons, of cuprates such as the high T(c) superconductors. We compute the relevant local and momentum dependent magnetic scattering amplitudes, which we compare to the elastic and dd-excitation scattering intensities.