Valley selectivity of soft x-ray excitations of core electrons in two-dimensional transition metal dichalcogenides

Optical properties of semiconducting monolayer transition metal dichalcogenides have received a lot of attention in recent years, following the discovery of the valley selective optical population of either K+ or K− valleys at the direct band gap, depending on the polarization of the incoming light. We use group theoretical selection rules, as well as ab initio DFT calculations, to investigate whether this valley selectivity effect is also present in x-ray optical transitions from the flat core level of the transition metal atom to the valence and conduction band K valleys. Valley selectivity is predicted for s, p1/2, and p3/2 edges in transitions to and from the valence band edges with circularly polarized radiation. Possible novel applications to the diagnostics of valleytronic properties and intervalley dynamics are investigated and the feasibility of ultrafast pump-probe and Kerr rotations experiments with suitable soft-x-ray free-electron laser sources is discussed

Probing Electron-Phonon Interactions Away from the Fermi Level with Resonant Inelastic X-Ray Scattering

Interactions between electrons and lattice vibrations are responsible for a wide range of material properties and applications. Recently, there has been considerable interest in the development of resonant inelastic x-ray scattering (RIXS) as a tool for measuring electron-phonon (e-ph) interactions. Here, we demonstrate the ability of RIXS to probe the interaction between phonons and specific electronic states both near to, and away from, the Fermi level. We perform carbon K-edge RIXS measurements on graphite, tuning the incident x-ray energy to separately probe the interactions of the π∗ and σ∗ electronic states. Our high-resolution data reveal detailed structure in the multiphonon RIXS features that directly encodes the momentum dependence of the e-ph interaction strength. We develop a Green’s-function method to model this structure, which naturally accounts for the phonon and interaction-strength dispersions, as well as the mixing of phonon momenta in the intermediate state. This model shows that the differences between the spectra can be fully explained by contrasting trends of the e-ph interaction through the Brillouin zone, being concentrated at the Γ and K points for the π∗ states while being significant at all momenta for the σ∗ states. Our results advance the interpretation of phonon excitations in RIXS and extend its applicability as a probe of e-ph interactions to a new range of out-of-equilibrium situations

Probing electron-phonon interactions away from the Fermi level with resonant inelastic x-ray scattering

Interactions between electrons and lattice vibrations are responsible for a wide range of material properties and applications. Recently, there has been considerable interest in the development of resonant inelastic x-ray scattering (RIXS) as a tool for measuring electron-phonon ( e -ph) interactions. Here, we demonstrate the ability of RIXS to probe the interaction between phonons and specific electronic states both near to, and away from, the Fermi level. We perform carbon K -edge RIXS measurements on graphite, tuning the incident x-ray energy to separately probe the interactions of the π ∗ and σ ∗ electronic states. Our high-resolution data reveal detailed structure in the multiphonon RIXS features that directly encodes the momentum dependence of the e -ph interaction strength. We develop a Green’s-function method to model this structure, which naturally accounts for the phonon and interaction-strength dispersions, as well as the mixing of phonon momenta in the intermediate state. This model shows that the differences between the spectra can be fully explained by contrasting trends of the e -ph interaction through the Brillouin zone, being concentrated at the Γ and K points for the π ∗ states while being significant at all momenta for the σ ∗ states. Our results advance the interpretation of phonon excitations in RIXS and extend its applicability as a probe of e -ph interactions to a new range of out-of-equilibrium situations

Driving the europium valence state in EuCo2As2\mathrm{EuCo_{2}As_{2}} by external and internal impact

The pressure effect on the europium valence state was investigated by means of X-ray absorption spectroscopy in the series of samples based on the EuCo$_2$As$_2$ compound. The pressure dependence of the europium valence was obtained in a wide range of applied and chemical pressures; the latter was reached by substituting Ca for Eu. The correlation between these two ways to impact the Eu valence was established. The data obtained allow estimating the pressure effect on the electronic structure of EuCo$_2$As$_2$ and other related properties such as the change in magnetic ordering

Local Electronic and Crystal Structure of Magnetic RCo2As2\mathrm{RCo_{2}As_{2}} (R = La, Ce, Pr, Eu)

The experimental X-ray absorption data on the recently prepared magnetic arsenides RCo$_{2}$As$_{2}$ (R = rare earth) are provided. The oxidation state of rare-earth ions is explored by means of the X-ray absorption near edge structure (XANES) spectroscopy. Ce and Eu exhibit the intermediate valence state. The rare-earth valence instability sensitive to external perturbations opens the way for theinfluencing the magnetic state of RCo$_{2}$As$_{2}$compounds

Pressure-induced electronic phase transition in compound EuCu2Ge2\mathrm{EuCu_2Ge_2}

We report the high-pressure XANES study of the electronic phase transition from 4f$^7$ to 4f$^6$ configuration of europium in the rare-earth compound EuCu$_2$Ge$_2$. The hydrostatic pressure dependence of the europium valence was obtained in a wide pressure range (1-30 GPa) at room temperature. It was found that upon the pressure increase above 20 GPa the europium valence does not reach the integer value +3 but stabilizes at 2.87. The experimental results were supported by the band structure calculations in the framework of DFT, which allowed us to discuss the features of 3d-4f hybridization in this system. The study also compares the mechanisms of external and "chemical" pressure by the Si substitution in Ge site in series EuCu$_2$(Si$_x$Ge$_{1-x})_2$

Large polarons as key quasiparticles in SrTiO3 and SrTiO3-based heterostructures

Despite its simple structure and low degree of electronic correlation, SrTiO3 (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 LaAlO3/SrTiO3 two-dimensional electron gas (2DEG) by Ti L3 edge resonant inelastic x-ray scattering. In all samples, we find the hallmark of polarons in the form of intense dd+phonon excitations, and a decrease of the LO3-mode electron-phonon coupling when going from insulating to highly conducting STO single crystals and heterostructures. Both results are attributed to the dynamic screening of the large polaron self-induced polarization, showing that the low-temperature physics of STO and STO-based 2DEGs is dominated by large polaron quasiparticles

Controlling Magnetic Ordering in Ca_1–<i>x</i>Eu_<i>x</i>Co₂As₂ by Chemical Compression

To investigate the interplay between electronic structure and itinerant magnetism, Ca_1–<i>x</i>Eu_<i>x</i>Co₂As₂ solid solutions (<i>x</i> = 0, 0.1, 0.3, 0.4, 0.5, 0.6, 0.65, 0.7, 0.9, 1.0) were prepared by reactions between constituent elements in molten Bi. All of the samples crystallize in the ThCr₂Si₂ structure type. The crystal structure refinement revealed the formation of Co vacancies, the concentration of which decreases as the Eu content increases. The Eu site exhibits mixed valence in all samples. X-ray absorption near-edge structure spectroscopy revealed that the average Eu oxidation state decreases from +2.17 at 0 < <i>x</i> ≤ 0.6 to +2.14 at <i>x</i> ≥ 0.65. The same borderline behavior is observed in magnetic properties. The substitution of Eu for Ca causes the transition from the antiferromagnetic (AFM) ordering of Co moments in CaCo₂As₂ to ferromagnetic (FM) ordering of Co moments in Ca_1–<i>x</i>Eu_<i>x</i>Co₂As₂ with 0.1 ≤ <i>x</i> ≤ 0.6. At higher Eu content, AFM ordering of Eu moments is observed, whereas the Co sublattice exhibits only paramagnetic behavior. Single-crystal neutron diffraction studies revealed that both Co and Eu sublattices order FM in Ca_0.5Eu_0.5Co₂As₂ with the magnetic moments aligned along the tetragonal <i>c</i> axis. In the AFM phases with <i>x</i> ≥ 0.65, only Eu moments are ordered in a helical spin structure defined by an incommensurate propagation vector <i>k</i> = [00<i>q</i>], with the moment lying in the <i>ab</i> plane. The changes in magnetic behavior are well-justified by the analysis of the electronic density of states and crystal orbital Hamilton population

Synthesis, crystal structure, and magnetism of A2Co12As7 A Ca, Y, Ce Yb

Ternary intermetallics, $A_{2}Co_{12}As_{7}$ (A=Ca, Y, Ce–Yb), have been synthesized by annealing mixtures of elements in molten Bi at 1223 K. The materials obtained crystallize in the P63/m variant of the $Zr_{2}Fe_{12}P_{7}$ structure type. The unit cell volume shows a monotonic decrease with the increasing atomic number of the rare-earth metal, with the exception of Ce-, Eu-, and Yb-containing compounds. An examination of these outliers with X-ray absorption near edge structures (XANES) spectroscopy revealed mixed valence of Ce, Eu, and Yb, with the average oxidation states of +3.20(1), +2.47(5), and +2.91(1), respectively, at room temperature. Magnetic behavior of $A_{2}Co_{12}As_{7}$ is generally characterized by ferromagnetic ordering of Co 3d moments at 100–140 K, followed by low-temperature ordering of rare-earth 4f moments. The 3d-4f magnetic coupling changes from antiferromagnetic for A=Pr–Sm to ferromagnetic for A=Ce and Eu–Yb. Polarized neutron scattering experiments were performed to support the postulated ferro- and ferrimagnetic ground states for $Ce_{2}Co_{12}As_{7}$ and $Nd_{2}Co_{12}As_{7}$, respectively