Search

Graphene quality indicators obtained by Raman spectroscopy have been correlated to the structural changes of the graphene/Germanium interface as a function of in-vacuum thermal annealing. Specifically, it is found that graphene becomes markedly defected at 650 {\deg}C. By combining scanning tunneling microscopy, x-Ray Photoelectron Spectroscopy and Near Edge x-ray Absorption Fine Structure Spectroscopy, we conclude that these defects are due to the release of H_{2} gas trapped at the graphene/Germanium interface. The H_{2} gas was produced following the transition from the as-grown hydrogen-termination of the Ge(110) surface to the emergence of surface reconstructions in the substrate. Interestingly, a complete self-healing process is observed in graphene upon annealing to 800 {\deg}C. The subtle interplay revealed between the microscopic changes occurring at the graphene/Germanium interface and graphene's defect density is valuable for advancing graphene growth, controlled 2D-3D heterogeneous materials interfacing and integrated fabrication technology on semiconductors.

The chiral helimagnet, $Cr_{1/3}NbS_{2}$, hosts exotic spin textures, whose influence on the magneto-transport properties, make this material an ideal candidate for future spintronic applications. To date, the interplay between macroscopic magnetic and transport degrees of freedom is believed to result from a reduction in carrier scattering following spin order. Here, we present electronic structure measurements through the helimagnetic transition temperature, $T_{C}$ that challenges this view by showing a Fermi surface comprised of strongly hybridized Nb- and Cr- derived electronic states, and spectral weight in proximity to the Fermi level to anomalously increases as temperature is lowered below $T_{C}$. These findings are rationalized on the basis of first principle, density functional theory calculations, which reveal a large nearest-neighbor exchange energy, suggesting the interaction between local spin moments and hybridized Nb- and Cr- derived itinerant states to go beyond the perturbative interaction of Ruderman-Kittel-Kasuya-Yosida, suggesting instead a mechanism rooted in a Hund's exchange interaction.

We have experimentally determined the lateral registry and geometric structure of free-base porphine (2H-P) and copper-metalated porphine (Cu-P) adsorbed on Cu(111), by means of energy-scanned photoelectron diffraction (PhD), and compared the experimental results to density functional theory (DFT) calculations that included van der Waals corrections within the Tkatchenko-Scheffler approach. Both 2H-P and Cu-P adsorb with their center above a surface bridge site. Consistency is obtained between the experimental and DFT-predicted structural models, with a characteristic change in the corrugation of the four N atoms of the molecule's macrocycle following metalation. Interestingly, comparison with previously published data for cobalt porphine adsorbed on the same surface evidences a distinct increase in the average height of the N atoms above the surface through the series 2H-P, Cu-P, cobalt porphine. Such an increase strikingly anti-correlates the DFT-predicted adsorption strength, with 2H-P having the smallest adsorption height despite the weakest calculated adsorption energy. In addition, our findings suggest that for these macrocyclic compounds, substrate-to-molecule charge transfer and adsorption strength may not be univocally correlated.

We report x-ray absorption and photoemission spectroscopy of the electronic structure in the normal state of metallic YFe2Ge2. The data reveal evidence for large fluctuating spin moments on the Fe sites, as indicated by exchange multiplets appearing in the Fe 3s core level photoemission spectra, even though the compound does not show magnetic order. The magnitude of the multiplet splitting is comparable to that observed in the normal state of the Fe-pnictide superconductors. This shows a connection between YFe2Ge2 and the Fe-based superconductors even though it contains neither pnictogens nor chalcogens. The implication is that the chemical range of compounds showing at least one of the characteristic magnetic signatures of the Fe-based superconductors is broader than previously thought., Comment: To appear

Bulk Rashba systems BiTeX (X = I, Br, Cl) are emerging as important candidates for developing spintronics devices, because of the coexistence of spin-split bulk and surface states, along with the ambipolar character of the surface charge carriers. The need of studying the spin texture of strongly spin-orbit coupled materials has recently promoted circular dichroic Angular Resolved Photoelectron Spectroscopy (cd-ARPES) as an indirect tool to measure the spin and the angular degrees of freedom. Here we report a detailed photon energy dependent study of the cd-ARPES spectra in BiTeX (X = I, Br and Cl). Our work reveals a large variation of the magnitude and sign of the dichroism. Interestingly, we find that the dichroic signal modulates differently for the three compounds and for the different spin-split states. These findings show a momentum and photon energy dependence for the cd-ARPES signals in the bulk Rashba semiconductor BiTeX (X = I, Br, Cl). Finally, the outcome of our experiment indicates the important relation between the modulation of the dichroism and the phase differences between the wave-functions involved in the photoemission process. This phase difference can be due to initial or final state effects. In the former case the phase difference results in possible interference effects among the photo-electrons emitted from different atomic layers and characterized by entangled spin-orbital polarized bands. In the latter case the phase difference results from the relative phases of the expansion of the final state in different outgoing partial waves., Comment: 6 pages, 4 figures

The electronic properties of the Mn:GaSe interface, produced by evaporating Mn at room temperature on an epsilon-GaSe(0001) single crystal surface, have been studied by soft X-ray spectroscopies. Substitutional effects of Mn replacing Ga cations and Mn-Se hybridization effects are found both in core level and valence band photoemission spectra. The Mn cation valence state is probed by XAS measurements at the Mn L-edge, which indicate that Mn diffuses into the lattice as a Mn2+ cation with negligible crystal field effects. The Mn spectral weight in the valence band is probed by resonant photoemission spectroscopy at the Mn L-edge, which also allowed an estimation of the charge transfer and Mott-Hubbard energies on the basis of impurity-cluster configuration-interaction model of the photoemission process. The charge transfer energy is found to scale with the energy gap of the system. Competing effects of Mn segregation on the surface have been identified, and the transition from the Mn diffusion through the surface to the segregation of metallic layers on the surface has been tracked by core-level photoemission., Comment: 9 pages,9 figures

We use core-valence-valence (CVV) Auger spectra to probe the Coulomb repulsion between holes in the valence band of Fe pnictide superconductors. By comparing the two-hole final state spectra to density functional theory calculations of the single particle density of states, we extract a measure of the electron correlations that exist in these systems. Our results show that the Coulomb repulsion is highly screened and can definitively be considered as weak. We also find that there are differences between the 1111 and 122 families and even a small variation as a function of the doping, x, in Ba(Fe1 xCox)2As2. We discuss how the values of the hole-hole Coulomb repulsion obtained from our study relate to the onsite Coulomb parameter "U" used in model and first principles calculations based on dynamical mean field theory, and establish an upper bound for its effective value. Our results impose stringent constraints on model based phase diagrams, Comment: 16 pages, 4 figures

A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO1-xFx (x = 0, 0.11) and Sr(Fe1 xCox)2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10-15 s time scale in the paramagnetic, anti-ferromagnetic and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3 \muB in CeFeAsO and 2.1 \muB in SrFe2As2. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9 \muB in CeFeAsO0.89F0.11 and 1.3 \muB in Sr(Fe0.9Co0.1)2As2. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic super-exchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high temperature superconductivity.

We report an extensive study on the intrinsic bulk electronic structure of the high-temperature superconductor CeFeAsO0.89F0.11 and its parent compound CeFeAsO by soft and hard x-ray photoemission, x-ray absorption and soft-x-ray emission spectroscopies. The complementary surface/bulk probing depth, and the elemental and chemical sensitivity of these techniques allows resolving the intrinsic electronic structure of each element and correlating it with the local structure, which has been probed by extended-x-ray absorption fine structure spectroscopy. The measurements indicate a predominant 4f1 (i.e. Ce3+) initial state configuration for Cerium and an effective valence-band-to-4f charge-transfer screening of the core hole. The spectra also reveal the presence of a small Ce f0 initial state configuration, which we assign to the occurrence of an intermediate valence state. The data reveal a reasonably good agreement with the partial density of states as obtained in standard density functional calculations over a large energy range. Implications for the electronic structure of these materials are discussed., Comment: Accepted for publication in Phys. Rev. B

The orbital symmetries of electron doped iron-arsenide superconductors Ba(Fe1-xCox)2As2 have been measured with x-ray absorption spectroscopy. The data reveal signatures of Fe d electron itinerancy, weak electronic correlations, and a high degree of Fe-As hybridization related to the bonding topology of the Fe dxz+yz states, which are found to contribute substantially at the Fermi level. The energies and detailed orbital character of Fe and As derived unoccupied s and d states are found to be in remarkably good agreement with the predictions of standard density functional theory., Comment: Accepted for publication in Phys. Rev. B, 3 figures. Minor corrections added

The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity., Comment: Shorter version. Accepted in Phys. Rev. Lett

We present an experimental study of the electronic structure of MnSi. Using X-ray Absorption Spectroscopy, X-ray photoemission and X-ray fluorescence we provide experimental evidence that MnSi has a mixed valence ground state. We show that self consistent LDA supercell calculations cannot replicate the XAS spectra of MnSi, while a good match is achieved within the atomic multiplet theory assuming a mixed valence ground state. We discuss the role of the electron-electron interactions in this compound and estimate that the valence fluctuations are suppressed by a factor of 2.5, which means that the Coulomb repulsion is not negligible.