Correlation-Driven Charge and Spin Fluctuations in LaCoO_{3}.

The spin transition in LaCoO_{3} has been investigated using density-functional theory in combination with dynamical mean-field theory employing continuous time quantum Monte Carlo and exact diagonalization impurity solvers. Calculations on the experimental rhombohedral atomic structure with two Co sites per unit cell show that an independent treatment of the Co atoms results in a ground state with strong charge fluctuations induced by electronic correlations. Each atom shows a contribution from either a d^{5} or a d^{7} state in addition to the main d^{6} state. These states play a relevant role in the spin transition which can be understood as a low spin-high spin (LS-HS) transition with significant contributions (~10%) to the LS and HS states of d^{5} and d^{7} states, respectively. We report spectra as well as optical conductivity data for all cases. A thermodynamic analysis reveals a significant kinetic energy gain through introduction of charge fluctuations, which in addition to the potential energy reduction lowers the total energy of the system.

Temporal cross-correlation of x-ray free electron and optical lasers using soft x-ray pulse induced transient reflectivity.

The recent development of x-ray free electron lasers providing coherent, femtosecond-long pulses of high brilliance and variable energy opens new areas of scientific research in a variety of disciplines such as physics, chemistry, and biology. Pump-probe experimental techniques which observe the temporal evolution of systems after optical or x-ray pulse excitation are one of the main experimental schemes currently in use for ultrafast studies. The key challenge in these experiments is to reliably achieve temporal and spatial overlap of the x-ray and optical pulses. Here we present measurements of the x-ray pulse induced transient change of optical reflectivity from a variety of materials covering the soft x-ray photon energy range from 500eV to 2000eV and outline the use of this technique to establish and characterize temporal synchronization of the optical-laser and FEL x-ray pulses.

Insight into the f-derived Fermi surface of the heavy-fermion compound YbRh2Si2.

Angle-resolved photoelectron spectroscopy (ARPES) was used to study the Fermi surface of the heavy-fermion system YbRh(2)Si(2) at a temperature of about 10 K, i.e., a factor of 2 below the Kondo energy scale. We observed sharp structures with a well-defined topology, which were analyzed by comparing with results of band-structure calculations based on the local-density approximation (LDA). The observed bulk Fermi surface presents strong similarities with that expected for a trivalent Yb state, but is slightly larger, has a strong Yb-4f character, and deviates from the LDA results by a larger region without states around the Γ point. These properties are qualitatively explained in the framework of a simple f-d hybridization model. Our analysis highlights the importance of taking into account surface states and doing an appropriate projection along k(z) when comparing ARPES data with results from theoretical calculations.

k Dependence of the crystal-field splittings of 4f states in rare-earth systems.

The occupation, energy separation, and order of the crystal-field-split 4f states are crucial for the understanding of the magnetic properties of rare-earth systems. We provide the experimental evidence that crystal-field-split 4f states exhibit energy dispersion in momentum space leading to variations of energy spacings between them and even of their energy sequence across the Brillouin zone. These observations were made by performing angle-resolved photoemission experiments on YbRh(2)Si(2) and properly simulated within a simple model based on results obtained by inelastic neutron scattering experiments and band structure calculations. Our findings should be generally applicable to rare-earth systems and have considerable impact on the understanding of magnetism and related phenomena.

CeFePO: f-d hybridization and quenching of superconductivity.

As a homologue to the new, Fe-based type of high-temperature superconductors, the electronic structure of the heavy-fermion compound CeFePO was studied by means of angle-resolved resonant photoemission. It was experimentally found-and later on confirmed by local-density approximation (LDA) as well as dynamical mean-field theory (DMFT) calculations-that the Ce 4f states hybridize to the Fe 3d states of d{3z{2}-r{2}} symmetry near the Fermi level that discloses their participation in the occurring electron-correlation phenomena and provides insight into mechanism of superconductivity in oxopnictides.

Method of measurements with random perturbation: application in photoemission experiments.

We report on an application of a simultaneous perturbation stochastic approximation (SPSA) algorithm to filtering systematic noise (SN) with nonzero mean value in photoemission data. In our analysis, we have used a series of 50 single-scan photoemission spectra of W(110) surface where different SNs were added. It was found that the SPSA-evaluated spectrum is in good agreement with the spectrum measured without SN. On the basis of our results, a wide application of SPSA algorithm for evaluation of experimental data is anticipated.

Ultrafast dynamics of localized magnetic moments in the unconventional Mott insulator Sr2IrO4.

We report a time-resolved study of the ultrafast dynamics of the magnetic moments formed by the [Formula: see text] states in Sr2IrO4 by directly probing the localized iridium 5d magnetic state through resonant x-ray diffraction. Using optical pump-hard x-ray probe measurements, two relaxation time scales were determined: a fast fluence-independent relaxation is found to take place on a time scale of 1.5 ps, followed by a slower relaxation on a time scale of 500 ps-1.5 ns.

Interplay of Dirac fermions and heavy quasiparticles in solids.

Many-body interactions in crystalline solids can be conveniently described in terms of quasiparticles with strongly renormalized masses as compared with those of non-interacting particles. Examples of extreme mass renormalization are on the one hand graphene, where the charge carriers obey the linear dispersion relation of massless Dirac fermions, and on the other hand heavy-fermion materials where the effective electron mass approaches the mass of a proton. Here we show that both extremes, Dirac fermions, like they are found in graphene and extremely heavy quasiparticles characteristic for Kondo materials, may not only coexist in a solid but can also undergo strong mutual interactions. Using the example of EuRh2Si2, we explicitly demonstrate that these interactions can take place at the surface and in the bulk. The presence of the linear dispersion is imposed solely by the crystal symmetry, whereas the existence of heavy quasiparticles is caused by the localized nature of the 4f states.

Discovery of 505-million-year old chitin in the basal demosponge Vauxia gracilenta.

Sponges are probably the earliest branching animals, and their fossil record dates back to the Precambrian. Identifying their skeletal structure and composition is thus a crucial step in improving our understanding of the early evolution of metazoans. Here, we present the discovery of 505-million-year-old chitin, found in exceptionally well preserved Vauxia gracilenta sponges from the Middle Cambrian Burgess Shale. Our new findings indicate that, given the right fossilization conditions, chitin is stable for much longer than previously suspected. The preservation of chitin in these fossils opens new avenues for research into other ancient fossil groups.

Three-dimensional chitin-based scaffolds from Verongida sponges (Demospongiae: Porifera). Part II: Biomimetic potential and applications.

In order to evaluate the biomedical potential of three-dimensional chitinous scaffolds of poriferan origin, chondrocyte culturing experiments were performed. It was shown for the first time that freshly isolated chondrocytes attached well to the chitin scaffold and synthesized an extracellular matrix similar to that found in other cartilage tissue engineering constructs. Chitin scaffolds also supported deposition of a proteoglycan-rich extracellular matrix of chondrocytes seeded bioconstructs in an in vivo environment. We suggest that chitin sponge scaffolds, apart from the demonstrated biomedical applications, are highly optimized structures for use as filtering systems, templates for biomineralization as well as metallization in order to produce catalysts.

Three-dimensional chitin-based scaffolds from Verongida sponges (Demospongiae: Porifera). Part I. Isolation and identification of chitin.

Marine invertebrate organisms including sponges (Porifera) not only provide an abundant source of biologically active secondary metabolites but also inspire investigations to develop biomimetic composites, scaffolds and templates for practical use in materials science, biomedicine and tissue engineering. Here, we presented a detailed study of the structural and physico-chemical properties of three-dimensional skeletal scaffolds of the marine sponges Aiolochroia crassa, Aplysina aerophoba, A. cauliformis, A. cavernicola, and A. fulva (Verongida: Demospongiae). We show that these fibrous scaffolds have a multilayered design and are made of chitin. (13)C solid-state NMR spectroscopy, NEXAFS, and IR spectroscopy as well as chitinase digestion and test were applied in order to unequivocally prove the existence of alpha-chitin in all investigated species.

Charge transport in proteins probed by resonant photoemission.

The degrees of charge localization in the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of the bacterial surface layer protein of Bacillus sphaericus NCTC 9602 were studied by resonant photoemission. In agreement with a charge transport hopping mechanism that involves torsional motions of the peptide backbone, the lifetime of electrons excited into the LUMO was found to be approximately 100 fs.

Tuning the hybridization at the surface of a heavy-fermion system.

Electron-hybridization phenomena in YbRh_{2}Si_{2} were probed by angle-resolved photoemission. It was shown that the Yb 4f-Rh 4d hybridization strength in the surface region of this heavy-fermion material can be varied by deposition of Ag. Site-specific charge transfer from adatoms leads to change of the energy overlap of the interacting states close to the Fermi energy. Our study demonstrates a new way to tune the hybridization between 4f and valence electrons as well as the induced strong correlation effects at the surface of heavy-fermion systems.

Hybridization phenomena in nearly-half-filled f-shell electron systems: photoemission study of EuNi2P2.

The mixed-valent compound EuNi2P2 was studied by photoemission. Observed splittings and dispersions of the Eu 4f;{6} final state close to energy crossings of the Eu 4f and Ni 3d states are explained in terms of hybridization by a momentum and energy dependence of the electron hopping matrix element. These data obtained for a system with more than one 4f electron (hole) show that dispersions and hybridization gaps related to Kondo and heavy-fermion behavior can be found in other rare-earth-metal compounds apart from Ce and Yb-based ones.

Electron-electron correlation in graphite: a combined angle-resolved photoemission and first-principles study.

The full three-dimensional dispersion of the pi bands, Fermi velocities, and effective masses are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The band structure by density-functional theory underestimates the slope of the bands and the trigonal warping effect. Including electron-electron correlation on the level of the GW approximation, however, yields remarkable improvement in the vicinity of the Fermi level. This demonstrates the breakdown of the independent electron picture in semimetallic graphite and points toward a pronounced role of electron correlation for the interpretation of transport experiments and double-resonant Raman scattering for a wide range of carbon based materials.

Photoemission insight into heavy-fermion behavior in YbRh2Si2.

As shown by angle-resolved photoemission (PE), hybridization of bulk Yb 4f(2+) states with a shallow-lying valence band of the same symmetry leads in YbRh2Si2 to dispersion of a 4f PE signal in the region of the Kondo resonance with a Fermi-energy crossing close to Gamma[over ]. Additionally, renormalization of the valence state results in the formation of a heavy band that disperses parallel to the 4f originating signal. The symmetry and character of the states are probed by circular dichroism and the photon-energy dependence of the PE cross sections.

Wave-vector conservation upon hybridization of 4f and valence-band states observed in photoemission spectra of a Ce monolayer on W(110).

Angle-resolved resonant photoemission data for a hexagonally ordered monolayer of Ce on W(110) are presented. The spectra reveal a splitting of the 4f(0) ionization peak around a point in k space where a degeneracy with a valence-band state is expected. The phenomenon is described within a simple approach to the periodic Anderson model. It is found that the Ce 4f state forms a band and hybridization predominantly occurs between the 4f and the valence-band states at the same wave vector.

Energy dispersion of 4f-derived emissions in photoelectron spectra of the heavy-fermion compound YbIr2Si2.

Angle-resolved photoemission spectra of the heavy-fermion system YbIr(2)Si(2) are reported that reveal strong momentum (k) dependent splittings of the 4f(13) bulk and surface emissions around the expected intersection points of the 4f final states with valence bands in the Brillouin zone. The obtained dispersion is explained in terms of a simplified periodic Anderson model by a k dependence of the electron hopping matrix element disregarding clearly interpretation in terms of a single-impurity model.

Interpretation of resonant photoemission spectra of solid actinide systems.

It is shown that valence-band photoemission (PE) spectra of epitaxial hcp films of U metal may be understood within a one-step model of PE based on an itinerant description of the U 5f states. Particularly, the PE intensities of the f-derived bands are correctly reproduced for photon energies between 60 and 103 eV. It is found that a cross-section minimum of the Fermi-energy feature at h(nu) = 90-94 eV, which previously was assigned to a Fano antiresonance at the 5d-->5f excitation threshold, is mainly caused by a Cooper minimum. Analogous 5f cross-section variations in the region of the 5d-->5f resonances were obtained for UPd3 and Pu metal.

Cooper minima in the photoemission spectra of solids.

Variations of the photoionization cross section of valence states as a function of interatomic distance are studied by means of atomic and solid-state density functional approaches and compared with photoemission data. In contrast to the free atom case, a series of Cooper minima is found for 4d, 5d, and 5f states in Pd, Ag, Au, and U metals. The discovered fundamental phenomenon is of high importance for the correct interpretation of photoemission data.