J. van den Brink

Theoretical prediction of perfect spin filtering at interfaces between close-packed surfaces of Ni or Co and graphite or graphene

The in-plane lattice constants of close-packed planes of fcc and hcp Ni and Co match that of graphite almost perfectly so that they share a common two-dimensional reciprocal space. Their electronic structures are such that they overlap in this reciprocal space for one spin direction only allowing us to predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. First-principles calculations of the scattering matrix show that the spin filtering is quite insensitive to amounts of interface roughness and disorder which drastically influence the spin-filtering properties of conventional magnetic tunnel junctions or interfaces between transition metals and semiconductors. When a single graphene sheet is adsorbed on these open d-shell transition-metal surfaces, its characteristic electronic structure, with topological singularities at the K points in the two-dimensional Brillouin zone, is destroyed by the chemical bonding. Because graphene bonds only weakly to Cu which has no states at the Fermi energy at the K point for either spin, the electronic structure of graphene can be restored by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin-injection property.

Observation of phonons with resonant inelastic x-ray scattering

Phonons, the quantum mechanical representation of lattice vibrations, and their coupling to the electronic degrees of freedom are important for understanding thermal and electric properties of materials. For the first time, phonons have been measured using resonant inelastic x-ray scattering (RIXS) across the Cu K-edge in cupric oxide (CuO). Analyzing these spectra using an ultra-short core-hole lifetime approximation and exact diagonalization techniques, we can explain the essential inelastic features. The relative spectral intensities are related to the electron-phonon coupling strengths.

Ultrashort Lifetime Expansion for Indirect Resonant Inelastic X-ray Scattering

In indirect resonant inelastic x-ray scattering RIXS an intermediate state is created with a core-hole that has an ultrashort lifetime. The core-hole potential therefore acts as a femtosecond pulse on the valence electrons. We show that this fact can be exploited to integrate out the intermediate states from the expression for the scattering cross section. By doing so we obtain an effective scattering cross section that only contains the initial and final scattering states. This effective cross section turns out to be a linear combination of the charge response function Sq, and the dynamic longitudinal spin density correlation function, both with a resonant prefactor. This result is asymptotically exact for both strong and weak local core-hole potentials and ultrashort lifetimes. The resonant scattering prefactor is shown to be weakly temperature dependent. We also derive a sum rule for the total scattering intensity and generalize the results to multiband systems. One of the remarkable outcomes is that one can change the relative charge and spin contribution to the inelastic spectral weight by varying the incident photon energy.

Temperature-dependent electron spin polarization of the triplet state of the primary donor in Rhodopseudomonas viridis

Abstract The electron spin polarization (ESP) of the triplet of the primary donor ( 3 P) in reaction centers of the photosynthetic bacterium Rhodopseudomonas viridis shows an anisotropic temperature dependence (Van Wijk, F.G.H. and Schaafsma, T.J. (1988) Biochim. Biophys. Acta 936, 236). The reported inversion of the initial electron spin polarization (IESP) for the canonical Y -direction of 3 P at 100 K has been explained by means of coherent S-T ± mixing in the radical pair, due to a fast relaxing electron spin on the iron-quinone acceptor complex X (Hore, P.J., Hunter, D.A., Van Wijk, F.G.H., Schaafsma, T.J. and Hoff, A.J. (1988) Biochim. Biophys. Acta 936, 249). Using direct-detection EPR, we show that at 100 K the IESP in randomly oriented samples is not inverted for the canonical Y -direction of 3 P. Furthermore, in single crystals the IESP at 100 K is shown to be almost zero for the complete YZ -plane of 3 P. Since X ·− shows a strong g -anisotropy, the model of Hore et al., in which polarization-inversion only occurs when the effective g -value of X ·− is around g = 2.00, is inadequate to explain the temperature-dependent changes of the IESP. Therefore, we conclude that anisotropic fast spin-lattice relaxation in the radical pair triplet state is the origin of the temperature dependence of the ESP. The inversion for the canonical Y -direction under continuous illumination is the result of the interplay of spin-lattice relaxation in 3 P and its triplet decay rates, in combination with changes in the IESP.

Magnetic excitations in La2CuO4 probed by indirect resonant inelastic x-ray scattering

Recent experiments on La2CuO4 suggest that indirect resonant inelastic x-ray scattering RIXS might provide a probe for transversal spin dynamics. We present in detail a systematic expansion of the relevant magnetic RIXS cross section by using the ultrashort core-hole lifetime UCL approximation. We compute the scattering intensity and its momentum dependence in leading order of the UCL expansion. The scattering is due to two-magnon processes and is calculated within a linear spin-wave expansion of the Heisenberg spin model for this compound, including longer range and cyclic spin interactions. We observe that the latter terms in the Hamiltonian enhance the first moment of the spectrum if they strengthen the antiferromagnetic ordering. The theoretical spectra agree very well with experimental data, including the observation that scattering intensity vanishes for the transferred momenta q=0,0 and q=,. We show that at finite temperature, there is an additional single-magnon contribution to the scattering with a spectral weight proportional to T 3 .W e also compute the leading corrections to the UCL approximation and find them to be small, setting the UCL results on a solid basis. All this univocally points to the conclusion that the observed low temperature RIXS intensity in La2CuO4 is due to two-magnon scattering.

Site-Directed Isotope Labelling as a Tool in Spectroscopy of Photosynthetic Preparations. Investigations on Quinone Binding in Bacterial Reaction Centers

The strategy of combining site-directed isotope labelling with various spectroscopic techniques for investigating structural details of complex biosystems is illustrated with a study of the binding of selectively 13C-labelled ubiquinone-10 cofactors to the bacterial photosynthetic reaction center protein. It is shown that the (math) group of Q A - of Rb. sphaeroides is hydrogen-bonded to the imidazole Nδ(1) of HisM219. For the neutral QA moiety, FTIR shows a distinct asymmetry of the 1- and (math) groups, the latter carbonyl having a bond order about 0.1 lower than that of the (math) group. On the basis of solid-state NMR spectroscopy and quantum chemical calculations it is suggested that this bond order difference might be largely due to a protein-induced change in sp 2 hybridization of the 4-carbon, and that formation of the semiquinone leads to a considerable strengthening of hydrogen bond(s) to the 4-carbonyl group.

Polarization suppression and nonmonotonic local two-body correlations in the two-component Bose gas in one dimension

We study the interplay of quantum statistics, strong interactions, and finite temperatures in the twocomponent spinor Bose gas with repulsive delta-function interactions in one dimension. Using the Thermodynamic Bethe Ansatz, we obtain the equation of state, population densities, and local density correlation numerically as a function of all physical parameters interaction, temperature, and chemical potentials, quantifying the full crossover between low-temperature ferromagnetic and high-temperature unpolarized regimes. In contrast to the single component, Lieb-Liniger gas, nonmonotonic behavior of the local density correlation as a function of temperature is observed.

Orbital-assisted Peierls state in NaTiSi2O6

Does the quasi–one-dimensional titanium pyroxene NaTiSi2O6 exhibit the novel orbital-assisted Peierls state? We calculate its groundstate properties by three methods: Monte Carlo simulations, a spin-orbital decoupling scheme and a mapping onto a classical model. The results show univocally that for the spin and orbital ordering to occur at the same temperature—an experimental observation—the crystal field needs to be small and the orbitals are active. We also find that fluctuations in the spin-orbital sector drive the transition, explaining why bandstructure methods fail to find it. The conclusion that NaTiSi2O6 shows an orbital assisted Peierls transition is therefore inevitable.

Temperature-dependent electron spin polarization of the triplet state of the primary donor in photosynthetic reaction centers ofRhodopseudomonas viridis: A time-resolved EPR study of single crystals and solid solution

The electron spin polarization (ESP) of triplet of the primary donor (3P) ofRhodopseudomonas viridis reaction centers (RCs) is anomalous at temperatures above 25 K, i.e. the steady-state ESP changes from AEEAAE to AEAEAE. Fast, time-resolved EPR measurements in solid solution and single crystals of RCs show that this phenomenon results most probably from fast anisotropic spin-lattice relaxation in the radical pair triplet state (kr≈ 5·109s−1 at 25 K).