Highlights

Effective operator for dd transition in nonresonant inelastic x-ray scattering
M. van Veenendaal and M. Haverkort
Phys. Rev. B. 77, 224107 (2008)

Recent experiments by Larson et al. demonstrate the feasibility of measuring local d–d excitations using nonresonant inelastic x-ray scattering (IXS). We establish a general framework for the interpretation where the d–d transitions created in the scattering process are expressed in effective one-particle operators that follow a simple selection rule. The different operators can be selectively
probed by employing their different dependence on the direction and magnitude of the transferred momentum. We use the operators to explain the presence of nodal directions and the nonresonant IXS in specific directions and planes. We demonstrate how nonresonant IXS can be used to extract valuable ground state information for orbiton excitations in manganite.

Orbital reconstruction and covalent bonding at an oxide interface
J. Chakhalian, J. W. Freeland, H.-U. Habermeier, G. Cristiani, G. Khaliullin, M. van Veenendaal, and B. Keimer (12.2007)
Science 318, 114 (2007)

Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba2Cu₃O₇ and metallic La0._0.67Ca._0.33MnO₃ by resonant x-ray spectroscopy. A charge of about -0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu 3z²-r² orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures.

APS HIGHLIGHT
UNIVERSITY OF ARKANSAS PRESS RELEASE
SCIENCE BREAKTHROUGH OF THE YEAR Nr. 5
NIU's NORTHERN TODAY (10/22/2007) AND (1/22/2008)

Prediction of strong dichroism induced by x-rays carrying orbital momentum
M. van Veenendaal and I. McNulty
Phys. Rev. Lett. 98 , 157401 (2007)

We predict the presence of strong dichroic effects induced by x-ray beams carrying orbital angular momentum (OAM). Taking the difference between spectra obtained with positive and negative OAM states allows the separation of quadrupolar from dipolar transitions at, e.g., the transition-metal K edges, enabling the study of the unoccupied states in the absence of strong core-hole effects. We study the dependence of OAM-induced x-ray dichroism on different polarization vectors and derive sum rules that relate the integrated intensity to ground-state hole densities. Calculations of spectral line shapes for cuprates, manganites, and ruthenates confirm the strong OAM-induced dichroism and indicate the potential of this new spectroscopy in, e.g., the fields of orbital physics and magnetism.

Generation of spin currents and spin densities in systems with reduced symmetry
D. Culcer and R.Winkler
Phys. Rev. Lett. 99, 226601 (2007)

We show that the spin-current response of a semiconductor crystal to an external electric field is considerably more complex than previously assumed. While in systems of high symmetry only the spin-Hall components are allowed, in systems of lower symmetry other non-spin-Hall components may be present. We argue that, when spin-orbit interactions are present only in the band structure, the distinction between intrinsic and extrinsic contributions to the spin current is not useful. We show that the generation of spin currents and that of spin densities in an electric field are closely related, and that our general theory provides a systematic way to distinguish between them in experiment. We discuss also the meaning of vertex corrections in systems with spin-orbit interactions.