High multipole transitions in NIXS: valence and hybridization in 4f systems

Momentum-transfer (q) dependent non-resonant inelastic x-ray scattering measurements were made at the N4,5 edges for several rare earth compounds. With increasing q, giant dipole resonances diminish, to be replaced by strong multiplet lines at lower energy transfer. These multiplets result from two different orders of multipole scattering and are distinct for systems with simple 4f^0 and 4f^1 initial states. A many-body theoretical treatment of the multiplets agrees well with the experimental data on ionic La and Ce phosphate reference compounds. Comparing measurements on CeO2 and CeRh3 to the theory and the phosphates indicates sensitivity to hybridization as observed by a broadening of 4f^0-related multiplet features. We expect such strong, nondipole features to be generic for NIXS from f-electron systems

Two-dimensional superconductor-insulator transition in bulk single-crystal YBa_2Cu_3O_(6.38)

We use a magnetic field to tune a highly anisotropic single crystal of oxygen-deficient YBa_2Cu_3O_(7-δ) with a transition temperature of 2 K through the superconductor-insulator transition. The sheet resistance scales with temperature, 0.05≤T≤1.0 K, and field, 0≤H≤94 kOe, in a manner predicted by a theory for quantum phase transitions in disordered two-dimensional superconductors