M. Grioni

Measurement of Magnetic Excitations in the Two-Dimensional Antiferromagnetic Sr2CuO2Cl2 Insulator Using Resonant X-Ray Scattering: Evidence for Extended Interactions

We measured the momentum dependence of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr2CuO2Cl2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves display a large (70 meV) dispersion between the zone-boundary points (π, 0) and (π/2, π/2). Employing an extended t-t-t-U one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions, indicative of extended magnetic interactions. The spectral line shape at (π, 0) indicates sizable quantum effects in SCOC and probably more generally in the cuprates.

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

We exploit time- and angle-resolved photoemission spectroscopy to determine the evolution of the out-of-equilibrium electronic structure of the topological insulator Bi2Se3. The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modeling the dynamics of hot electrons after optical excitation. We disentangle a large increase in the effective temperature (T*) from a shift of the chemical potential (mu*), which is consequence of the ultrafast photodoping of the conduction band. The relaxation dynamics of T* and mu* are k independent and these two quantities uniquely define the evolution of the excited charge population. We observe that the energy dependence of the nonequilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.

Spin-orbit-induced orbital excitations in Sr2RuO4 and Ca2RuO4: A resonant inelastic x-ray scattering study

High-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen K edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism x-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λso=200 meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.

Electronic properties of layered oxides: Pulsed laser deposition of YBCO films for in-situ studies by photoemission spectroscopy

Due to imperfect surfaces of most cuprate samples, almost all photoemission studies in the past decade were performed on Bi2Sr2CaCu2O8+x, even though a large fraction of other studies and electronic applications was reported for YBa2Cu3O7-delta (YBCO) family of superconducting compounds. In order to systematically study the gap parameter and the Fermi surface variation in high symmetry directions of YBCO and related oxide films we have constructed a new facility at the Wisconsin Synchrotron Radiation Center. We use the pulsed laser ablation (PLD) system that is directly linked to the photoemission chamber. In our unique approach, the samples never leave the controlled ambient and we oxidize our films, either by molecular oxygen or by ozone. Tn this paper, we summarize some of the most relevant recent results on electronic properties of layered oxides and describe our new facility for the study of YBCO and related oxide films.

Electronic and magnetic excitations in the half-stuffed Cu-O planes of Ba2Cu3O4Cl2 measured by resonant inelastic x-ray scattering

We use resonant inelastic x-ray scattering (RIXS) at the Cu L3 edge to measure the charge and spin excitations in the “half-stuffed” Cu-O planes of the cuprate antiferromagnet Ba2Cu3O4Cl2. The RIXS line shape reveals distinct contributions to the dd excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy response exhibits magnetic excitations. We find a spin-wave branch whose dispersion follows the symmetry of a CuO2 sublattice, similar to the case of the “fully stuffed” planes of tetragonal CuO (T-CuO). Its bandwidth is closer to that of a typical cuprate material, such as Sr2CuO2Cl2, than it is to that of T-CuO. We interpret this result as arising from the absence of the effective four-spin inter-sublattice interactions that act to reduce the bandwidth in T-CuO.