Atomically resolved spectroscopic study of Sr2IrO4: experiment and theory.

Particularly in Sr2IrO4, the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion stabilizes a J(eff) = 1/2 spin-orbital entangled insulating state at low temperatures. Whether this insulating phase is Mott- or Slater-type, has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy (STM/S). STS results clearly illustrate the opening of an insulating gap (150 ~ 250 meV) below the Néel temperature (TN), in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the temperature dependence of the gap is qualitatively consistent with our DFT + dynamical mean field theory (DMFT) results, both showing a continuous transition from a gapped insulating ground state to a non-gap phase as temperatures approach TN. These results indicate a significant Slater character of gap formation, thus suggesting that Sr2IrO4 is a uniquely correlated system, where Slater and Mott-Hubbard-type behaviors coexist.

Mechanism of enhanced optical second-harmonic generation in the conducting pyrochlore-type Pb2Ir2O7-x oxide compound.

The structural, electronic, and optical properties of pyrochlore-type Pb(2)Ir(2)O(6)O(0.55)('), which is a metal without spatial inversion symmetry at room temperature, were investigated. Structural analysis revealed that the structural distortion relevant to the breakdown of the inversion symmetry is dominated by the Pb-O' network but is very small in the Ir-O network. At the same time, gigantic second-harmonic generation signals were observed, which can only occur if the local environment of the Ir 5d electrons features broken inversion symmetry. First-principles electronic structure calculations reveal that the underlying mechanism for this phenomenon is the induction of the noncentrosymmetricity in the Ir 5d bands by the strong hybridization with O' 2p orbitals. Our results stimulate theoretical study of inversion-broken iridates, where exotic quantum states such as a topological insulator and Dirac semimetal are anticipated.

Plasmon response of a quantum-confined electron gas probed by core-level photoemission.

We demonstrate the existence of quantized "bulk" plasmons in ultrathin magnesium films on Si(111) by analyzing plasmon-loss satellites in core-level photoemission spectra, recorded as a function of the film thickness d. Remarkably, the plasmon energy is shown to vary as 1/d2 all the way down to three atomic layers. The loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. With decreasing film thickness, spectral weight is gradually transferred from the plasmon modes to the low-energy single-particle excitations. These results represent striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures.

Tuning the electronic coupling and magnetic moment of a metal nanoparticle dimer in the nonlinear dielectric-response regime.

We show that the electronic coupling and magnetic moment of a silver nanoparticle dimer can be readily tuned by applying an electric field in the nonlinear dielectric-response regime. For a given interparticle separation, the electronic coupling becomes tunable as soon as the system crosses over from the linear to nonlinear regime. Remarkably, this transition takes place for modest strengths of the electric field. Further increase of the field strength may close the HOMO-LUMO gap of the dimer due to Stark shifts, accompanied by the emergence of a net magnetic moment from two nonmagnetic building blocks. These findings, obtained within density functional theory, exhibit the delicate coupling between the electronic and magnetic degrees of freedom and point to new approaches to gain multifunctionality of nanoparticle aggregates.

Electronic coupling and optimal gap size between two metal nanoparticles.

We study the electronic coupling between two silver nanoparticles using ab initio density functional theory for real atoms. We show that the electronic coupling depends on both the gap size of the dimer system and the relative orientation of the particles. As the two particles are separated from touching contact, the dimer undergoes a bond-breaking step, which also establishes the striking existence of an optimal gap size defined by a maximal static polarizability of the dimer. For some dimers, the electronic coupling before the bond breaking can be strong enough to give rise to a net magnetic moment of the dimer, even though the isolated particles are nonmagnetic. These findings may be instrumental in understanding and controlling the physical and chemical properties of closely packed nanoparticle aggregates.

Band-gap problem in semiconductors revisited: effects of core states and many-body self-consistency.

A novel picture of the quasiparticle (QP) gap in prototype semiconductors Si and Ge emerges from an analysis based on all-electron, self-consistent, GW calculations. The deep-core electrons are shown to play a key role via the exchange diagram-if this effect is neglected, Si becomes a semimetal. Contrary to current lore, the Ge 3d semicore states (e.g., their polarization) have no impact on the GW gap. Self-consistency improves the calculated gaps-a first clear-cut success story for the Baym-Kadanoff method in the study of real-materials spectroscopy; it also has a significant impact on the QP lifetimes. Our results embody a new paradigm for ab initio QP theory.