We have synthesized core/shell NaGdF4:Nd3+/NaGdF4 nanocrystals with an average size of 15 nm and exceptionally high photoluminescence (PL) quantum yield. When excited at 740 nm, the nanocrystals manifest spectrally distinguished, near infrared to near infrared (NIR-to-NIR) downconversion PL peaked at ~900, ~1050, and ~1300 nm. The absolute quantum yield of NIR-to-NIR PL reached 40% for core-shell nanoparticles dispersed in hexane. Time-resolved PL measurements revealed that this high quantum yield was achieved through suppression of nonradiative recombination originating from surface states and cross relaxations between dopants. NaGdF4:Nd3+/NaGdF4 nanocrystals, synthesized in organic media, were further converted to be water-dispersible by eliminating the capping ligand of oleic acid. NIR-to-NIR PL bioimaging was demonstrated both in vitro and in vivo through visualization of the NIR-to-NIR PL at ~900 nm under incoherent lamp light excitation. The fact that both excitation and the PL of these nanocrystals are in the biological window of optical transparency, combined with their high quantum efficiency, spectral sharpness and photostability, makes these nanocrystals extremely promising as optical biomaging probes.

We report intense upconversion photoluminescence (PL) in colloidal LiYF4:Er3+ nanocrystals under excitation with telecom-wavelength at 1490 nm. The intensities of two- and three-photon anti-Stokes upconversion PL bands are higher than or comparable to that of the Stokes emission under excitation with low power density in the range of 5–120 W/cm2. The quantum yield of the upconversion PL was measured to be as high as ~1.2±0.1%, which is almost 4 times higher than the highest upconversion PL quantum yield reported up to date for lanthanide-doped nanocrystals in 100 nm sized hexagonal NaYF4:Yb3+20%, Er3+2% using excitation at ~980 nm. Power dependence study revealed that the intensities of all PL bands have linear dependence on the excitation power density, which was explained by saturation effects in the intermediate energy states.

The discovery of the Dirac electron dispersion in graphene [1] led to the question of the Dirac cone stability with respect to interactions. Coulomb interactions between electrons were shown to induce a logarithmic renormalization of the Dirac dispersion. With a rapid expansion of the list of compounds and quasiparticle bands with linear band touching [2], the concept of bosonic Dirac materials has emerged. We consider a specific case of ferromagnets consisting of the Van der Waals-bonded stacks of honeycomb layers, e.g chromium trihalides CrX3 (X = F, Cl, Br and I), that display two spin wave modes with energy dispersion similar to that for the electrons in graphene. At the single particle level, these materials resemble their fermionic counterparts. However, how different particle statistics and interactions affect the stability of Dirac cones has yet to be determined. To address the role of interacting Dirac magnons, we expand the theory of ferromagnets beyond the standard Dyson theory [3, 4] to a case of non-Bravais honeycomb layers. We demonstrate that magnon-magnon interactions lead to a significant momentumdependent renormalization of the bare band structure in addition to strongly momentumdependent magnon lifetimes. We show that our theory qualitatively accounts for hitherto unexplained anomalies in a nearly half century old magnetic neutron scattering data for CrBr3 [5,6]. We also show that honeycomb ferromagnets display dispersive surface and edge states, unlike their electronic analogs.

We present theory and implementation for a new approach for studying solvent effects: the multiconfigurational self-consistent reaction-field (MCSCRF) method. The atom, molecule, or supermolecule is assumed to be surrounded by a linear, homogeneous, continuous medium described by its macroscopic dielectric constant. The electronic structure of the compound is described by a multiconfigurational self-consistent field (MCSCF) wave function. The wave function is fully optimized with respect to all variational parameters in the presence of the surrounding polarizable dielectric medium. We develop a second-order convergent optimization procedure for the solvent states. The solvent integrals are evaluated by an efficient and general algorithm. The flexible description of the electronic structure allows us to accurately describe ground, excited, or ionized states of the solute. Deficiencies in the calculation can therefore be assigned to the cavity model rather than the description of the solute.

The interpretation of molecular valence Auger spectra by means of ab initio computational methods is discussed. As alternatives to self-consistent-field optimizations of the full Auger spectrum, the feasibility of a procedure based on single ionization potentials and of CI calculations using frozen orbitals is tested. Numerical calculations are performed for the CO, N2, NO, and CO2 molecules, which show especially structure-rich Auger spectra. These spectra are analyzed in detail and they are found to contain three nonoverlapping regions of transitions corresponding to final state vacancies in outer–outer, outer–inner, and inner–inner valence orbitals. It is investigated whether this division of the transitions also is relevant with respect to the magnitude of dynamical relaxation errors in the single ionization potential procedure or with respect to the choice of molecular orbital basis in the CI calculations. CI effects are found to be important for intensities and energies of the Auger transitions in the major energy interval of the spectra. These effects are mainly due to near degeneracies between the main double hole states or between these states and configuration states formed by internal–external double excitations. In particular, configuration states involving π–π excitations are found to give important contributions. The one-center intensity model for Auger transitions is evaluated in the case of an open-shell molecule.