The many-body formalism for dynamical mean-field theory is extended to treat nonequilibrium problems. We illustrate how the formalism works by examining the transient decay of the oscillating current that is driven by a large electric field turned on at time t=0. We show how the Bloch oscillations are quenched by the electron-electron interactions, and how their character changes dramatically for a Mott insulator.

The high intrinsic spin and long spin relaxation time of manganese-12-acetate (Mn(12)) makes it an archetypical single molecular magnet. While these characteristics have been measured on bulk samples, questions remain whether the magnetic properties replicate themselves in surface supported isolated molecules, a prerequisite for any application. Here we demonstrate that electrospray ion beam deposition facilitates grafting of intact Mn(12) molecules on metal as well as ultrathin insulating surfaces enabling submolecular resolution imaging by scanning tunneling microscopy. Using scanning tunneling spectroscopy we detect spin excitations from the magnetic ground state of the molecule at an ultrathin boron nitride decoupling layer. Our results are supported by density functional theory based calculations and establish that individual Mn(12) molecules retain their intrinsic spin on a well chosen solid support.

The exact nonlinear response of noninteracting (Bloch) electrons is examined within a nonequilibrium formalism on the infinite-dimensional hypercubic lattice. We examine the effects of a spatially uniform, but time-varying electric field (ignoring magnetic-field effects). The electronic Green's functions, Wigner density of states, and time-varying current are all determined and analyzed. We study both constant and pulsed electric fields, focusing on the transient response region. These noninteracting Green's functions are an important input into nonequilibrium dynamical mean field theory for the nonlinear response of strongly correlated electrons.

It has been well established that bimetallic systems can exhibit activity different than that of pure metals, and there are many examples in the catalysis literature illustrating the ability of a second metal to promote the desired catalytic activity and selectivity. 1À6 Consequently, there is much interest in basic understanding of the chemical activity on bimetallic surfaces in order to develop catalysts with properties that can be tuned by changing compositions. In some cases, reactions are promoted via a bifunctional mechanism, in which the reaction requires the different activities provided by each metal. 7À12 Furthermore, electronic effects associated with the formation of new me-talÀmetal bonds may alter surface chemical properties, such as CO adsorption strength, 6,13À16 hydrogenation activity, 3,17À19 dehydrogenation activity, 20,21 and reforming selectivity. 3,22,23 Bimetallic surfaces may also provide mixed-metal sites with activity different from that of the pure metal sites, such as on the SnÀPt alloy surfaces. 24À26 In addition, interactions between the metal clusters and the oxide support may also be used to control surface chemistry on the clusters, with lattice oxygen participating in reactions on the oxide-supported clusters. For example, atomic carbon on Ni clusters recombine with lattice oxygen from the titania support to produce gaseous CO, 27,28 and gaseous products containing lattice oxygen are observed in reactions on metal clusters supported on ceria. 29À34 Also for noble metals on ceria supports, ceria plays an important role in oxygen storage in the three-way catalysts for the conversion of CO, NO x , and hydrocarbons into CO 2 , water, and N 2 . 35À37 In other cases, it has been reported that chemical activity occurs at metal clusterÀoxide interfacial sites. 29,38À42 In order to probe the nature of metalÀmetal and metalÀsupport interactions, we have chosen to study a model system consisting of vapor-deposited NiÀAu bimetallic clusters supported on rutile TiO 2 (110). In this model system, the relationships between morphology, composition, and chemical activity can be explored on a fundamental level. The AuÀtitania system is one of chemical interest due to the unusual catalytic properties of

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Optical processes in insulators and semiconductors, including excitonic effects, can be described in principle exactly using time-dependent density-functional theory ͑TDDFT͒. Starting from a linearization of the TDDFT semiconductor Bloch equations in a two-band model, we derive a simple formalism for calculating exciton binding energies. This formalism leads to a generalization of the standard Wannier equation for excitons, featuring a nonlocal effective electron-hole interaction determined by long-range and dynamical exchangecorrelation ͑XC͒ effects. We calculate exciton binding energies in several direct-gap semiconductors using exchange-only and model XC kernels.

We derive exact operator average expressions for the first two spectral moments of nonequilibrium Green's functions for the Falicov-Kimball model and the Hubbard model in the presence of a spatially uniform, time-dependent electric field. The moments are similar to the well-known moments in equilibrium, but we extend those results to systems in arbitrary time-dependent electric fields. Moment sum rules can be employed to estimate the accuracy of numerical calculations; we compare our theoretical results to numerical calculations for the nonequilibrium dynamical mean-field theory solution of the Falicov-Kimball model at half-filling.

Dynamical Mean-Field Theory (DMFT) has established itself as a reliable and well-controlled approximation to study correlation effects in bulk solids and also two-dimensional systems. In combination with standard density-functional theory (DFT) it has been successfully applied to study materials in which localized electronic states play an important role. There are several evidences that for extended systems this DMFT+DFT approach is more accurate than the traditional DFT+U approximation, particularly because of its ability to take into account dynamical effects, such as the time-resolved double occupancy of the electronic orbitals. It was recently shown that this approach can also be successfully applied to study correlation effects in nanostructures. Here, we present a brief review of the recently proposed generalizations of the DFT+DMFT method. In particular, we discuss in details our recently proposed DFT+DMFT approach to study the magnetic properties of nanosystems[1] and present its application to small (up to five atoms) Fe andFePt clusters. We demonstrate that being a mean-field approach, DMFT produces meaningful results even for such small systems. We compare our results with those obtained using DFT+U and find that, as in the case of bulk systems, the latter approach tends to overestimate correlation effects in nanostructures. Finally, we discuss possible ways to farther improve the nano-DFT+DMFT approximation and to extend its application to molecules and nanoparticles on substrates and to nonequilibrium phenomena.