Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study

Using the recently developed N-th order muffin-tin orbital-based downfolding technique in combination with the Dynamical Mean Field theory, we investigate the electronic properties of the much discussed Mott insulator TiOCl in the undimerized phase. Inclusion of correlation effects through this approach provides a description of the spectral function into an upper and a lower Hubbard band with broad valence states formed out of the orbitally polarized, lower Hubbard band. We find that these results are in good agreement with recent photo-emission spectra.Comment: 4 pages, 3 figure

Competing itinerant and localized states in strongly correlated BaVS3_3

The electronic structure of the quasi-lowdimensional vanadium sulfide \bavs3 is investigated for the different phases above the magnetic ordering temperature. By means of density functional theory and its combination with dynamical-mean field theory, we follow the evolution of the relevant low-energy electronic states on cooling. Hence we go in the metallic regime from the room temperature hexagonal phase to the orthorhombic phase after the first structural transition, and close with the monoclinic insulating phase below the metal-insulator transition. Due to the low symmetry and expected intersite correlations, the latter phase is treated within cellular dynamical mean-field theory. It is generally discussed how the intriguing interplay between band-structure and strong-correlation effects leads to the stabilization of the various electronic phases with decreasing temperature.Comment: 12 pages, submitted to PR

A New Approach to Understanding Biological Control of Quinone Electrochemistry

Oxidoreductases play pivotal roles in energy capturing and converting processes of life. During these processes, quinones shuttle protons and reducing equivalents between membrane-bound oxidoreductases that generate the proton motive force during oxidative phosphorylation and photophosphorylation. A key mechanistic feature of these oxidoreductases is their ability to tune the reduction potentials of the hydroquinone, semiquinone and oxidized states of their substrate quinones. This level of control allows for maximization of conversion efficiency between the energy of the quinone reducing equivalents and the proton motive force, and prevents side reactions that may be fatal to cells. A half-century of experimental study and computational modeling of the respiratory and photosynthetic complexes has revealed little information on how this mechanistic control is accomplished. To obtain mechanistic insights into the control process, it is necessary to eliminate the biological complexity intrinsic to natural quinone oxidoreductases and create experimental systems that are simplified maquettes of quinone active sites. In this work, development of a naphthoquinone amino acid (Naq), modeled after vitamin K, allowed the creation of a range of quinone peptide maquettes designed to address uncertain mechanistic details of biological quinone control. In a simple heptamer, Naq acquires properties of quinone cofactors found in the three distinct classes of active sites of membrane oxidoreductases under different experimental conditions. Study of Naq in a lanthanide ion binding EF hand peptide allowed observation of the effect of a structural transition from coil to alpha-helix on the aqueous midpoint potential of Naq and measurement of the rate of electron transfer between reduced and oxidized Naq. Naq was also incorporated into a structured miniprotein based upon the TrpCage using a combination of the SCADS computational approach and iterative redesign by hand, creating a simple scaffold for evaluating effects of changing the local environment on Naq. Finally, using expressed protein ligation, Naq was incorporated into a single chain heme-binding maquette. Studies using this multi-cofactor protein to explore electron transfer reactions to and from Naq like those critical to respiration and photosynthesis are underway

Controlling the Kondo Effect in CoCu_n Clusters Atom by Atom

Clusters containing a single magnetic impurity were investigated by scanning tunneling microscopy, spectroscopy, and ab initio electronic structure calculations. The Kondo temperature of a Co atom embedded in Cu clusters on Cu(111) exhibits a non-monotonic variation with the cluster size. Calculations model the experimental observations and demonstrate the importance of the local and anisotropic electronic structure for correlation effects in small clusters.Comment: 4 pages, 4 figure