Cedric Weber

Strength of correlations in electron-and hole-doped cuprates

Copper oxide superconductors do not superconduct unless electrons or holes are added to the parent compounds. A theoretical study reveals how the electrons or holes affect the host material microscopically in an asymmetric way.

Vanadium Dioxide: A Peierls-Mott Insulator Stable against Disorder

Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this Letter, we develop and carry out state-of-the-art linear scaling density-functional theory calculations refined with nonlocal dynamical mean-field theory. We identify a complex mechanism, a Peierls-assisted orbital selection Mott instability, which is responsible for the insulating M(1) phase, and which furthermore survives a moderate degree of disorder.

Apical oxygens and correlation strength in electron- and hole-doped copper oxides

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to carry out a comparative investigation of a typical electron doped and a typical hole doped copper oxide, NCCO and LSCO respectively. The parent compounds of both materials are strongly correlated electron systems in the vicinity of the metal to charge transfer insulator transition. In NCCO the magnetic long range order is essential to open a charge transfer gap, while Mott physics is responsible for the gap in LSCO. We highlights the role of the apical oxygens in determining the strength of the correlations and obtaining overall good agreement between theory and several experimentally determined quantities. Results for optical conductivity, polarized X-ray absorption and angle resolved photoemission are presented and compared with experiments.

Orbital Currents in Extended Hubbard Models of High-T c Cuprate Superconductors

Motivated by the recent report of broken time-reversal symmetry and zero momentum magnetic scattering in underdoped cuprates, we investigate under which circumstances orbital currents circulating inside a unit cell might be stabilized in extended Hubbard models that explicitly include oxygen orbitals. Using Gutzwiller projected variational wave functions that treat on an equal footing all instabilities, we show that orbital currents indeed develop on finite clusters and that they are stabilized in the thermodynamic limit if additional interactions, e.g., strong hybridization with apical oxygens, are included in the model.

Local selection rules that can determine specific pathways of DNA unknotting by type II DNA topoisomerases

We performed numerical simulations of DNA chains to understand how local geometry of juxtaposed segments in knotted DNA molecules can guide type II DNA topoisomerases to perform very efficient relaxation of DNA knots. We investigated how the various parameters defining the geometry of inter-segmental juxtapositions at sites of inter-segmental passage reactions mediated by type II DNA topoisomerases can affect the topological consequences of these reactions. We confirmed the hypothesis that by recognizing specific geometry of juxtaposed DNA segments in knotted DNA molecules, type II DNA topoisomerases can maintain the steady-state knotting level below the topological equilibrium. In addition, we revealed that a preference for a particular geometry of juxtaposed segments as sites of strand-passage reaction enables type II DNA topoisomerases to select the most efficient pathway of relaxation of complex DNA knots. The analysis of the best selection criteria for efficient relaxation of complex knots revealed that local structures in random configurations of a given knot type statistically behave as analogous local structures in ideal geometric configurations of the corresponding knot type.

Numerical Simulation of Gel Electrophoresis of DNA Knots in Weak and Strong Electric Fields

Gel electrophoresis allows to separate knotted DNA (nicked circular) of equal length according to the knot type. At low electric fields, complex knots being more compact, drift faster than simpler knots. Recent experiments have shown that the drift velocity dependence on the knot type is inverted when changing from low to high electric fields. We present a computer simulation on a lattice of a closed, knotted, charged DNA chain drifting in an external electric field in a topologically restricted medium. Using a simple Monte Carlo algorithm, the dependence of the electrophoretic migration of the DNA molecules on the type of knot and on the electric field intensity was investigated. The results are in qualitative agreement with electrophoretic experiments done under conditions of low and high electric fields: especially the inversion of the behavior from low to high electric field could be reproduced. The knot topology imposes on the problem the constrain of self-avoidance, which is the final cause of the observed behavior in strong electric field.

Scaling of the transition temperature of hole-doped cuprate superconductors with the charge-transfer energy

We use first-principles calculations to extract two essential microscopic parameters, the charge-transfer energy and the inter-cell oxygen-oxygen hopping, which correlate with the maximum superconducting transition temperature Tc,max across the cuprates. We explore the superconducting state in the three-band model of the copper-oxygen planes using cluster Dynamical Mean-Field Theory. We find that the variation in the charge-transfer energy largely accounts for the empirical trend in Tc,max, resolving a long-standing contradiction with theoretical calculations.

Optical weights and waterfalls in doped charge-transfer insulators: A local density approximation and dynamical mean-field theory study of La2−xSrxCuO4

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to investigate intermediate energy properties of the copper oxides. We identify coherent and incoherent spectral features that results from doping a charge transfer insulator, namely quasiparticles, Zhang-Rice singlet band, and the upper and lower Hubbard bands. Angle resolving these features, we identify a waterfall like feature, between the quasiparticle part and the incoherent part of the Zhang-Rice band. We investigate the assymetry between particle and hole doping. On the hole doped side, there is a very rapid transfer of spectral weight upon doping in the one particle spectra. The optical spectral weight increases superlinearly on the hole doped side in agreement with experiments.

Ising transition driven by frustration in a 2D classical model with continuous symmetry

We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nearest (J1) and next-nearest (J2) exchange couplings on the square lattice by extensive Monte Carlo simulations. We show that, for J2/J1>1/2, thermal fluctuations give rise to an effective Z2 symmetry leading to a finite-temperature phase transition. We provide strong numerical evidence that this transition is in the 2D Ising universality class, and that T(c)-->0 with an infinite slope when J2/J1-->1/2.

Importance of many-body effects in the Kernel of hemoglobin for ligand binding.

We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selection process. This scenario may be described as bonding determined by local valence fluctuations. We support this model using linear-scaling first-principles calculations, in combination with dynamical mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein central to human respiration. We find that variations in Hunds exchange coupling induce a reduction of the iron 3d density, with a concomitant increase of valence fluctuations. We discuss the comparison between our computed optical absorption spectra and experimental data, our picture accounting for the observation of optical transitions in the infrared regime, and how the Hunds coupling reduces, by a factor of 5, the strong imbalance in the binding energies of heme with CO and O(2) ligands.