Kinks: Fingerprints of strong electronic correlations

The textbook knowledge of solid state physics is that the electronic specific heat shows a linear temperature dependence with the leading corrections being a cubic term due to phonons and a cubic-logarithmic term due to the interaction of electrons with bosons. We have shown that this longstanding conception needs to be supplemented since the generic behavior of the low-temperature electronic specific heat includes a kink if the electrons are sufficiently strongly correlatedComment: 4 pages, 1 figure, ICM 2009 conference proceedings (to appear in Journal of Physics: Conference Series

Regulation of the \u3cem\u3eEscherichia coli\u3c/em\u3e Tryptophan Operon by Early Reactions in the Aromatic Pathway

7-Methyltryptophan (7MT) or compounds which can be metabolized to 7MT, 3-methylanthranilic acid (3MA) and 7-methylindole, cause derepression of the trp operon through feedback inhibition of anthranilate synthetase. Tyrosine reverses 3MA or 7-methylindole derepression, apparently by increasing the amount of chorismic acid available to the tryptophan pathway. A mutant isolated on the basis of 3MA resistance (MAR 13) was found to excrete small amounts of chorismic acid and to have a feedback-resistant phenylalanine 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase. Genetic evidence indicates that the mutation conferring 3MA resistance and feedback resistance is very closely linked to aroG, the structural gene for the DAHP synthetase (phe). Since feedback inhibition of anthranilate synthetase by l-tryptophan (or 7MT) is competitive with chorismic acid, alterations in growth conditions (added tyrosine) or in a mutant (MAR 13) which increase the amount of chorismic acid available to the tryptophan pathway result in resistance to 7MT derepression. Owing to this competitive nature of tryptophan feedback inhibition of anthranilate synthetase by chorismic acid, the early pathway apparently serves to exert a regulatory influence on tryptophan biosynthesis

Mechanism of 3-Methylanthranilic Acid Derepression of the Tryptophan Operon in \u3cem\u3eEscherichia coli\u3c/em\u3e

3-Methylanthranilic acid (3MA) inhibits growth and causes derepression of the tryptophan biosynthetic enzymes in wild-type strains of Escherichia coli. Previous reports attributed this effect to an inhibition of the conversion of 1-(o-carboxyphenylamino)-1-deoxyribulose 5-phosphate to indole-3-glycerol phosphate and a consequent reduction in the concentration of endogenous tryptophan. Our studies have shown that 3MA-resistant mutants linked to the tryptophan operon have a feedback-resistant anthranilate synthetase; mutants with an altered indole-3-glycerol phosphate synthetase were not found. 3MA or 7-methylindole can be metabolized to 7-methyltryptophan, and 3MA, 7-methylindole, and 7-methyltryptophan lead to derepression of the tryptophan operon. Furthermore, 3MA-resistant mutants are also resistant to 7-methylindole derepression. These results strongly suggest that the primary cause of derepression by 3MA is through its conversion to 7-methyltryptophan, which can inhibit anthranilate synthetase, thereby decreasing the concentration of endogenous tryptophan. Unlike 5- or 6-methyltryptophan, 7-methyltryptophan does not appear to function as an active corepressor

Comparing pertinent effects of antiferromagnetic fluctuations in the two and three dimensional Hubbard model

We use the dynamical vertex approximation (D$\Gamma$A) with a Moriyaesque $\lambda$ correction for studying the impact of antiferromagnetic fluctuations on the spectral function of the Hubbard model in two and three dimensions. Our results show the suppression of the quasiparticle weight in three dimensions and dramatically stronger impact of spin fluctuations in two dimensions where the pseudogap is formed at low enough temperatures. Even in the presence of the Hubbard subbands, the origin of the pseudogap at weak-to-intermediate coupling is in the splitting of the quasiparticle peak. At stronger coupling (closer to the insulating phase) the splitting of Hubbard subbands is expected instead. The $\mathbf{k}$-dependence of the self energy appears to be also much more pronounced in two dimensions as can be observed in the $\mathbf{k}$-resolved D$\Gamma$A spectra, experimentally accessible by angular resolved photoemission spectroscopy in layered correlated systems.Comment: 10 pages, 12 figure

Cluster Dynamical Mean-Field Methods for d-wave Superconductors: the Role of Geometry

We compare the accuracy of two cluster extensions of Dynamical Mean-Field Theory in describing d-wave superconductors, using as a reference model a saddle-point t-J model which can be solved exactly in the thermodynamic limit and at the same time reasonably describes the properties of high-temperature superconductors. The two methods are Cellular Dynamical Mean-Field Theory, which is based on a real-space perspective, and Dynamical Cluster Approximation, which enforces a momentum-space picture by imposing periodic boundary conditions on the cluster, as opposed to the open boundary conditions of the first method. We consider the scaling of the methods for large cluster size, but we also focus on the behavior for small clusters, such as those accessible by means of present techniques, with particular emphasis on the geometrical structure, which is definitely a relevant issue in small clusters.Comment: 11 pages, 10 figure

Quantum criticality with a twist - interplay of correlations and Kohn anomalies in three dimensions

A general understanding of quantum phase transitions in strongly correlated materials is still lacking. By exploiting a cutting-edge quantum many-body approach, the dynamical vertex approximation, we make an important progress, determining the quantum critical properties of the antiferromagnetic transition in the fundamental model for correlated electrons, the Hubbard model in three dimensions. In particular, we demonstrate that -in contradiction to the conventional Hertz-Millis-Moriya theory- its quantum critical behavior is driven by the Kohn anomalies of the Fermi surface, even when electronic correlations become strong.Comment: 6 pages, 4 figures (8 pages Supplemental Material

Streamline integration as a method for structured grid generation in X-point geometry

We investigate structured grids aligned to the contours of a two-dimensional flux-function with an X-point (saddle point). Our theoretical analysis finds that orthogonal grids exist if and only if the Laplacian of the flux-function vanishes at the X-point. In general, this condition is sufficient for the existence of a structured aligned grid with an X-point. With the help of streamline integration we then propose a numerical grid construction algorithm. In a suitably chosen monitor metric the Laplacian of the flux-function vanishes at the X-point such that a grid construction is possible. We study the convergence of the solution to elliptic equations on the proposed grid. The diverging volume element and cell sizes at the X-point reduce the convergence rate. As a consequence, the proposed grid should be used with grid refinement around the X-point in practical applications. We show that grid refinement in the cells neighboring the X-point restores the expected convergence rate