Study of the charge correlation function in one-dimensional Hubbard heterostructures

We study inhomogeneous one-dimensional Hubbard systems using the density matrix renormalization group method. Different heterostructures are investigated whose configuration is modeled varying parameters like the on-site Coulomb potential and introducing local confining potentials. We investigate their Luttinger liquid properties through the parameter K_rho, which characterizes the decay of the density-density correlation function at large distances. Our main goal is the investigation of possible realization of engineered materials and the ability to manipulate physical properties by choosing an appropriate spatial and/or chemical modulation.Comment: 6 pages, 7 figure

Die Hochtemperatur-Helium-Versuchsanlage (HHV): Aufbau und Beschreibung der Anlage

Die Hochtemperatur-Helium-Versuchsanlage (HHV-Anlage) ist Bestandteil des Entwicklungsprogramms für einen Hochtemperaturreaktor mit Heliumturbine großer Leistung (HHT-Projekt). Ziel des Projektes ist das Aufzeigen des technischen und wirtschaftlichen Potentials sowie der Realisierbarkeit von Kernkraftwerken mit Hochtemperaturreaktor und Heliumturbine. Die Arbeiten des HHT-Projektes werden mit Förderungsmitteln der Bundesrepublik Deutschland, des Landes Nordrhein-Westfalen und der Schweizerischen Eidgenossenschaftin Zusammenarbeit von 8 Partnern aus der Bundesrepublik, der Schweiz und den USA durchgeführt. [...

Uniform and staggered magnetizations induced by Dzyaloshinskii-Moriya interactions in isolated and coupled spin 1/2 dimers in a magnetic field

We investigate the interplay of Dzyaloshinskii-Moriya interactions and an external field in spin 1/2 dimers. For isolated dimers and at low field, we derive simple expressions for the staggered and uniform magnetizations which show that the orientation of the uniform magnetization can deviate significantly from that of the external field. In fact, in the limit where the ${\bf D}$ vector of the Dzyaloshinskii-Moriya interaction is parallel to the external field, the uniform magnetization actually becomes {\it perpendicular} to the field. For larger fields, we show that the staggered magnetization of an isolated dimer has a maximum close to one-half the polarization, with a large maximal value of $0.35 g\mu_B$ in the limit of very small Dzyaloshinskii-Moriya interaction. We investigate the effect of inter-dimer coupling in the context of ladders with Density Matrix Renormalization Group (DMRG) calculations and show that, as long as the values of the Dzyaloshinskii-Moriya and of the exchange interaction are compatible with respect to the development of a staggered magnetization, the simple picture that emerges for isolated dimers is also valid for weakly coupled dimers with minor modifications. The results are compared with torque measurements on Cu$_{2}$(C$_{5}$H$_{12}$N$_{2}$)$_{2}$Cl$_{4}$.Comment: 8 pages, 9 figure

Possible high TcT_c superconductivity mediated by antiferromagnetic spin fluctuations in systems with Fermi surface pockets

We propose that if there are two small pocket-like Fermi surfaces, and the spin susceptibility is pronounced around a wave vector {\bf Q} that bridges the two pockets, the spin-singlet superconductivity mediated by spin fluctuations may have a high transition temperature. Using the fluctuation exchange approximation, this idea is confirmed for the Hubbard on a lattice with alternating hopping integrals, for which $T_c$ is estimated to be almost an order of magnitude larger than those for systems with a large connected Fermi surface.Comment: 5 pages, uses RevTe

The relation between the number of symptoms and other health indicators in working men and women

Simple symptom counts are widely used Indicators of ill-health in health survey research. However, there has only been little research aimed at a better understanding of symptom counts. The purpose of this study was to explore the number of symptoms (a summary score derived from 10 unspecific self-reported symptoms) in a ‘healthy' population. We reanalysed data on a sample of 850 employed men and women (Berne Workplace Health Project). Stepwise ordinal logistic regression analysis was used to study the relation of the number of symptoms to contextual, person-related, behavioural, and health-related variables. In neither men nor women were any of the sociodemographic Indicators significantly associated with the number of symptoms. Of the work-related variables, low job discretion and adverse work environment were significantly associated with the number of symptoms and there was a statistical trend for high job demands. In women, dissatisfaction with salary was the only work-related variable which was significant. The private context, on the other hand, seemed to be more influential for women than for men and there was a statistical trend for strain at home. Among the person-related factors studied, age and emotional problems were significantly associated to the number of symptoms in both men and women. Psychological ill-health and negative future orientation were significant only in men and immersion only in women. Physical inactivity in leisure time was the only behavioural variable found to be significantly associated to the number of symptoms. Among the health-related factors, poor self-rated health was significant for both gender groups, while obesity was significant only in men. Only categories of variables being hypothesized to be unfavourable in terms of health were associated with a higher number of symptoms. Most of the associations found seem to be theoretically meaningful. Despite methodological provisos, our findings support a conception of the number of symptoms as an indicator of general ill-healt

Condensation of magnons and spinons in a frustrated ladder

Motivated by the ever-increasing experimental effort devoted to the properties of frustrated quantum magnets in a magnetic field, we present a careful and detailed theoretical analysis of a one-dimensional version of this problem, a frustrated ladder with a magnetization plateau at m=1/2. We show that even for purely isotropic Heisenberg interactions, the magnetization curve exhibits a rather complex behavior that can be fully accounted for in terms of simple elementary excitations. The introduction of anisotropic interactions (e.g., Dzyaloshinskii-Moriya interactions) modifies significantly the picture and reveals an essential difference between integer and fractional plateaux. In particular, anisotropic interactions generically open a gap in the region between the plateaux, but we show that this gap closes upon entering fractional plateaux. All of these conclusions, based on analytical arguments, are supported by extensive Density Matrix Renormalization Group calculations.Comment: 15 pages, 15 figures. minor changes in tex

Charge order induced by electron-lattice interaction in NaV2O5

We present Density Matrix Renormalization Group calculations of the ground-state properties of quarter-filled ladders including static electron-lattice coupling. Isolated ladders and two coupled ladders are considered, with model parameters obtained from band-structure calculations for $\alpha^\prime$-NaV$_2$O$_5$. The relevant Holstein coupling to the lattice causes static out-of-plane lattice distortions, which appear concurrently with a charge-ordered state and which exhibit the same zigzag pattern observed in experiments. The inclusion of electron-lattice coupling drastically reduces the critical nearest-neighbor Coulomb repulsion $V_c$ needed to obtain the charge-ordered state. No spin gap is present in the ordered phase. The charge ordering is driven by the Coulomb repulsion and the electron-lattice interaction. With electron-lattice interaction, coupling two ladders has virtually no effect on $V_c$ or on the characteristics of the charge-ordered phase. At V=0.46\eV, a value consistent with previous estimates, the lattice distortion, charge gap, charge order parameter, and the effective spin coupling are in good agreement with experimental data for NaV$_2$O_5$.Comment: 7 pages, 9 figure

Sleep strengthens integration of spatial memory systems

Spatial memory comprises different representational systems that are sensitive to different environmental cues, like proximal landmarks or local boundaries. Here we examined how sleep affects the formation of a spatial representation integrating landmark-referenced and boundary-referenced representations. To this end, participants (n = 42) were familiarized with an environment featuring both a proximal landmark and a local boundary. After nocturnal periods of sleep or wakefulness and another night of sleep, integration of the two representational systems was tested by testing the participant's flexibility to switch from landmark-based to boundary-based navigation in the environment, and vice versa. Results indicate a distinctly increased flexibility in relying on either landmarks or boundaries for navigation, when familiarization to the environment was followed by sleep rather than by wakefulness. A second control study (n = 45) did not reveal effects of sleep (vs. wakefulness) on navigation in environments featuring only landmarks or only boundaries. Thus, rather than strengthening isolated representational systems per se, sleep presumably through forming an integrative representation, enhances flexible coordination of representational subsystems