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R Oppermann

Publications and source records attributed to R Oppermann.

8 recordsLinked to original sources
PubMed2005

Scaling and renormalization group in replica-symmetry-breaking space: evidence for a simple analytical solution of the Sherrington-Kirkpatrick model at zero temperature.

Using numerical self-consistent solutions of a sequence of finite replica symmetry breakings (RSB) and Wilson's renormalization group but with the number of RSB steps playing a role of decimation scales, we report evidence for a nontrivial T-->0 limit of the Parisi order function q(x) for the Sherrington-Kirkpatrick spin glass. Supported by scaling in RSB space, the fixed point order function is conjectured to be q*(a)=sqrt[pi]/2 a/xi erf(xi/a) on 0 a at T =0 and xi approximately 1.13+/-0.01. Xi plays the role of a correlation length in a-space. q*(a) may be viewed as the solution of an effective 1D field theory.

Journal Article↗
PubMed2000

Schwinger-Keldysh semionic approach for quantum spin systems.

We derive a path-integral Schwinger-Keldysh approach for quantum spin systems. This is achieved by means of a semionic representation of spins as fermions with imaginary chemical potential. The major simplifying feature in comparison with other representations (Holstein-Primakoff, Dyson-Maleev, slave bosons/fermions, etc.) is that the local constraint is taken into account exactly. As a result, the standard diagram technique with the usual Feynman codex is constructed. We illustrate the application of this technique for the Néel and spin-liquid states of the antiferromagnetic Heisenberg model.

Journal Article↗
PubMed2000

Density of states of disordered two-dimensional crystals with half-filled band

A diagrammatic method is applied to study the effects of commensurability in two-dimensional disordered crystalline metals by using the particle-hole symmetry with respect to the nesting vector P0 = +/-pi / a,pi / a for a half-filled electronic band. The density of electronic states (DoS) is shown to have nontrivial quantum corrections due to both nesting and elastic impurity scattering processes, and as a result the Van Hove singularity is preserved in the center of the band. However, the energy dependence of the DoS is significantly changed. A small offset from the middle of the band gives rise to the disappearance of quantum corrections to the DoS.

Journal Article↗
PubMed2000

Stress, burnout and locus of control in German nurses.

The aim of this study was to evaluate the effects of locus of control and work-related stress on burnout in hospital staff nurses. A convenience sample of 361 staff nurses from nine units in five German hospitals were surveyed using the Maslach Burnout Inventory, the Locus of Control Questionnaire and a Work-Related Stress Inventory. Causal modeling was used to explore the moderating effect of locus of control on burnout. Results support the hypothesized model and suggest that greater work-related stress and burnout would be associated with poorer locus of control in nurses. The findings supported the notion that perceived degree of control is instrumental in enabling nurses to cope with stress and burnout.

Adaptation, Psychological↗