Equation of motion approach to the Hubbard model in infinite dimensions

We consider the Hubbard model on the infinite-dimensional Bethe lattice and construct a systematic series of self-consistent approximations to the one-particle Green's function, $G^{(n)}(\omega),\ n=2,3,\dots\$ . The first $n-1$ equations of motion are exactly fullfilled by $G^{(n)}(\omega)$ and the $n$'th equation of motion is decoupled following a simple set of decoupling rules. $G^{(2)}(\omega)$ corresponds to the Hubbard-III approximation. We present analytic and numerical results for the Mott-Hubbard transition at half filling for $n=2,3,4$.Comment: 10pager, REVTEX, 8-figures not available in postscript, manuscript may be understood without figure

Charge-density-wave order parameter of the Falicov-Kimball model in infinite dimensions

In the large-U limit, the Falicov-Kimball model maps onto an effective Ising model, with an order parameter described by a BCS-like mean-field theory in infinite dimensions. In the small-U limit, van Dongen and Vollhardt showed that the order parameter assumes a strange non-BCS-like shape with a sharp reduction near T approx T_c/2. Here we numerically investigate the crossover between these two regimes and qualitatively determine the order parameter for a variety of different values of U. We find the overall behavior of the order parameter as a function of temperature to be quite anomalous.Comment: (5 pages, 3 figures, typeset with ReVTeX4

Cone opsins and response of female chamois ( Rupicapra rupicapra ) to differently coloured raincoats

Alpine species are often exposed to intense levels of human recreational activities. Exactly how human disturbances influence the behaviour of these species is still open to much debate. For example, little is known regarding how the colourful clothing often worn by tourists influences the behaviour of animals. Tourists wearing colourful clothing may be more conspicuous to local wildlife and thus cause more disturbances. We therefore investigated this question in female chamois (Rupicapra rupicapra) in the Swiss Alps. We firstly investigated, via a morphological and an immunohistochemical approach, whether chamois are likely to have colour vision and would therefore be more likely to respond to different coloured clothing. We detected evidence of two cone types—short-wavelength-sensitive cones (S-cones, JH 455) and middle-wavelength-sensitive cones (M-cones, JH492) in the chamois retina—suggesting that chamois have dichromatic vision, similar to other ungulates. Secondly, via behavioural assays where a person wearing one of three coloured coats commonly worn by tourists (red, yellow and blue) approached a female chamois, we show that neither the alert and flight initiation distance nor the site of refuge were influenced by the raincoat colour. In addition, behavioural responses of the chamois were neither influenced by animal group size nor the presence of kids nor the time of the experiment. The results suggest that, although chamois possess colour vision, they do not react more strongly towards conspicuous colours worn by hikers. We discuss our results in light of what is already known about chamois biology and suggest implications for future studie

Work in progress: developing tools that support the design of easily maintainable rolling stock

Maintenance activities strongly influence the operational availability and life cycle costs of rolling stock. To achieve an effective and efficient maintenance process, it is essential that the design of the trains and its maintenance service are well-aligned. We conduct research on how the design of industrial equipment, such as rolling stock, and the design of its maintenance service influence the maintenance process. We use our findings for the development of methods and tools that support the use of maintenance knowledge in the equipment design process. An example of a past train development project shows that re- designing the mounting construction of a compressor has led to lower maintenance costs and a higher availability of both the trains and the maintenance resources. Based on such examples from industry and from findings in the literature, we expect that our research can contribute to better design decisions, which ultimately results in production systems that are cost-effective over their whole life tim

Towards structured integration of maintenance knowledge in industrial equipment design

Industrial equipment requires maintenance to remain operational. The level of maintenance that is required, and how easily it can be executed, is affected by the characteristics of the equipment. Therefore, design decisions have a strong influence on the effectiveness and efficiency of the maintenance process. Ideally, the design of the equipment should be aligned with the design of the maintenance service. Relevant knowledge about the relationships between them is available in both the literature and in practice. It is essential to bring this knowledge into the equipment design process, but suitable design support for this remains lacking. Therefore, we propose to conduct research on how this knowledge can be systematically integrated into the design process. The final goal of the research is to develop design support that leads to an improved quality of design decisions, so that production systems with an increased life cycle performance will be achieved

Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3

The effect of in-chain and off-chain substitutions on 1D spin fluctuations in the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in order to understand the interplay between defect induced states, enhanced spin-spin correlations and the ground state of low dimensional systems. In-chain and off-chain substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D gap-induced mode as well as a constant intensity of a spinon continuum are observed at low temperatures. A 3D two-magnon density of states now gradually extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This effect is more pronounced in the case of off-chain substitutions (Si) for which a N\'eel state occurs over a larger substitution range, starting at very low concentrations. Besides, additional low energy excitations are induced. These effects, i.e. the shift of a dimensional crossover to higher temperatures are due to an enhancement of the spin-spin correlations induced by a small amount of substitutions. The results are compared with recent Monte Carlo studies on substituted spin ladders, pointing to a similar instability of coupled, dimerized spin chains and spin ladders upon substitution.Comment: 14 pages, 6 eps figures, to be published in PR

Kinetic Anomalies in Addition-Aggregation Processes

We investigate irreversible aggregation in which monomer-monomer, monomer-cluster, and cluster-cluster reactions occur with constant but distinct rates K_{MM}, K_{MC}, and K_{CC}, respectively. The dynamics crucially depends on the ratio gamma=K_{CC}/K_{MC} and secondarily on epsilon=K_{MM}/K_{MC}. For epsilon=0 and gamma<2, there is conventional scaling in the long-time limit, with a single mass scale that grows linearly in time. For gamma >= 2, there is unusual behavior in which the concentration of clusters of mass k, c_k decays as a stretched exponential in time within a boundary layer k<k* propto t^{1-2/gamma} (k* propto ln t for gamma=2), while c_k propto t^{-2} in the bulk region k>k*. When epsilon>0, analogous behaviors emerge for gamma<2 and gamma >= 2.Comment: 6 pages, 2 column revtex4 format, for submission to J. Phys.

Patient versus general population health state valuations:a case study of non-specific low back pain

PURPOSE: The purpose of this study was twofold: (1) to compare non-specific low back pain (LBP) patients' health state valuations with those of the general population, and (2) to explore how aspects of health-related quality of life as measured by the EQ-5D-3L impact non-specific LBP patient valuations. METHODS: Data were used of a randomized controlled trial, including 483 non-specific LBP patients. Outcomes included the EQ-VAS and the EQ-5D-3L. Patient valuations were derived from the EQ-VAS. Population valuations were derived from the EQ-5D-3L using a Dutch VAS-based tariff. The difference between patient and population valuations was assessed using t tests. An OLS linear regression model was constructed to explore how various aspects of health-related quality of life as measured by the ED-5D-3L impact non-specific LBP patient valuations. RESULTS: Non-specific LBP patients valued their health states 0.098 (95% CI 0.082-0.115) points higher than the general population. Only 22.2% of the variance in patient valuations was explained by the patients' EQ-5D-3L health states (R (2) = 0.222). Non-specific LBP patients gave the most weight to the anxiety/depression dimension. CONCLUSIONS: This study demonstrated that non-specific LBP patients value their health states higher than members of the general population and that the choice of valuation method could have important implications for cost-effectiveness analyses and thus for clinical practice