510 research outputs found
On the origine of the Boson peak
We show that the phonon-saddle transition in the ensemble of generalized
inherent structures (minima and saddles) happens at the same point as the
dynamical phase transition in glasses, that has been studied in the framework
of the mode coupling approximation. The Boson peak observed in glasses at low
temperature is a remanent of this transition.Comment: Proceeding of the Pisa conference September 2002, 13 pages+ 4
figures, To be publiched by Journal of Physic
Frequency behavior of Raman coupling coefficient in glasses
Low-frequency Raman coupling coefficient of 11 different glasses is
evaluated. It is found that the coupling coefficient demonstrates a universal
linear frequency behavior near the boson peak maximum and a superlinear
behavior at very low frequencies. The last observation suggests vanishing of
the coupling coefficient when frequency tends to zero. The results are
discussed in terms of the vibration wavefunction that combines features of
localized and extended modes.Comment: 8 pages, 9 figure
Pseudopotential study of binding properties of solids within generalized gradient approximations: The role of core-valence exchange-correlation
In ab initio pseudopotential calculations within density-functional theory
the nonlinear exchange-correlation interaction between valence and core
electrons is often treated linearly through the pseudopotential. We discuss the
accuracy and limitations of this approximation regarding a comparison of the
local density approximation (LDA) and generalized gradient approximations
(GGA), which we find to describe core-valence exchange-correlation markedly
different. (1) Evaluating the binding properties of a number of typical solids
we demonstrate that the pseudopotential approach and namely the linearization
of core-valence exchange-correlation are both accurate and limited in the same
way in GGA as in LDA. (2) Examining the practice to carry out GGA calculations
using pseudopotentials derived within LDA we show that the ensuing results
differ significantly from those obtained using pseudopotentials derived within
GGA. As principal source of these differences we identify the distinct behavior
of core-valence exchange-correlation in LDA and GGA which, accordingly,
contributes substantially to the GGA induced changes of calculated binding
properties.Comment: 13 pages, 6 figures, submitted to Phys. Rev. B, other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Building democracy from below : lessons from Western Uganda
How to achieve democratisation in the neopatrimonial and agrarian environments that predominate in sub-Saharan Africa continues to present a challenge for both development theory and practice. Drawing on intensive fieldwork in Western Uganda, this paper argues that Charles Tillyâs âdemocratisation as processâ provides us with the framework required to explain the ways in which particular kinds of association can advance democratisation from below. Moving beyond the current focus on how elite-bargaining and certain associational forms may contribute to liberal forms of democracy, this approach helps identify the intermediate mechanisms involved in building democracy from below, including the significance of challenging categorical inequalities, notably through the role of producer groups, and of building trust networks, cross-class alliances and synergistic relations between civil and political society. The evidence and mode of analysis deployed here help suggest alternative routes for supporting local efforts to build democracy from below in sub-Saharan Africa
Moral hazard in marriage: the use of domestic labor as an incentive device
This paper argues that some women in developing countries use domestic labor as a tool to incentivize husbands. A theoretical model is derived based on the traditions of rural Malawi, where men often supplement farm income with wage labor. As wage labor is not observed by the wife, this creates moral hazard: husbands may not make enough effort to bring home wages. The model predicts that women overcome this by using domestic labor as an incentive device: they increase their domestic labor and reduce their leisure in response to good consumption outcomes, but only if they cannot rely on divorce threat as an alternative source of incentives. This prediction is confirmed using survey data from Malawi. Identification is based on the fact that Malawi's kinship traditions exogenously determine women's accessibility to divorce. Where divorce is not an option, women make inefficient labor choices in order to provide incentives
Elevated glutamine/glutamate ratio in cerebrospinal fluid of first episode and drug naive schizophrenic patients
BACKGROUND: Recent magnetic resonance spectroscopy (MRS) studies report that glutamine is altered in the brains of schizophrenic patients. There were also conflicting findings on glutamate in cerebrospinal fluid (CSF) of schizophrenic patients, and absent for glutamine. This study aims to clarify the question of glutamine and glutamate in CSF of first episode and drug naive schizophrenic patients. METHOD: Levels of glutamine and glutamate in CSF of 25 first episode and drug-naive male schizophrenic patients and 17 age-matched male healthy controls were measured by a high performance liquid chromatography. RESULTS: The ratio (126.1 (median), 117.7 ± 27.4 (mean ± S.D.)) of glutamine to glutamate in the CSF of patients was significantly (z = -3.29, p = 0.001) higher than that (81.01 (median), 89.1 ± 22.5 (mean ± S.D.)) of normal controls although each level of glutamine and glutamate in patients was not different from that of normal controls. CONCLUSION: Our data suggests that a disfunction in glutamate-glutamine cycle in the brain may play a role in the pathophysiology of schizophrenia
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