15,531 research outputs found
Momentum-Resolved Ultrafast Electron Dynamics in Superconducting Bi2Sr2CaCu2O8+delta
The non-equilibrium state of the high-Tc superconductor Bi2Sr2CaCu2O8+delta
and its ultrafast dynamics have been investigated by femtosecond time- and
angle-resolved photoemission spectroscopy well below the critical temperature.
We probe optically excited quasiparticles at different electron momenta along
the Fermi surface and detect metastable quasiparticles near the antinode. Their
decay through e-e scattering is blocked by a phase space restricted to the
nodal region. The lack of momentum dependence in the decay rates is in
agreement with relaxation dominated by Cooper pair recombination in a boson
bottleneck limit
Spin polarization of light atoms in jellium: Detailed electronic structures
We revisit the problem of the spontaneous magnetization of an {\em sp}
impurity atom in a simple metal host. The main features of interest are: (i)
Formation of the spherical spin density/charge density wave around the
impurity; (ii) Considerable decrease in the size of the pseudoatom in the
spin-polarized state as compared with the paramagnetic one, and (iii) Relevance
of the electron affinity of the isolated atom to this spin polarization, which
is clarified by tracing the transformation of the pseudoatom into an isolated
negative ion in the low-density limit of the enveloping electron gas.Comment: 4 pages, 4 figures, accepted to Phys. Rev.
Electron-phonon coupling in 122 Fe pnictides analyzed by femtosecond time-resolved photoemission
Based on results from femtosecond time-resolved photoemission, we compare
three different methods for determination of the electron-phonon coupling
constant {\lambda} in Eu and Ba-based 122 FeAs compounds. We find good
agreement between all three methods, which reveal a small {\lambda} < 0.2. This
makes simple electron-phonon mediated superconductivity unlikely in these
compounds.Comment: 11 pages, 3 figure
Momentum dependent ultrafast electron dynamics in antiferromagnetic EuFe2As2
Employing the momentum-sensitivity of time- and angle-resolved photoemission
spectroscopy we demonstrate the analysis of ultrafast single- and many-particle
dynamics in antiferromagnetic EuFe2As2. Their separation is based on a
temperature-dependent difference of photo-excited hole and electron relaxation
times probing the single particle band and the spin density wave gap,
respectively. Reformation of the magnetic order occurs at 800 fs, which is four
times slower compared to electron-phonon equilibration due to a smaller
spin-dependent relaxation phase space
Pressure Effect and Specific Heat of RBa2Cu3Ox at Distinct Charge Carrier Concentrations: Possible Influence of Stripes
In YBa2Cu3Ox, distinct features are found in the pressure dependence of the
transition temperature, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in
the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal,
whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes.
Substituting Nd for Y and doping with Ca leads to a shift of these specific
oxygen contents, since oxygen order and hole doping by Ca influences the hole
content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh)
behavior, the features coalesce for all samples at nh=0.11 and nh=0.175,
irrespective of substitution and doping. Hence, this behavior seems to reflect
an intrinsic property of the CuO2 planes. Analyzing our results we obtain
different mechanisms in three doping regions: Tc changes in the optimally doped
and overdoped region are mainly caused by charge transfer. In the slightly
underdoped region an increasing contribution to dTc/dp is obtained when well
ordered CuO chain fragments serve as pinning centers for stripes. This behavior
is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the
well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content
below nh=0.11 our results point to a crossover from an underdoped
superconductor to a doped antiferromagnet, changing completely the physics of
these materials.Comment: 6 pages, 5 figures Proccedings of the 'Stripes 2000' Conference, Rome
(2000
Critical properties of the double exchange ferromagnet Nd0.4Pb0.4MnO3
Results of a study of dc-magnetization M(T, H), performed on a Nd0.6Pb0.4MnO3
single crystal in the temperature range around T_C (Curie temperature) which
embraces the critical region | epsilon | = |T -T_C |/T_C <= 0.05 are reported.
The magnetic data analyzed in the critical region using the Kouvel-Fisher
method give the values for the T_C =156.47 +/- 0.06 K and the critical
exponents, beta = 0.374 +/- 0.006 (from the temperature dependence of
magnetization), and gamma = 1.329 +/- 0.003 (from the temperature dependence of
initial susceptibility). The critical isotherm M(T_C, H) gives delta = 4.547
+/- 0.1. Thus the scaling law gamma+beta=delta beta is fulfilled. The critical
exponents obey the single scaling-equation of state M(H, epsilon) = epsilon^b
f_+/- (H/epsilon^(beta + gamma)) where, f_+ for T > T_C and f_- for T< T_C. The
exponent values are very close to those expected for the universality class of
3D Heisenberg ferromagnets with short-range interactions.Comment: 19 pages, including 6 figure
Step-wedge cluster-randomised community-based trials: An application to the study of the impact of community health insurance
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.BACKGROUND: We describe a step-wedge cluster-randomised community-based trial which has been conducted since 2003 to accompany the implementation of a community health insurance (CHI) scheme in West Africa. The trial aims at overcoming the paucity of evidence-based information on the impact of CHI. Impact is defined in terms of changes in health service utilisation and household protection against the cost of illness. Our exclusive focus on the description and discussion of the methods is justified by the fact that the study relies on a methodology previously applied in the field of disease control, but never in the field of health financing. METHODS: First, we clarify how clusters were defined both in respect of statistical considerations and of local geographical and socio-cultural concerns. Second, we illustrate how households within clusters were sampled. Third, we expound the data collection process and the survey instruments. Finally, we outline the statistical tools to be applied to estimate the impact of CHI. CONCLUSION: We discuss all design choices both in relation to methodological considerations and to specific ethical and organisational concerns faced in the field. On the basis of the appraisal of our experience, we postulate that conducting relatively sophisticated trials (such as our step-wedge cluster-randomised community-based trial) aimed at generating sound public health evidence, is both feasible and valuable also in low income settings. Our work shows that if accurately designed in conjunction with local health authorities, such trials have the potential to generate sound scientific evidence and do not hinder, but at times even facilitate, the implementation of complex health interventions such as CHI
- …