72 research outputs found
Critical spin-flip scattering at the helimagnetic transition of MnSi
We report spherical neutron polarimetry (SNP) and discuss the spin-flip
scattering cross sections as well as the chiral fraction close to the
helimagnetic transition in MnSi. For our study, we have developed a
miniaturised SNP device that allows fast data collection when used in small
angle scattering geometry with an area detector. Critical spin-flip scattering
is found to be governed by chiral paramagnons that soften on a sphere in
momentum space. Carefully accounting for the incoherent spin-flip background,
we find that the resulting chiral fraction decreases gradually above the
helimagnetic transition reflecting a strongly renormalised chiral correlation
length with a temperature dependence in excellent quantitative agreement with
the Brazovskii theory for a fluctuation-induced first order transition.Comment: 5 pages, 3 figure
Monopolar and dipolar relaxation in spin ice HoTiO
When degenerate states are separated by large energy barriers, the approach
to thermal equilibrium can be slow enough that physical properties are defined
by the thermalization process rather than the equilibrium. The exploration of
thermalization pushes experimental boundaries and provides refreshing insights
into atomic scale correlations and processes that impact steady state dynamics
and prospects for realizing solid state quantum entanglement. We present a
comprehensive study of magnetic relaxation in HoTiO based on
frequency-dependent susceptibility measurements and neutron diffraction studies
of the real-time atomic-scale response to field quenches. Covering nearly ten
decades in time scales, these experiments uncover two distinct relaxation
processes that dominate in different temperature regimes. At low temperatures
(0.6K<T<1K) magnetic relaxation is associated with monopole motion along the
applied field direction through the spin-ice vacuum. The increase of the
relaxation time upon cooling indicates reduced monopole conductivity driven by
decreasing monopole concentration and mobility as in a semiconductor. At higher
temperatures (1K<T<2K) magnetic relaxation is associated with the reorientation
of monopolar bound states as the system approaches the single-spin tunneling
regime. Spin fractionalization is thus directly exposed in the relaxation
dynamics
Kinetic small angle neutron scattering of the skyrmion lattice in MnSi
We report a kinetic small angle neutron scattering study of the skyrmion
lattice (SL) in MnSi. Induced by an oscillatory tilting of the magnetic field
direction, the elasticity and relaxation of the SL along the magnetic field
direction have been measured with microsecond resolution. For the excitation
frequency of 325 Hz the SL begins to track the tilting motion of the applied
magnetic field under tilting angles exceeding > 0.4{\deg}.
Empirically the associated angular velocity of the tilting connects
quantitatively with the critical charge carrier velocity of approx. 0.1mm/s
under current driven spin transfer torques, for which the SL unpins. In
addition, a pronounced temperature dependence of the skyrmion motion is
attributed to the variation of the skyrmion stiffness. Taken together our study
highlights the power of kinetic small angle neutron scattering as a new
experimental tool to explore, in a rather general manner, the elasticity and
impurity pinning of magnetic textures across a wide parameter space without
parasitic signal interferences due to ohmic heating or Oersted magnetic fields
Appraisal of health care: from patient value to societal benefit
Aim: This paper summarizes the deficiencies and weaknesses of the most frequently used methods for the allocation of health-care resources. New, more transparent and practical methods for optimizing the allocation of these resources are proposed. Method: The examples of quality-adjusted life years (QALYs) and efficiency frontier (EF) are analyzed to describe weaknesses and problems in decisions regulating health-care provision. After conducting a literature search and discussions with an international group of professionals, three groups of professionals were formed to discuss the assessment and appraisal of health-care services and allocation of available resources. Results: At least seven essential variables were identified that should be heeded when applying the concept of QALYs for decisions concerning health-care provision. The efficiency frontier (EF) concept can be used to set a ceiling price and perform a cost-benefit analysis of provision, but different stakeholders—a biostatistician (efficacy), an economist (costs), a clinician (effectiveness), and the patient (value)—could provide a fairer appraisal of health-care services. Efficacy and costs are often based on falsifiable data. Effectiveness and value depend on the success with which a particular clinical problem has been solved. These data cannot be falsified. The societal perspective is generated by an informal cost-benefit analysis including appraisals by the above-mentioned stakeholders and carried out by an authorized institution. Conclusion: Our analysis suggests that study results expressed in QALYs or as EF cannot be compared unless the variables included in the calculation are specified. It would be far more objective and comprehensive if an authorized institution made an informal decision based on formal assessments of the effectiveness of health-care services evaluated by health-care providers, of the value assessed by consumers, of efficacy described by biostatisticians, and of costs calculated by economists
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