309 research outputs found
Magnetic and structural properties of GeMn films: precipitation of intermetallic nanomagnets
We present a comprehensive study relating the nanostructure of Ge_0.95Mn_0.05
films to their magnetic properties. The formation of ferromagnetic nanometer
sized inclusions in a defect free Ge matrix fabricated by low temperature
molecular beam epitaxy is observed down to substrate temperatures T_S as low as
70 deg. Celsius. A combined transmission electron microscopy (TEM) and electron
energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions
as precipitates of the ferromagnetic compound Mn_5Ge_3. The volume and amount
of these precipitates decreases with decreasing T_S. Magnetometry of the films
containing precipitates reveals distinct temperature ranges: Between the
characteristic ferromagnetic transition temperature of Mn_5Ge_3 at
approximately room temperature and a lower, T_S dependent blocking temperature
T_B the magnetic properties are dominated by superparamagnetism of the Mn_5Ge_3
precipitates. Below T_B, the magnetic signature of ferromagnetic precipitates
with blocked magnetic moments is observed. At the lowest temperatures, the
films show features characteristic for a metastable state.Comment: accepted for publication in Phys. Rev. B 74 (01.12.2006). High
resolution images ibide
Reciprocal space mapping of magnetic order in thick epitaxial MnSi films
We report grazing incidence small angle neutron scattering (GISANS) and
complementary off-specular neutron reflectometry (OSR) of the magnetic order in
a single-crystalline epitaxial MnSi film on Si(111) in the thick film limit.
Providing a means of direct reciprocal space mapping, GISANS and OSR reveal a
magnetic modulation perpendicular to the films under magnetic fields parallel
and perpendicular to the film, where additional polarized neutron reflectometry
(PNR) and magnetization measurements are in excellent agreement with the
literature. Regardless of field orientation, our data does not suggest the
presence of more complex spin textures, notably the formation of skyrmions.
This observation establishes a distinct difference with bulk samples of MnSi of
similar thickness under perpendicular field, in which a skyrmion lattice
dominates the phase diagram. Extended x-ray absorption fine structure
measurements suggest that small shifts of the Si positions within the
unstrained unit cell control the magnetic state, representing the main
difference between the films and thin bulk samples
Skyrmion Lattice in a Doped Semiconductor
We report a comprehensive small angle neutron scattering study (SANS) of the
magnetic phase diagram of the doped semiconductor Fe_{1-x}Co_{x}Si for x=0.2
and 0.25. For magnetic field parallel to the neutron beam we observe a six-fold
intensity pattern under field-cooling, which identifies the A-phase of
Fe_{1-x}Co_{x}Si as a skyrmion lattice. The regime of the skyrmion lattice is
highly hysteretic and extents over a wide temperature range, consistent with
the site disorder of the Fe and Co atoms. Our study identifies Fe_{1-x}Co_{x}Si
is a second material after MnSi in which a skyrmion lattice forms and
establishes that skyrmion lattices may also occur in strongly doped
semiconductors
Digital phenotyping: towards replicable findings with comprehensive assessments and integrative models in bipolar disorders
Background:
Digital phenotyping promises to unobtrusively obtaining a continuous and objective input of symptomatology from patients’ daily lives. The prime example are bipolar disorders, as smartphone parameters directly reflect bipolar symptomatology. Empirical studies, however, have yielded inconsistent findings. We believe that three main shortcomings have to be addressed to fully leverage the potential of digital phenotyping: short assessment periods, rare outcome assessments, and an extreme fragmentation of parameters without an integrative analytical strategy.
Methods:
To demonstrate how to overcome these shortcomings, we conducted frequent (biweekly) dimensional and categorical expert ratings and daily self-ratings over an extensive assessment period (12 months) in 29 patients with bipolar disorder. Digital phenotypes were monitored continuously. As an integrative analytical strategy, we used structural equation modelling to build latent psychopathological outcomes (mania, depression) and latent digital phenotype predictors (sleep, activity, communicativeness).
Outcomes:
Combining gold-standard categorical expert ratings with dimensional self and expert ratings resulted in two latent outcomes (mania and depression) with statistically meaningful factor loadings that dynamically varied over 299 days. Latent digital phenotypes of sleep and activity were associated with same-day latent manic psychopathology, suggesting that psychopathological alterations in bipolar disorders relate to domains (latent variables of sleep and activity) and not only to specific behaviors (such as the number of declined incoming calls). The identification of latent psychopathological outcomes that dimensionally vary on a daily basis will enable to empirically determine which combination of digital phenotypes at which days prior to an upcoming episode are viable as digital prodromal
predictors
Digital phenotyping: Towards replicable findings with comprehensive assessments and integrative models in bipolar disorders
Background: Digital phenotyping promises to unobtrusively obtaining a continuous and objective input of symptomatology from patients\u27 daily lives. The prime example are bipolar disorders, as smartphone parameters directly reflect bipolar symptomatology. Empirical studies, however, have yielded inconsistent findings. We believe that three main shortcomings have to be addressed to fully leverage the potential of digital phenotyping: short assessment periods, rare outcome assessments, and an extreme fragmentation of parameters without an integrative analytical strategy. Methods: To demonstrate how to overcome these shortcomings, we conducted frequent (biweekly) dimensional and categorical expert ratings and daily self-ratings over an extensive assessment period (12 months) in 29 patients with bipolar disorder. Digital phenotypes were monitored continuously. As an integrative analytical strategy, we used structural equation modelling to build latent psychopathological outcomes (mania, depression) and latent digital phenotype predictors (sleep, activity, communicativeness). Outcomes: Combining gold-standard categorical expert ratings with dimensional self and expert ratings resulted in two latent outcomes (mania and depression) with statistically meaningful factor loadings that dynamically varied over 299 days. Latent digital phenotypes of sleep and activity were associated with same-day latent manic psychopathology, suggesting that psychopathological alterations in bipolar disorders relate to domains (latent variables of sleep and activity) and not only to specific behaviors (such as the number of declined incoming calls). The identification of latent psychopathological outcomes that dimensionally vary on a daily basis will enable to empirically determine which combination of digital phenotypes at which days prior to an upcoming episode are viable as digital prodromal predictors. (DIPF/Orig.
Orbital textures and charge density waves in transition metal dichalcogenides
Low-dimensional electron systems, as realized naturally in graphene or
created artificially at the interfaces of heterostructures, exhibit a variety
of fascinating quantum phenomena with great prospects for future applications.
Once electrons are confined to low dimensions, they also tend to spontaneously
break the symmetry of the underlying nuclear lattice by forming so-called
density waves; a state of matter that currently attracts enormous attention
because of its relation to various unconventional electronic properties. In
this study we reveal a remarkable and surprising feature of charge density
waves (CDWs), namely their intimate relation to orbital order. For the
prototypical material 1T-TaS2 we not only show that the CDW within the
two-dimensional TaS2-layers involves previously unidentified orbital textures
of great complexity. We also demonstrate that two metastable stackings of the
orbitally ordered layers allow to manipulate salient features of the electronic
structure. Indeed, these orbital effects enable to switch the properties of
1T-TaS2 nanostructures from metallic to semiconducting with technologically
pertinent gaps of the order of 200 meV. This new type of orbitronics is
especially relevant for the ongoing development of novel, miniaturized and
ultra-fast devices based on layered transition metal dichalcogenides
Theory of current-driven motion of Skyrmions and spirals in helical magnets
We study theoretically the dynamics of the spin textures, i.e., Skyrmion
crystal (SkX) and spiral structure (SS), in two-dimensional helical magnets
under external current. By numerically solving the Landau-Lifshitz-Gilbert
equation, it is found that (i) the critical current density of the motion is
much lower for SkX compared with SS in agreement with the recent experiment,
(ii) there is no intrinsic pinning effect for SkX and the deformation of the
internal structure of Skyrmion reduces the pinning effect dramatically, (iii)
the Bragg intensity of SkX shows strong time-dependence as can be observed by
neutron scattering experiment.Comment: 4 pages, 3 figure
An intrinsic circadian clock of the pancreas is required for normal insulin release and glucose homeostasis in mice
AIMS/HYPOTHESIS: Loss of circadian clocks from all tissues causes defective glucose homeostasis as well as loss of feeding and activity rhythms. Little is known about peripheral tissue clocks, so we tested the hypothesis that an intrinsic circadian clock of the pancreas is important for glucose homeostasis. METHODS: We monitored real-time bioluminescence of pancreas explants from circadian reporter mice and examined clock gene expression in beta cells by immunohistochemistry and in situ hybridisation. We generated mice selectively lacking the essential clock gene Bmal1 (also known as Arntl) in the pancreas and tested mutant mice and littermate controls for glucose and insulin tolerance, insulin production and behaviour. We examined islets isolated from mutants and littermate controls for glucose-stimulated insulin secretion and total insulin content. RESULTS: Pancreas explants exhibited robust circadian rhythms. Clock genes Bmal1 and Per1 were expressed in beta cells. Despite normal activity and feeding behaviour, mutant mice lacking clock function in the pancreas had severe glucose intolerance and defective insulin production; their isolated pancreatic islets had defective glucose-stimulated insulin secretion, but normal total insulin content. CONCLUSIONS/INTERPRETATION: The mouse pancreas has an autonomous clock function and beta cells are very likely to be one of the pancreatic cell types possessing an intrinsic clock. The Bmal1 circadian clock gene is required in the pancreas, probably in beta cells, for normal insulin secretion and glucose homeostasis. Our results provide evidence for a previously unrecognised molecular regulator of pancreatic glucose-sensing and/or insulin secretion
Structure and degeneracy of vortex lattice domains in pure superconducting niobium: A small-angle neutron scattering study
High-purity niobium exhibits a surprisingly rich assortment of vortex lattice (VL) structures for fields applied parallel to a fourfold symmetry axis, with all observed VL phases made up of degenerate domains that spontaneously break some crystal symmetry. Yet a single regular hexagonal VL domain is observed at all temperatures and fields parallel to a threefold symmetry axis. We report a detailed investigation of the transition between these lush and barren VL landscapes, discovering new VL structures and phase transitions at high fields. We show that the number and relative population of VL domains is intrinsically tied to the underlying crystal symmetry. We discuss how subtle anisotropies of the crystal may generate the remarkable VLs observed. © 2009 The American Physical Society
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