494 research outputs found
Antiferromagnetic Critical Fluctuations in BaFeAs
Magnetic correlations near the magneto-structural phase transition in the
bilayer iron pnictide parent compound, BaFeAs, are measured. In close
proximity to the antiferromagnetic phase transition in BaFeAs, a
crossover to three dimensional critical behavior is anticipated and has been
preliminarily observed. Here we report complementary measurements of
two-dimensional magnetic fluctuations over a broad temperature range about
T. The potential role of two-dimensional critical fluctuations in the
magnetic phase behavior of BaFeAs and their evolution near the
anticipated crossover to three dimensional critical behavior and long-range
order are discussed.Comment: 6 pages, 4 figures; Accepted for publication in Physical Review
Heat capacity study of BaFeAs: effects of annealing
Heat-capacity, X-ray diffraction, and resistivity measurements on a
high-quality BaFeAs sample show an evolution of the
magneto-structural transition with successive annealing periods. After a 30-day
anneal the resistivity in the (ab) plane decreases by more than an order of
magnitude, to 12 cm, with a residual resistance ratio 36; the
heat-capacity anomaly at the transition sharpens, to an overall width of less
than K, and shifts from 135.4 to 140.2 K. The heat-capacity anomaly in both the
as-grown sample and after the 30-day anneal shows a hysteresis of 0.15 K,
and is unchanged in a magnetic field H = 14 T. The X-ray and
heat-capacity data combined suggest that there is a first order jump in the
structural order parameter. The entropy of the transition is reported
Structural and Antiferromagnetic Properties of Ba(Fe1−x−yCoxRhy)2As2 compounds
We present a systematic investigation of the electrical, structural, and antiferromagnetic properties for the series of Ba(Fe1 −x−yCoxRhy)2As2 compounds with fixed x ≈ 0.027 and 0 ≤ y ≤ 0.035. We compare our results for the Co-Rh doped Ba(Fe1−x−yCoxRhy)2As2 compounds with the Co doped Ba(Fe1−xCox)2As2 compounds. We demonstrate that the electrical, structural, antiferromagnetic, and superconducting properties of the Co-Rh doped compounds are similar to the properties of the Co doped compounds. We find that the overall behaviors of Ba(Fe1−x−yCoxRhy)2As2 and Ba(Fe1−xCox)2As2 compounds are very similar when the total number of extra electrons per Fe/TM (TM=transition metal) site is considered, which is consistent with the rigid band model. Despite the similarity, we find that the details of the transitions, for example, the temperature difference between the structural and antiferromagnetic transition temperatures and the incommensurability of the antiferromangetic peaks, are different between Ba(Fe1−x−yCoxRhy)2As2 and Ba(Fe1−xCox)2As2 compounds
Conformity of spin fluctuations in alkali-metal iron selenide superconductors inferred from the observation of a magnetic resonant mode in K(x)Fe(2-y)Se(2)
Spin excitations stemming from the metallic phase of the ferrochalcogenide
superconductor K(0.77)Fe(1.85)Se(2) (T_c=32 K) were mapped out in the ab plane
by means of the time-of-flight neutron spectroscopy. We observed a magnetic
resonant mode at Q_res=(1/2 1/4), whose energy and in-plane shape are almost
identical to those in the related compound Rb(0.8)Fe(1.6)Se(2). This lets us
infer that there is a unique underlying electronic structure of the bulk
superconducting phase K(x)Fe(2)Se(2), which is universal for all alkali-metal
iron selenide superconductors and stands in contrast to the doping-tunable
phase diagrams of the related iron pnictides. Furthermore, the spectral weight
of the resonance on the absolute scale, normalized to the volume fraction of
the superconducting phase, is several times larger than in optimally doped
BaFe(2-x)Co(x)As(2). We also found no evidence for any additional low-energy
branches of spin excitations away from Q_res. Our results provide new input for
theoretical models of the spin dynamics in iron based superconductors
Spin dynamics near a putative antiferromagnetic quantum critical point in Cu substituted BaFeAs and its relation to high-temperature superconductivity
We present the results of elastic and inelastic neutron scattering
measurements on non-superconducting
Ba(FeCu)As, a composition close to a
quantum critical point between AFM ordered and paramagnetic phases. By
comparing these results with the spin fluctuations in the low Cu composition as
well as the parent compound BaFeAs and superconducting
Ba(FeNi)As compounds, we demonstrate that paramagnon-like
spin fluctuations are evident in the antiferromagnetically ordered state of
Ba(FeCu)As, which is distinct from the AFM-like
spin fluctuations in the superconducting compounds. Our observations suggest
that Cu substitution decouples the interaction between quasiparticles and the
spin fluctuations. We also show that the spin-spin correlation length,
, increases rapidly as the temperature is lowered and find
scaling behavior, the hallmark of quantum criticality, at an
antiferromagnetic quantum critical point.Comment: 10 pages, 7 figure
Autism as a disorder of neural information processing: directions for research and targets for therapy
The broad variation in phenotypes and severities within autism spectrum disorders suggests the involvement of multiple predisposing factors, interacting in complex ways with normal developmental courses and gradients. Identification of these factors, and the common developmental path into which theyfeed, is hampered bythe large degrees of convergence from causal factors to altered brain development, and divergence from abnormal brain development into altered cognition and behaviour. Genetic, neurochemical, neuroimaging and behavioural findings on autism, as well as studies of normal development and of genetic syndromes that share symptoms with autism, offer hypotheses as to the nature of causal factors and their possible effects on the structure and dynamics of neural systems. Such alterations in neural properties may in turn perturb activity-dependent development, giving rise to a complex behavioural syndrome many steps removed from the root causes. Animal models based on genetic, neurochemical, neurophysiological, and behavioural manipulations offer the possibility of exploring these developmental processes in detail, as do human studies addressing endophenotypes beyond the diagnosis itself
Recommended from our members
Scintillation Properties of Eu2+-Activated Barium Fluoroiodide
The scintillation properties of powders and single-crystals of BaFI doped with Eu2+ are presented. Single crystals were grown by the vertical Bridgman technique. Under optical and X-ray excitation, the samples exhibit a narrow E2+ 5d-4f transition emission centered at 405 nm. The scintillation light output is estimated to be 55,000+-5,000 photons/MeV at 662 keV with 85percent of the light decaying within 600 ns. An energyresolution of 8.5percent full width at half maximum (FWHM) has been achieved using this scintillator for 662 keV excitation (137Cs source) at room temperature
Growth and morphology of 0.80eV photoemitting indium nitride nanowires
InN nanowires with high efficiency photoluminescence emission at 0.80 eV are reported for the first time. InN nanowires were synthesized via a vapor solid growth mechanism from high purity indium metal and ammonia. The products consist of only hexagonal wurtzite phase InN. Scanning electron microscopy showed wires with diameters of 50-100nm and having fairly smooth morphologies. High-resolution transmission electron microscopy revealed high quality, single crystal InN nanowires which grew in the <0001> direction. The group-III nitrides have become an extremely important technological material over the past decade. They are commonly used in optoelectronic devices, such as high brightness light-emitting diodes (LEDs) and low wavelength laser diodes (LDs), as well as high power/high frequency electronic devices. Recently InN thin films grown by MOCVD and MBE were found to have a bandgap energy in the range of 0.7-0.9 eV, much lower than the value of {approx}1.9 eV found for InN films grown by sputtering. This large decrease in the direct bandgap transition energy and the ability to form ternary (InGaN) and quaternary (AlInGaN) alloys increases the versatility of group-III nitride optoelectronic devices, ranging from the near IR to the UV. Additionally, InN has some promising transport and electronic properties. It has the smallest effective electron mass of all the group-III nitrides which leads to high mobility and high saturation velocity10 and a large drift velocity at room temperature. As a result of these unique properties, there has been a large increase in interest in InN for potential use in optoelectronic devices, such as LDs and high efficiency solar cells, as well as high frequency/high power electronic devices
P300 and uncertainty reduction in a concept identification task.
The relationship between the amplitude of P300, the mean amplitude of the Slow Wave, and uncertainty reduction after (dis)confirmation of hypotheses was studied in a Concept-Identification task. The subjects had to categorize stimuli according to a conceptual rule (joint denial or exclusion) and to rate the confidence that their classification was correct. Three types of feedback were distinguished: confirming (subject's categorization was correct), disconfirming (subject's categorization was incorrect), and non-informative feedback. The EEG was averaged separately according to the three types of feedback and the two confidence ratings (low, high).
The data showed the predicted interaction between type of feedback and confidence level. A larger P300 amplitude turned up after confirming feedback when the subject was less confident, than when he was more confident. The reverse was found after disconfirming feedback. The P300 amplitude after non-informative feedback was not influenced by confidence. The mean amplitude of the Slow Wave showed approximately the same interaction pattern.
The results were interpreted in terms of changes in the probability of hypotheses which subjects use to categorize stimuli in a Concept-Identification task
- …