3,638 research outputs found
Toward Perfection: Kapellasite, Cu3Zn(OH)6Cl2, a New Model S = 1/2 Kagome Antiferromagnet
The search for the resonating valence bond (RVB) state continues to underpin
many areas of condensed matter research. The RVB is made from the dimerisation
of spins on different sites into fluctuating singlets, and was proposed by
Anderson to be the reference state from which the transition to BCS
superconductivity occurs. Little is known about the state experimentally, due
to the scarcity of model materials. Theoretical work has put forward the S =
1/2 kagome antiferromagnet (KAFM) as a good candidate for the realization of
the RVB state. In this paper we introduce a new model system, the S = 1/2 KAFM
Kapellasite, Cu3Zn(OH)6Cl2. We show that its crystal structure is a good
approximation to a 2-dimensional kagome antiferromagnet and that susceptibility
data indicate a collapse of the magnetic moment below T = 25 K that is
compatible with the spins condensing into the non-magnetic RVB state.Comment: Communication, 3 pages, 3 figure
Predictive learning, prediction errors, and attention: evidence from event-related potentials and eye tracking
Prediction error (‘‘surprise’’) affects the rate of learning: We learn more rapidly about cues for which we initially make incorrect predictions than cues for which our initial predictions are correct. The current studies employ electrophysiological measures to reveal early attentional differentiation of events that differ in their previous involvement in errors of predictive judgment.
Error-related events attract more attention, as evidenced by features of event-related scalp potentials previously implicated in selective visual attention (selection negativity, augmented anterior N1). The earliest differences detected occurred around 120 msec after stimulus onset, and distributed source localization (LORETA)
indicated that the inferior temporal regions were one source of the earliest differences. In addition, stimuli associated with the production of prediction errors show higher dwell times in an eyetracking procedure. Our data support the view that early attentional processes play a role in human associative learning
Prototype Backscatter Moessbauer Spectrometer for Measurement of Martian Surface Mineralogy
We have designed and successfully tested a prototype of a backscatter Moessbauer spectrometer (BaMS) targeted for use on the Martian surface to (1) determine oxidation states of iron, and (2) identify and determine relative abundances of iron-bearing mineralogies. No sample preparation is required to perform measurements; it is only necessary to bring sample and instrument into physical contact. The prototype meets our projected specification for a flight instrument in terms of mass, power, and volume. A Moessbauer spectrometer on the Martian surface would provide wide variety of information about the current state of the Martian surface, and this information is described
Absence of Visual Input Results in the Disruption of Grid Cell Firing in the Mouse
Grid cells are spatially modulated neurons within the medial entorhinal cortex whose firing fields are arranged at the vertices of tessellating equilateral triangles [1]. The exquisite periodicity of their firing has led to the suggestion that they represent a path integration signal, tracking the organism's position by integrating speed and direction of movement [2-10]. External sensory inputs are required to reset any errors that the path integrator would inevitably accumulate. Here we probe the nature of the external sensory inputs required to sustain grid firing, by recording grid cells as mice explore familiar environments in complete darkness. The absence of visual cues results in a significant disruption of grid cell firing patterns, even when the quality of the directional information provided by head direction cells is largely preserved. Darkness alters the expression of velocity signaling within the entorhinal cortex, with changes evident in grid cell firing rate and the local field potential theta frequency. Short-term (<1.5 s) spike timing relationships between grid cell pairs are preserved in the dark, indicating that network patterns of excitatory and inhibitory coupling between grid cells exist independently of visual input and of spatially periodic firing. However, we find no evidence of preserved hexagonal symmetry in the spatial firing of single grid cells at comparable short timescales. Taken together, these results demonstrate that visual input is required to sustain grid cell periodicity and stability in mice and suggest that grid cells in mice cannot perform accurate path integration in the absence of reliable visual cues
Moessbauer Spectroscopy for Lunar Resource Assessment: Measurement of Mineralogy and Soil Maturity
First-order assessment of lunar soil as a resource includes measurement of its mineralogy and maturity. Soils in which the mineral ilmenite is present in high concentrations are desirable feedstock for the production of oxygen at a lunar base. The maturity of lunar soils is a measure of their relative residence time in the upper 1 mm of the lunar surface. Increasing maturity implies increasing load of solar wind species (e.g., N, H, and He-3), decreasing mean grain size, and increasing glass content. All these physicochemical properties that vary in a regular way with maturity are important parameters for assessing lunar soil as a resource. For example, He-3 can be extracted and potentially used for nuclear fusion. A commonly used index for lunar soil maturity is I(sub s)/FeO, which is the concentration of fine-grained metal determined by ferromagnetic resonance (I(sub s)) normalized to the total iron content (as FeO). I(sub s)/FeO has been measured for virtually every soil returned by the Apollo and Luna missions to the Moon. Because the technique is sensitive to both oxidation state and mineralogy, iron Moessbauer spectroscopy (FeMS) is a viable technique for in situ lunar resource assessment. Its utility for mineralogy is apparent from examination of published FeMS data for lunar samples. From the data published, it can be inferred that FeMS data can also be used to determine soil maturity. The use of FeMS to determine mineralogy and maturity and progress on development of a FeMS instrument for lunar surface use are discussed
Field induced magnetic order in the frustrated magnet Gadolinium Gallium Garnet
Gd3Ga5O12, (GGG), has an extraordinary magnetic phase diagram, where no long
range order is found down to 25 mK despite \Theta_CW \approx 2 K. However, long
range order is induced by an applied field of around 1 T. Motivated by recent
theoretical developments and the experimental results for a closely related
hyperkagome system, we have performed neutron diffraction measurements on a
single crystal sample of GGG in an applied magnetic field. The measurements
reveal that the H-T phase diagram of GGG is much more complicated than
previously assumed. The application of an external field at low T results in an
intensity change for most of the magnetic peaks which can be divided into three
distinct sets: ferromagnetic, commensurate antiferromagnetic, and
incommensurate antiferromagnetic. The ferromagnetic peaks (e.g. (112), (440)
and (220)) have intensities that increase with the field and saturate at high
field. The antiferromagnetic reflections have intensities that grow in low
fields, reach a maximum at an intermediate field (apart from the (002) peak
which shows two local maxima) and then decrease and disappear above 2 T. These
AFM peaks appear, disappear and reach maxima in different fields. We conclude
that the competition between magnetic interactions and alternative ground
states prevents GGG from ordering in zero field. It is, however, on the verge
of ordering and an applied magnetic field can be used to crystallise ordered
components. The range of ferromagnetic and antiferromagnetic propagation
vectors found reflects the complex frustration in GGG.Comment: 6 pages, 7 figures, HFM 2008 conference pape
Threaded-nail Fasteners - Research and Standardization Needs
Threaded nail fasteners are commonly used in agricultural and commercial post-frame structures, yet there has been insufficient research to fully understand the effect of threads on withdrawal and lateral load resistance. The objective of this article is to review technical information on threaded nail fasteners and to identify problems facing manufacturers, designers, and users of threaded nail fasteners for engineering applications. Recommendations are given concerning research and standardization needs
- …