Composite defect extends analogy between cosmology and 3He

Spin-mass vortices have been observed to form in rotating superfluid 3He-B, following the absorption of a thermal neutron and a rapid transition from the normal to the superfluid state. The spin-mass vortex is a composite defect which consists of a planar soliton (wall) which terminates on a linear core (string). This observation fits well within the framework of a cosmological scenario for defect formation, known as the Kibble-Zurek mechanism. It suggests that in the early Universe analogous cosmological defects might have formed.     

Random replicators with high-order interactions

We use tools of the equilibrium statistical mechanics of disordered systems to study analytically the statistical properties of an ecosystem composed of N species interacting via random mutual interactions, as well as via deterministic self-interactions of order p>/=2. We show that the main effect of increasing the order of the interactions among the species is to make the system less competitive, in the sense that the fraction of extinct species is greatly reduced. In addition, we find that for p>2 there is a threshold value which gives a lower bound to the concentration of the surviving species, preventing then the existence of rare species and, consequently, increasing the robustness of the ecosystem to external perturbations.     

Measurements on the vortex sheet in rotating superfluid He-A

When a cylindrical container filled with superfluid ³He---A is rotated around its symmetry axis, several different configurations of quantized vorticity are possible: which of them will be preferred depends on the specifics how the rotating state is formed. The most unusual is the vortex sheet, a domain wall in the order parameter texture into which vortex lines are confined. This metastable structure has the lowest critical velocity of formation if a domain wall with the appropriate orientation is already present in the container. In this case the vortex sheet becomes the preferred rotating state which provides the solid-body rotation of the superfluid component on an averaged scale. Its presence can be identified from the cw NMR spectrum which samples the order parameter texture. Here the experimental properties of the vortex sheet are reviewed, as deduced from NMR measurements.     

Annihilation of quantized vortex lines in rotating<Superscript>3</Superscript>He-A

The maximum number of vortex lines at a given angular velocity Ω is obtained during deceleration when vortices annihilate at the wall of the container. The width of the vortex-free region in the decelerated state has been measured by cw NMR for vortex lines with singular and continuous core structures. It corresponds to the limit of stability of peripheral vortices for single and double quantization respectively.     

Vortex arrays of coexisting singly and doubly quantized vortex lines in<Superscript>3</Superscript>He-A

In superfluid³He-A singly and doubly quantized vortex lines can coexist in a rotating container. We measure with NMR techniques the radial distribution of the two vortex types in an array of vortex lines. The radial composition is found to depend on the procedure by which the array has been formed. The result shows that in superfluid³He the energy barriers separating different configurations of the vortex array are inpenetrably high for a metastable state to relax.     

Maturation of auditory temporal integration and inhibition assessed with event-related potentials (ERPs)

Background

We examined development of auditory temporal integration and inhibition by assessing electrophysiological responses to tone pairs separated by interstimulus intervals (ISIs) of 25, 50, 100, 200, 400, and 800 ms in 28 children aged 7 to 9 years, and 15 adults.

Results

In adults a distinct neural response was elicited to tones presented at ISIs of 25 ms or longer, whereas in children this was only seen in response to tones presented at ISIs above 100 ms. In adults, late N1 amplitude was larger for the second tone of the tone pair when separated by ISIs as short as 100 ms, consistent with the perceptual integration of successive stimuli within the temporal window of integration. In contrast, children showed enhanced negativity only when tone pairs were separated by ISIs of 200 ms. In children, the amplitude of the P1 component was attenuated at ISIs below 200 ms, consistent with a refractory process.

Conclusions

These results indicate that adults integrate sequential auditory information into smaller temporal segments than children. These results suggest that there are marked maturational changes from childhood to adulthood in the perceptual processes underpinning the grouping of incoming auditory sensory information, and that electrophysiological measures provide a sensitive, non-invasive method allowing further examination of these changes.