No abstract

ac losses and self-field critical currents were measured in a Nb3Sn tape between 4.2 and 17 K. The observed dependence of losses on current at 4.2 K is adequately reproduced by assuming a surface barrier ΔH = 710 Oe and a bulk critical current density Jc = 5.7×106 A/cm2. The temperature dependence of losses above 4.2 K can also be reproduced by taking into account the observed parabolic dependence of Jc on temperature and by postulating a [1 − (T/Tc)4] dependence for ΔH. A new technique is described for measuring the critical current.

This letter gives experimental verification of Burgess' theory of the flux-flow noise spectrum in Type II superconductors. The measurements made on vanadium foil biased in linear V-I region shows quantative agreement with the modified theory.

We have studied the effect of an external electric field on the dc fractional quantum Hall conductivity of a two-dimensional system with the dynamic structure function in the single-mode approximation. In analogy with the integral quantum Hill effect and superconductivity, a gap-dependent critical electric current J e also exists in the case of fracctional quantum Hall conduction, above which is the maximum current for system to maintain superfluidity. We also find that, depending on the gap width and the phonon velocity, phonon may or may not contribute to the conductivity even at a finite lattice temperature, in agreement with experiments.

The vein pattern in insect wings allows this lightweight structure to carry multiple biological functions. An investigation of the angular distribution of the vein struts in dragonfly wings revealed that the golden angle or golden ratio dominates the venation patterns in two closely related ways. First, the golden angle dominates the intervein angles in regions where thin veins and membranes demand strength reinforcement. Second, a golden ratio partition method has been developed to explain a set of preferred intervein angles in distorted polygon-shaped venation cells throughout the venation pattern in dragonfly wings. These observations provide new evidence that the wing structure is spatially optimized, by the golden rule in nature, for supporting biomechanical functions of dragonfly wings.