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Topological insulators have been a burgeoning area of condensed-matter physics since they were proposed in 2005 (SN: 5/22/10, p. 22). Typical materials are either conductors or insulators, but topological insulators such as bismuth telluride are exotic hybrids: They block electric current yet allow electrons to flow along their surfaces.
What’s more, these surface electrons can move unimpeded through bumps and grooves that would normally block their path. That useful property makes topological insulators intriguing candidates for future electronics.
The ability to enable electrons to surf along the surface and avoid obstacles is so enticing that some physicists have investigated whether other particles, particularly photons, could do the same thing. Along with electrons, photons are an essential element of modern technology. Electrons flow through chips in our computers and smartphones, while photons are the information carriers that enable high-speed communication over fiber-optic cables. The key to faster, more efficient communication networks is minimizing the scattering of photons when they encounter obstacles.
With that goal in mind, physicist Mordechai Segev and his team at the Technion-Israel Institute of Technology in Haifa set out to demonstrate the first photonic topological insulator. They started with a block of glass and etched in hundreds of helical waveguides, which are essentially wires for light. The waveguides were tightly packed in a honeycomb-like structure so that light trying to make its way through one waveguide interfered with light in the others and canceled out.
The only part of each waveguide that did not cancel out light was its outer edge. As a result, photons got steered along the outside of the bundle of waveguides, confining them to the surface of the glass block.
When the researchers shined a beam of red light on one face of the glass, the photons moved along the surface of the glass, easily made a turn once they reached an edge of the glass and then continued on their way along the surface. None of the light got scattered by surface imperfections.
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Spooning soft honey from a jar generates long, dangling threads of the liquid, but researchers have not understood why columns of a viscous fluid get so long without breaking up. In Physical Review Letters, a team explains the mystery using a combination of theory and experiments with dripping silicone oil. They found that while viscosity doesn’t affect the large-scale motion of the falling liquid very much—consistent with previous theories—it does affect the way that small, random waves in the fluid become amplified over time, which leads to breakup.
The results may be relevant for industrial processes that involve pulling long strands of viscous fluid, such as the fabrication of optical fibers. For perturbations that start at the nozzle, this influence of viscosity doesn’t count for much, because they get rapidly stretched out as the jet descends, before they can grow and create pinch-off. But irregularities appearing further down the jet can grow in amplitude before they get stretched too much, so viscosity matters for them.
Delayed breakup of liquid columns is important for the industrial process of fiber-spinning, where a viscous liquid such as a polymer or molten glass is drawn out into a long, thin strand to make textiles or optical fibers. The theory should help to predict the maximum length for such fibers, Eggers says.
Stretching of liquid jets also happens naturally in some volcanic environments, where molten rock is formed into glass fibers known as “Pele’s hair.” “This phenomenon shares with our falling viscous jets the elements of strong stretching and high viscosity,” says team member Neil Ribe of the University of Paris and the French National Center for Scientific Research (CNRS). “However, it also involves fast cooling and consequent strong increases in viscosity. This means that threads that have been stretched very thin will tend to solidify before they can break up.”
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The Alpha Magnetic Spectrometer (AMS) Collaboration announces the publication of its first physics result in Physical Review Letters. The AMS Experiment is the most powerful and sensitive particle physics spectrometer ever deployed in space. As seen in Figure 1, AMS is located on the exterior of the International Space Station (ISS) and, since its installation on May 19, 2011 until the present, it has measured over 30 billion cosmic rays at energies up to trillions of electron volts.
In the initial 18 month period of space operations, from May 19, 2011 to December 10, 2012, AMS analyzed 25 billion primary cosmic ray events. Of these, an unprecedented number, 6.8 million, were unambiguously identified as electrons and their antimatter counterpart, positrons. The 6.8 million particles observed in the energy range 0.5 to 350 GeV are the subject of the precision study reported in this first paper.
AMS has measured the positron fraction (ratio of the positron flux to the combined flux of positrons and electrons) in the energy range 0.5 to 350 GeV. We have observed that from 0.5 to 10 GeV, the fraction decreases with increasing energy. The fraction then increases steadily between 10 GeV to ~250 GeV. Yet the slope (rate of growth) of the positron fraction decreases by an order of magnitude from 20 to 250 GeV. At energies above 250 GeV, the spectrum appears to flatten but to study the behavior above 250 GeV requires more statistics – the data reported represents ~10% of the total expected. The positron fraction spectrum exhibits no structure nor time dependence. The positron to electron ratio shows no anisotropy indicating the energetic positrons are not coming from a preferred direction in space. Together, these features show evidence of a new physics phenomena.
Video description translated by Google Translate: Levitation melting aluminum sample weighing 2.6 g. Power Inverter 1.6 kW. Frequency of 204 kHz. Temperature peaks 1200C.
The OPERA international experiment at the INFN Gran Sasso Laboratory (Italy) has observed a third neutrino tau candidate from “flavour” oscillation. The “muon-type” neutrino produced at CERN in Geneva arrived at the Gran Sasso laboratory as a “tau” neutrino. An extremely rare event observed only twice before, in 2010 and in 2012.
The data analysis will be pursued for two more years searching for other tau neutrinos that could definitely prove this very rare phenomenon.