Superconductivity

This is a phenomenon observed in several metals and ceramic materials. Some of these materials when cooled to temperatures ranging from near absolute zero (0K, i.e. -273°C) to liquid nitrogen temperatures (77K, i.e. -96°C), their electrical resistance drops suddenly to zero. Superconductors exhibit very strong diamagnetism. Superconductivity is manifested only below a certain critical temperature and a critical magnetic field

which varies with the material used. Before 1986, the highest critical temperature was 23.2K (-249.8°C/-417.6°F). Temperatures this low were achieved by the use of liquid helium an inefficient coolant. (Tinkham, 1986).

Superconductors are so called because they present no resistance to the movement of electric charge through them. When charge moves through a material, an electric current exists in the material. Ordinary materials, even good conductors tend to resist the flow of charge through them. In superconductors however, the resistance is zero. If current is set up in a superconductivity ring, it lasts forever, with no battery or other source of energy needed to maintain it. (Halliday, 2001)

Superconductivity found in neutron stars

NASA's Chandra X-ray Observatory has disco

vered the first direct evidence for a superfluid - a bizarre, friction-free state of matter - at the core of a neutron star.

Superfluids created in laboratories on Earth exhibit remarkable properties, such as the ability to climb upward and escape airtight containers. The finding has important implications for understanding nuclear interactions in matter at the highest known densities.

"The rapid cooling in Cas A's neutron star, seen with Chandra, is the first direct evidence that the cores of these neutron stars are, in fact, made of super fluid and superconducting material," said lead author Peter Shternin of the Ioffe Institute in St Petersburg, Russia, of a paper accepted in the Monthly Notices of the Royal Astronomical Society.

Unusual rapid decline in temperature

Neutron stars contain the densest known matter that is directly observable. One teaspoon of neutron star material weighs six

billion tonnes. The pressure in the star's core is so high that most of the charged particles, electrons and protons, merge resulting in a star composed mostly of uncharged particles called neutrons.

Two independent research teams studied the supernova remnant Cassiopeia A (or Cas A for short) - the remains of a massive star 11,000 light years away that would have appeared to explode about 330 years ago as observed from Earth.

Chandra data found a rapid decline in the temperature of the ultra-dense neutron star that remained after the supernova, showing that it had cooled by about 4% over a 10-year period.

Cooling neutron superfluid

"This drop in temperature, although it sounds small, was really dramatic and surprising to see," said Dany Page of the National Autonomous University in Mexico, leader of a team with a paper published inPhysical Review Letters. "This means that something unusual is happening within this neutron star."

Superfluids containing charged particles are also superconductors, meaning they act as perfect electrical conductors and never lose energy. The new results strongly suggest that the remaining protons in the star's core are in a superfluid state and, because they carry a charge, also form a superconductor.

Both Page’s and Shternin’s teams show that this rapid cooling is explained by the formation of a neutron superfluid in the core of the neutron star within about the last 100 years as seen from Earth. The rapid cooling is expected to continue for a few decades and then it should slow down.

A gift from the universe

"It turns out that Cas A may be a gift from the universe because we would have to catch a very young neutron star at just the right point in time," said Page's co-author Madappa Prakash, from Ohio University. "Sometimes a little good fortune can go a long way in science."

The onset of superfluidity in materials on Earth occurs at extremely low temperatures near absolute zero, but in neutron stars, it can occur at temperatures near a billion degrees Celsius. Until now there was a very large uncertainty in estimates of this critical temperature.

This new research constrains the critical temperature to between one half a billion to just under a billion degrees. Cas A will allow researchers to test models of how the strong nuclear force, which binds subatomic particles, behaves in ultradense matter.

Understanding neutron star behaviour

These results are also important for understanding a range of behavior in neutron stars, including ‘glitches’, neutron star precession and pulsation, magnetar outbursts and the evolution of neutron star magnetic fields.

Small sudden changes in the spin rate of rotating neutron stars - called glitches - have previously given evidence for superfluid neutrons in the crust of a neutron star, where densities are much lower than seen in the core of the star.

This latest news from Cas A unveils new information about the ultra-dense inner region of the neutron star. "Previously we had no idea how extended superconductivity of protons was in a neutron star," said Shternin's co-author Dmitry Yakovlev, also from the Ioffe Institute.

The cooling in the Cas A neutron star was first discovered by co-author Craig Heinke, from the University of Alberta, Canada, and Wynn Ho from the University of Southampton, UK, in 2010. It was the first time that astronomers have measured the rate of cooling of a young neutron star.

- article contributed by Ms.Rashiha Arsheen khan , I year Aerospace engg.