First detailed picture of the way a superfluid influences the behavior of a superconductor

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Washington, August 3 : Researchers at Washington University in St. Louis in the US have drawn the first detailed picture of the way a superfluid influences the behavior of a superconductor.

In addition to describing previously unknown superconductor behavior, these calculations could change scientists' understanding of the motion of neutron stars.

A neutron star, the high-density remnant of a former massive star, is thought to contain both a neutron superfluid and a proton superconductor at its core.

Despite widespread agreement that neutron stars contain both materials, superfluid-superconductors have not been widely studied.

"Not many people have thought seriously about the interactions between a superfluid and a superconductor that are co-existing like this," said Mark Alford, associate professor of physics at Washington University. "They tended to treat the two components separately," he added.

Separately, the two phenomena are well understood.

A superconductor allows a flow of current without resistance. Similarly, a superfluid flows without friction. Unlike superconductors and superfluids, a superfluid-superconductor does not exist on earth.

But, understanding its hybrid behavior may be a first step toward creating one in the lab and understanding what goes on inside neutron stars.

In addition to conducting current without resistance, superconductors also exclude magnetic fields. Neutron stars have massive magnetic fields, but scientists do not know how a superconductor behaves in the presence of this field, specifically whether it will be a type I or type II superconductor.

A type I superconductor forces a magnetic field around its exterior. A type II superconductor, however, strikes a compromise, letting the magnetic field pass through tiny non-superconducting holes called flux tubes.

Whether a superconductor is type I or type II depends on a value called kappa. If kappa is greater than a set critical value, the superconductor is type II. Likewise, if kappa is less than the critical value, the superconductor is type I.

Add a superfluid, however, and these calculations show that the superconductor's boundary shifts, changing the critical value of kappa and causing exotic behavior at the boundary.

To understand the boundary shift, Alford and Good examined two interactions between the superfluid and superconductor.

The first had a superconductor either attracting or repelling a superfluid. The second had a flowing superconductor causing a superfluid to flow either with it or against it.

Alford and Good found that the two superconductor-superfluid interactions (attractive/repulsive and flow) had opposite effects on the boundary shift and produced different, but equally exotic, boundary behavior.


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