Color Superconductivity

Color superconductivity, the hypothetical condensation of quark pairs at the cores of super-dense collapsed stars, might represent a unique example of superconductivity being made stronger, not weaker, by the presence of magnetism.

In ordinary electrical superconductivity, in a metallic lattice of atoms, free electrons can pair up through the agency of a very weak coupling force mediated by the subtle vibrations in the lattice itself, establishing a weakly attractive force between pairs of electrons. An external magnetic field is either repelled from such a superconducting environment (the Meissner effect) or can serve to undo the fragile superconducting state. On the other hand, if quark matter is realized inside the core of neutron stars -- with densities about 10 times the density of ordinary atomic nuclei -- or within the still hypothetical quark stars -- objects ranking somewhere between neutron stars and black holes in terms of matter density -- quarks will be pressed together so firmly that by the rules of asymptotic freedom (see the description of last year's physics Nobel prize in PNU 703) the force between the quarks will be quite weak and attractive.

This weakly interactive highly dense quark matter is expected to behave similarly to ordinary superconductors in condensed matter, and the quarks will form pairs as do the electrons in metallic superconductivity. Since quarks possess "color charge" ("colors" like red, green, or yellow are just another way of referring to a special type of charge, analogous to electric charge, carried by quarks) the quark-quark pair carries a net color charge; hence the phenomenon is called color superconductivity (for a detailed explanation of color superconductivity see this article in the August 2000 issue of Physics Today).

One might think that an applied magnetic field will produce in the color superconductor the same kind of counteracting effect that it does in ordinary superconductivity. However, a new study by Vivian de la Incera and Efrain Ferrer of Western Illinois University (Macomb, Ill., U.S.) and Cristina Manuel of the Instituto de Fisica Corpuscular (Valencia, Spain) shows theoretically that the powerful magnetic fields inside some super-compressed stars can actually enhance color superconductivity.

The authors say that, in the core of compact stars, the coming together of very high nuclear density, an enfeebled color nuclear force, and very strong magnetic fields (as high as 10¹⁷ gauss in some collapsed stars), enables the formation of a new phase of low-temperature color superconducting quark matter, one in which superconductivity and magnetism are on good terms (see figure).

Right now, the authors admit, testing this hypothesis will be difficult, as more investigations are still needed to identify signatures that can connect the inner phase of the star to its observable properties, such as the mass-to-radius ratio.

Ferrer et al., Physical Review Letters, 7 October 2005