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
the force between the quarks will be quite weak and
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
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.
al., Physical Review Letters, 7 October 2005