Experiment confirms existence of new electronic state in superconductors

The existence of a new electronic state in superconductors, materials that can carry an electric current without resistance, has been confirmed experimentally according to research to be published in the 12 November 2004 issue of the journal Science by a team led by Ying Liu, associate professor of physics at Penn State. “We have established direct evidence for the existence of an odd-parity superconductor, which previously had been theorized but never demonstrated in an unambiguous experiment,” says Liu. The results culminate six years of experimentation by Liu in collaboration with a former graduate student, Karl Nelson, and a former postdoctoral associate, Zhiqiang Mao at Penn State; and Yoshiteru Maeno, a professor of physics at Kyoto University.

In addition to their scientific interest, superconductors have a number of practical applications. These include superconducting magnets, which have enabled the development of high-resolution magnetic-resonance imaging in medicine, and superconducting wires, which transport electrical power without loss due to heating of the cable by electrical resistance.

A material becomes superconducting because electrons in the material form pairs, known as Cooper pairs. Liu likens the pairing process to dancers on a dance floor: “The electrons, crowded together, form pairs and move to the ’music’ of phase coherence, a quantummechanical property that synchronizes the steps of all the dancing pairs.” These pairs, described mathematically by a quantummechanical wave function, move tightly together despite tendencies that would force them apart.

Physicists theorize that there are two categories of electronic state in superconductors, based on the quantummechanical characteristics of the Cooper pairs. Although their properties vary widely, almost all superconductors found so far belong to the same category because they share a fundamental property, known as even-parity symmetry. “Each Cooper pair in a superconductor can be thought of as being born with a little one-handed internal clock that indicates the ’time’, or the phase, of the pair,” explains Liu. “When the hand points to midnight the phase of the Cooper pair is zero degrees. When the hand points to three, the phase is 90 degrees, at six it is 180 degrees. Quantum mechanics demands that the phase of two pairs moving in opposite directions be different by exactly zero or 180 degrees.” If the clocks of two Cooper pairs moving in opposite directions have the same time, the symmetry of the pairs is designated as even parity.

In elemental superconductors–first discovered almost 100 years ago–the two electrons in a pair tend to be close together without any relative motion. In so-called high-temperature superconductors–materials discovered a couple of decades ago that still are poorly understood–the electrons in a pair tend to be farther apart, with substantial relative motion. Although these Cooper pairs behave very differently and the superconductors exhibit rather different features, they share the same property of even-parity symmetry.

On the other hand, if the clocks for two pairs moving in the opposite directions are six hours apart–a phase difference of 180 degrees–the symmetry of the Cooper pairs is designated as odd-parity symmetry. These odd-parity Cooper pairs form a new electronic state in superconductors. “The pairing symmetry is important because it dictates many physical properties of a superconductor. An odd-parity superconductor behaves very differently from an even-parity superconductor,” says Liu. The article to appear in Science, “Odd-Parity Superconductivity in Sr2RuO4,” confirms unambiguously that strontium ruthenate, Sr2RuO4, which is the only known superconducting ruthenium oxide material, is a member of this category of odd-parity superconductors.

Although other experiments have indicated that odd-parity pairing was involved, Liu’s experiment provides the first definitive proof of this new type of pairing. “Theorists had predicted that superconductivity in strontium ruthenate could be associated with odd-parity pairing,” says Liu “Earlier experiments did provide plenty of evidence to support the prediction, but those results also could be questioned by counter examples and attributed to something else. Our experiment is a ’yes-or-no’ test of the odd-parity pairing that settles the issue.”

The basic idea of the experiment is to measure the dependence of the phase of the Cooper-pair wave function on the direction in which the Cooper pair moves, using the phenomenon of wave interference. “Essentially, we want to compare the clocks of the strontium ruthenate Cooper pairs moving in the opposite directions. We connected a strontium ruthenate superconductor to an even-parity, conventional superconductor through two parallel surfaces that are oppositely faced, forming two so-called Josephson junctions. This procedure makes a superconducting quantum-interference device, known as a SQUID. The clocks of the strontium ruthenate pairs moving into the conventional superconductor through the two junctions are then six hours apart, or 180 degrees different in phase. The Cooper-pair waves from the two junctions will then interfere destructively,” says Liu. This interference pattern was detected by measuring a current going through the SQUID as a function of an applied magnetic field. By confirming through the interference patterns that the oppositely moving Cooper pairs naturally position themselves in their respective time zones six hours apart–a 180 degree phase difference–Liu’s team demonstrated that strontium ruthenate does exhibit an odd-parity symmetry.

The discovery is of interest to physicists because it breaks new scientific ground that also could have useful applications. “In nature, particles can be paired in specific ways depending on the interactions that create the attractive force,” says Liu. “Odd-parity pairing has been found to exist in unusual systems ranging from small and cold–such as atoms of helium-3 at very low temperatures, a couple of thousandths of a degree above absolute zero, to large and hot–such as neutrons in neutron stars at hundreds of millions of degrees.”

The phenomenon of odd-parity superconductivity in strontium ruthenates occurs only below a temperature of about one and a half degrees above absolute zero, well below room temperature. However, Liu points out that now that the odd-parity superconductor has been shown to exist, the unique features of this type of superconductor can be studied for potential practical applications. In addition to possible expansion of current superconductor uses, odd-parity superconductors someday may be used for special purposes; for instance, in the research effort to develop quantum computers.