Pinning down a superconductivity theory

In conventional, low-temperature superconductors like niobium, electrons overcome their mutual repulsion and pair up to pass unhindered through the host material. This pairing is facilitated by vibrations of the material's crystal lattice.

 Quantum theory describes the pair by means of a single wave function, which mathematically specifies the probability distribution showing where the two electrons are most likely to be. In a conventional superconductor, the electrons' wave function is spherical, indicating that a pair has an equal chance of moving in any direction. Such a pairing is said to display s-wave symmetry.

 In copper oxide superconductors, lattice vibrations alone are not strong enough to maintain the necessary electron pairing at elevated temperatures. Some theorists have proposed that magnetic interactions between the electrons and copper atoms play a key role in forging electron pairs.

 In this case, an electron pair would instead have a wave function with d-wave symmetry, resembling a four-leaf clover that has its lobes aligned along the crystal axes.

 Researchers performed a number of experiments aimed at detecting d-wave pairing. The results pointed to the presence of d-wave symmetry, but they couldn't unambiguously rule out an additional contribution from s-wave pairing.

 Kirtley and his coworkers looked for d-wave pairing in a thin film of a thallium barium copper oxide that has a crystal structure known as tetragonal, which is difficult to make but highly symmetrical.

 In particular, the crystal geometry requires that the electron pairing be either s-wave or d-wave. "It can't be a combination of the two," Kirtley says.

 The results indicate that electron pairs in thallium barium copper oxide display d-wave symmetry. "This is the first time that an experiment has shown that s-wave behavior in electrons is not critical to high-temperature superconductivity," says Jui H. Wang of the State University of New York at Buffalo, a member of the team that fabricated the material.

 The identification of a superconductor displaying pure d-wave symmetry serves as a starting point for understanding the more complicated, mixed states that appear to characterize other high-temperature superconductors.