Unexpected Boost: A superconductivity killer’s silver lining

A bad neighbor sometimes has a good influence on the folks next door. Superconductivity researchers are discovering their own version of this experience.

Physicists have long been wary of ordinary metals that, when they share physical borders with superconductors, sap their neighbors of their no-resistance conductivity. This phenomenon is known as the proximity effect, and scientists have now found its opposite.

An ordinary metal that’s next door to one particular class of superconductors–those with so-called strongly correlated systems of electrons–can actually boost the neighboring material’s superconductivity. Among the superconductors that fall into this class are high-temperature superconductors (SN: 3/16/02, p. 173: Magnetism piece fits no-resistance puzzle). They superconduct in much warmer–though still bitterly cold–conditions than ordinary superconductors do.

Physicists have long sought ways of coaxing high-temperature superconductors to function at even higher temperatures (SN: 12/2/00, p. 359). The newfound “inverse proximity effect” may offer an avenue toward that goal, says Robert C. Dynes of the University of California, San Diego, leader of the new study. Also, since magnetic field sensors, extremely powerful magnets, and many of the other superconducting devices exploit the ordinary proximity effect, the inverse effect will probably lead to novel devices, he predicts.

Dynes and his colleagues have “discovered an anomaly in the proximity effect that challenges our understanding of superconductivity,” comments Robert J. Soulen of the Naval Research Laboratory (NRL) in Washington, D.C. He and Michael S. Osofsky, also of NRL, have developed a theoretical model to predict how structures of superconducting materials affect the so-called critical temperatures at which superconductivity kicks in. When the scientists plugged the new inverse-effect data into their model last week, “it fit like a glove,” Soulen says.

Dynes and his collaborators began their work on a hunch that there is an inverse proximity effect. However, because high-temperature superconductors are such complex materials, the team chose to start with simpler ingredients.

It zeroed in on lead, an ordinary superconductor. However, they knew it would become a strongly correlated system if they fabricated it in extremely thin layers–only several atoms thick–in which the atoms assumed a disorderly, noncrystalline atomic arrangement. The researchers then deposited silver on lead films to serve as the neighboring metal.

The team found that the temperatures at which the lead layers became superconducting increased ever so slightly as the silver overlayer increased to a thickness of 0.26 nanometer. That minute change in critical temperature–from about 1.6 kelvins to just over 1.8 K–signified that the lead film had become a better superconductor.

When the layers of either metal became too thick, the conventional proximity effect took over, and the lead’s critical temperature declined. Measurements indicating the amount of energy needed to break up the electronic pairings that underlie superconductivity–another gauge of how effectively a film superconducts–also showed a telltale rise and fall that indicates an inverse proximity effect. Dynes, Olivier S. Bourgeois, now of the National Center for Scientific Research in Grenoble, France, and Aviad Frydman of Bar Ilan University in Ramat Gan, Israel, report their findings in the May 6 Physical Review Letters.