Physicists have found a new way to test how one of the most puzzling families of superconductors behaves, and the results favor one of two competing explanations that have divided the field for years. A study led by Milan Allan of LMU Munich and Ingmar Swart of Utrecht used the faint electrical noise produced by moving electrons to show that electrons in these materials pair up.
The materials in question are copper-oxide compounds called cuprates, which were discovered in the late 1980s to superconduct, carrying electricity with no resistance, at temperatures above the boiling point of liquid nitrogen. That was a remarkable jump for the field, yet after roughly four decades the mechanism behind the effect remains unresolved. One of the central clues is the pseudogap, a range of energies above the critical temperature where fewer electrons than expected are observed, and researchers disagree about what it means.
One view holds that electrons settle into patterns that are separate from superconductivity itself. The rival view holds that the missing electrons are a sign that they have formed Cooper pairs, the paired states that make superconductivity possible. To tell the two apart, the team turned to shot noise, the small fluctuations in an electrical current that depend on how charges travel. They measured it with a scanning tunneling microscope on BSCCO crystals cooled close to absolute zero. Extra noise indicates that electrons are arriving in pairs rather than one at a time.
The researchers found that the noise rose at the local pseudogap energy, and the pattern held across different samples and different spots on each surface. That link between pairing and the pseudogap supports the pair-formation explanation. When the team applied magnetic fields up to six tesla, the noise dropped toward the level expected for single electrons, which further suggests the signal really comes from paired electrons being disrupted.
The findings also suggest a timeline. Pairs appear to form at higher temperatures, while phase coherence, the state in which the pairs act together across the material, arrives later as the temperature falls. The bottleneck may therefore be coherence rather than pairing itself. The authors judged that room-temperature superconductivity is unlikely to follow directly from this work, and noted that other coupled effects complicate the picture. The behavior of the material above its critical temperature was not measured with this technique, leaving that question open.
Credit. Based on reporting for WorldNL Magazine. Is this your work? Write to us.
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