In research results reported in the August 21 issue of the journal Nature, NHMFL scientists describe how sharp changes in the Hall Effect on studies of a high-temperature superconductor made from bismuth, strontium, lanthanum, copper and oxygen show signs of a phase transition consistent with the current belief in the scientific community that this could be the cause of superconductivity.

A phase transition is a sudden change in the physical properties of the material. In the case of this experiment, it is not clear what exactly the phase transition was, but its discovery and further study could lead to a better understanding of superconductivity.

The Hall Effect occurs when a magnetic field is applied to a sheet of conductive material through which an electrical current is flowing and the charge-carrying electrons are pushed perpendicular to the flow to the edges of the material.

The ratio of the voltage in those electrons to the level of current flowing through the material is known as the Hall resistance and is a measurable characteristic of the conductive material.

Based on the behavior of other materials, the Hall effect was expected to be proportional to the amount of charge carriers in the superconducting material the researchers studied. The experiment, however, revealed a surprisingly more complicated picture.

"We found a sharp anomaly in the Hall resistivity that is indicative of a phase transition," said NHMFL staff member Fedor Balakirev. Balakirev explained that the anomaly was at the point at which the superconductivity was most robust. The discovery may be a telltale indicator of the electron-electron interactions that could be responsible for superconductivity itself.

Although high-temperature superconductors have been widely studied since their discovery more than a decade ago, many aspects of their nature remain unknown. One reason for this is that the superconductivity itself masks the very characteristics scientists are trying to understand.

Subjecting the superconducting material to a high magnetic field at low temperatures essentially 'lifts' the veil of superconductivity, making it possible to observe the very qualities of the particles that contribute to the materials' superconductivity.

The NHMFL Pulsed Field Facility at Los Alamos is one of the few places in the world capable of conducting these types of experiments on high-temperature superconductors because of the high magnetic fields and unique instrumentation.

"The kind of custom advanced instrumentation needed to obtain clean enough data for an experiment like this is unique to our magnet lab," said Albert Migliori, NHMFL staff member and Laboratory Fellow.

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