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Scientists find evidence that a theoretical phenomenon is real

DOE/Brookhaven National Laboratory : 06 May, 2007  (Technical Article)
Recent research by a scientist at the U.S. Department of Energy
The Giant Proximity Effect is a theoretical relative of the established Proximity Effect, in which a very thin layer of ‘normal’ metal behaves like a superconductor when placed between two thicker superconductor slices. However, PE theory states that GPE, which occurs across a relatively thick normal metal layer, should not be possible.

“Our discovery indicates PE theory may need to be revised to incorporate GPE,” said Brookhaven physicist Ivan Bozovic, the study’s lead researcher. “While that is significant in itself, this observation may also lead to a critical step forward in the development of superconducting electronics.”

In GPE, the normal-metal barrier layer is much larger than in the PE case, as much as 100 times the thickness. In this experiment, the barrier layer was up to 20 nanometers, or billionths of a meter, thick. Having such dimensions makes these “sandwiches,” called Josephson junctions, the right size for manufacturing into components for “nano”-sized electric circuits.

Bozovic and his collaborators made a number of Josephson junctions with varying barrier thicknesses. They used a high-temperature superconducting material that contains lanthanum, strontium, copper, and oxygen and a ‘normal’ material called LCO, which lacks the strontium. LCO is technically a superconductor, but behaves like a regular metal above a certain “transition” temperature. Both LSCO and LCO are “cuprates,” a family of superconductors that contain copper oxide. In this experiment, the thick LCO barrier transmitted a superconducting current at temperatures well above its normal superconducting temperature.

“Our experiment shows that, under the right conditions, at least, GPE is no longer just a theoretical phenomenon,” said Bozovic. “In the cuprates we studied, relatively thick barriers of normal metals can conduct a superconducting current when sandwiched between two superconductors.”

In past experiments, other researchers have made the same claims, but have been met with skepticism by the scientific community. This is partly due to GPE’s utter inconsistency with the established theory, which states that the electron pairs that make up a supercurrent can travel only one or two tenths of a nanometer before separating. Additionally, possible experimental errors may have skewed the results of these previous experiments. One example would be “microshorts”, tiny superconductor filaments that pierce the barrier, causing the appearance of a superconducting current across it.

In light of this, Bozovic and his collaborators carefully chose materials and prepared their experimental setup to avoid these errors. LSCO and LCO are very similar, and match up well at the atomic level when sandwiched together. This results in an atomically smooth interface between the layers that lacks microshorts and “pinholes,” tiny unwanted holes in the junction that could cause a superconducting current to appear to pass through the normal metal layer.

In upcoming experiments, Bozovic and his colleagues plan to investigate how the current is transmitted across the LCO barrier to learn more about the mechanisms behind GPE. They will also look more closely at how the current flow depends on the thickness of the barrier, the temperature of the junction, and other factors.
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