Understanding of superconductivity may be closer
Physicists have long debated the causes of superconductivity, a phenomenon in which normal resistance to a flow of electrical current vanishes in certain materials when extremely cold. This allows hyper-efficient current transmission—offering the promise of a new electrical golden age with high-powered computers, magnetically levitating trains and super-efficient power lines.
But to put this effect to practical use, scientists have to understand it better, especially why it seems to occur only in such cold and whether that can be changed.
A new study may help clarify these questions, according to researchers who have found that superconductivity works differently in two slightly different temperature ranges.
Superconductivity was discovered by the Dutch physicist Heike Kamerlingh Onnes when in 1911 when he cooled mercury to barely above absolute zero, the lowest temperature theoretically possible.
Scientists later concluded that superconductivity at such rock-bottom temperatures occurs when vibrations of the grid-like atomic arrangement of the material affects its electrons, subatomic particles that carry electric charge. These, which normally repel each other because they have the same charge, then join up as pairs that glide effortlessly through the material without scattering off its atoms.
In 1986 came the discovery of a class of materials that allow superconductivity at somewhat less frigid temperatures: up to about 150 Kelvin (minus 253 F or minus 123 C), considerably higher than the 4 degrees Kelvin (minus 452F or minus 269 C) required in the original Onnes tests.
This advance allowed the materials to be cooled with liquid nitrogen, which costs less than the liquid helium needed to cool lower-temperature superconductivity.
Since that finding, scientists have debated whether in these higher-temperature superconductors—also called copper oxide superconductors—electrons bond in the same ways as in the lower-temperature superconductors.
The mechanism turns out to be different, according to the new study. Rather than atomic vibrations driving the electrons to join as pairs, the researchers said, higher-temperature superconductivity depends on electrons’ ability to take advantage of their natural repulsion in a complex situation.
This conclusion, investigators said, was based on experiments showing that the places in a sample where electrons form the most strongly bound pairs, are the same as where they show signs of stronger repulsion at higher, non-superconducting temperatures.
Surprisingly, in other words, it seems “the electrons with the strongest repulsion in one situation are the most adept at superconductivity in another,” said Princeton University physicist Ali Yazdani, one of the researchers. That’s unlike the behavior of electrons in lower-temperature superconductive materials, according to the group, which studied a compound made of strontium, bismuth, calcium and copper oxide and reported the findings in the April 11 issue of the research journal Science.
Although much remains to be explained, the researchers said their work may be a a useful step. “The data is a gold mine which we’re only beginning to exploit,” agreed Princeton physicist Philip Anderson, who won a physics Nobel in 1977 and wasn’t involved in the research.
The investigators used a specially rigged form of a device known as scanning tunneling microscope, which let them examine a single atom as electrons there went from repelling each other to pairing up. The microscope analyzes atoms by measuring current that flows between the surface of a sample, and a specially designed probe on the microscope. The probe, with a fine tip just one atom wide, is placed a hair’s breadth above the sample, and can move in increments smaller than an atom over the surface to take measurements.
April 10, 2008,Courtesy Princeton University and World Science staff
