Researchers warm up to superconductivity

Peter Kauffner

Even diehard Minnesotans consider 40 below to be a bit nippy. But researchers in the University’s Superconductivity Center conduct experiments at temperatures just above absolute zero, 460 degrees below zero Fahrenheit.
“If you make a mistake, you can warm up the system, and then your whole work is ruined,” said Nina Markovic, a physics research assistant who works at the lab. “You have to keep it cold all the time, which means transferring liquid helium twice a day, weekends and holidays and nights and days.”
Markovic is studying the electrical properties of a thin film of bismuth, a rare gray-white metal, cooled to a temperature just 0.27 degrees above absolute zero.
As more molecular layers are added to the film, it shifts from being an insulator to being a superconductor.
Electric current can flow through a conductor, but not through an insulator. In an ordinary conductor, such as copper wire, the resistance of the material causes some of the energy of the electric current to be converted into heat. But in a superconductor, electricity can flow without resistance or energy loss.
Markovic found that supercooled bismuth turns into a superconductor when it is at least 14 angstroms (5.6 trillionth of an inch) thick.
“The experiments show that the superconductor-insulator transition can be determined by thickness or by magnetic field,” Markovic said.
The lab also studies so-called high-temperature superconductivity. Traditionally, superconductivity was associated with extremely low temperatures. But since 1986, scientists have found that various unusual compounds may superconduct at much higher temperatures, as high as room temperature.
“In the case of high-temperature superconductivity, we are interested in the nature of the electron pairing in the superconducting state,” said Allen Goldman, professor of physics and director of the lab.
For a material to conduct electricity, there must be conduction electrons, atomic particles not bound to a specific atom. In an ordinary conductor, these electrons are scattered by impurities and vibrations. But in a low-temperature superconductor, they are arranged in very specific patterns, called Cooper pairs.
Goldman said he hopes to be able to use microwave tests to determine the nature of electron pairing in high-temperature superconductors.
It is possible the pairing has angular dependence, which would mean that resistance of the material is different depending on the direction of current flow.
The lab is also experimenting with mixed materials, called heterostructures, which consist of superconducting oxides and magnetic oxides.
“They are very strange materials,” Goldman said. Above room temperatures, they are insulators and nonmagnetic. At room temperature and below, they are magnetic and conduct electricity.
“We found that if you inject an electron from one of these magnetic materials into a superconductor, it can disrupt the superconductivity,” Goldman said. This discovery raises the possibility that heterostructures can be used to make switches that can turn the superconductivity of a material on and off.
Current superconducting switches rely on the Josephson junction, which consists of two superconductors separated by a thin insulating layer.
“People have not been able to miniaturize (Josephson junctions) the way they would like to miniaturize them,” Goldman said.
Superconducting switches would be faster and have lower power dissipation than the semiconductor switches found in current computers.
Lower power dissipation means that less heat is generated. This allows more switches to be crowded into the same area without overheating.