March 5, 2007

Unlocking the Secrets of High-temperature Superconductors

Although it was discovered more than 20 years ago, a particular type of high-temperature (Tc) superconductor – material that conducts electricity with almost zero resistance – is regaining the attention of scientists at the U.S. Department of Energy’s Brookhaven National Laboratory. Copper-oxide compounds, called cuprates, operate at temperatures warmer than traditional superconductors but still far below freezing. Understanding the mechanism for these superconductors may one day help scientists design superconductors able to function closer to room temperature for applications such as more-efficient power transmission.

Christopher Homes

Discovered in 1986, the most perplexing of these cuprate superconductors is “LBCO,” named for the elements it contains: lanthanum, barium, copper, and oxygen. After years of research on similar materials, Brookhaven researchers have learned how to “grow” better samples of LBCO, which has allowed for extensive studies on its intriguing properties. Three Brookhaven physicists, including NSLS user Christopher Homes, discussed their most recent findings about LBCO at the March meeting of the American Physical Society in Denver, Colorado. The details of Homes’ research is described below:

A Superconductor with Insulating Properties

One of the most perplexing findings involving LBCO is that the high-temperature superconductor actually has distinct insulating-like properties. Each barium atom has one fewer electron than lanthanum, so increasing barium adds electron “holes,” or the absence of electrons, to the system. The more barium that is “doped” into the material, the more holes, and the greater the superconductivity – until the composition reaches a point where there is exactly one barium atom for every eight copper atoms, a state known as the 1/8 doping. Then, oddly, the superconductivity disappears. Above this point, as more holes (barium atoms) are added, the superconductivity reappears.

Brookhaven physicist Christopher Homes discussed this odd phenomenon at 9:12 a.m. Mountain Time on March 5, 2007, in the Four Seasons Ballroom 2-3 at the Colorado Convention Center. At the National Synchrotron Light Source and other facilities on site, Homes investigates LBCO’s electronic properties by shining various types of light onto an LBCO crystal and measuring the intensity that is reflected back. This optical picture tells scientists about the behavior of the charge carriers – or holes – in LBCO. Most materials have a set number of carriers that scientists can count using these methods. As a material becomes a superconductor, some of the holes lower their energy by falling into a superconducting state that allows them to flow without resistance. As these carriers condense, there is a characteristic change in the optical conductivity. However, even though LBCO is not a superconductor at the 1/8 doping, the number of holes still decreases at low temperature. Homes and other researchers attribute this feature to the formation of the so-called “energy gap.” In semiconductors, the charge gap blocks the flow of current because of its isotropic nature (the gap spreads evenly in all directions). Superconductors also have energy gaps, but in the cuprates these gaps have different energies in different directions with respect to the copper-oxygen chemical bonds.

“The more we look at this charge gap, the more it looks like a superconducting gap,” Homes said. “It has the same magnitude, the same shape and symmetry. Yet, it doesn’t have superconductivity.” Homes and other BNL researchers continue to tackle this mysterious problem in order to understand why a material that wants to be a superconductor is behaving like an insulator.

This research is funded by the Office of Basic Energy Sciences within the U.S. Department of Energy’s Office of Science.