Researchers Seek Superconduction Secrets

Kevin Robinson

Angle-resolved photoemission spectroscopy, a tool that helped define semiconductor physics, is still proving useful to scientists working to explain the physics behind high-temperature superconductors -- basic research that may improve electric power transmission cable, electromagnets and electronics. Researchers from Lawrence Berkeley National Laboratory, Stanford University in Stanford, Calif., and the University of Tokyo reported that the technique has enabled them to study a superconductor model material, neodymium-substituted lanthanum strontium copper oxide (Nd-LSCO), that will help explain high-temperature superconductivity.

Superconductivity, the movement of current with virtually no resistance, is typically an extremely low-temperature phenomenon. Traditional superconductors work at 23 K or lower. High-temperature superconductors, however, work at 135 K at ambient pressure and 150 K at high pressure -- much higher than the temperature of liquid nitrogen (77 K). One theory suggests that the charge carriers in the superconductive material are arranged in one-dimensional stripes with insulating areas between them. These stripes are thought to move dynamically in the material and to allow free current flow.

"Whether the stripe is responsible for high-temperature superconductivity is currently the hottest topic in the field," said team member Xingjian Zhou.

To study the stripes more closely, the researchers chose Nd-LSCO for its static stripes. Zhou explained that, although LSCO is believed to contain dynamic stripes, neodymium can pin them down and make them static, producing a model system of stripes that can be easily studied.

Knocking out electrons

Angle-resolved photoemission spectroscopy uses light -- in this case a high-power synchrotron beam operating at 22.4 nm -- to knock electrons out of a sample. The researchers measured the intensity of the photoelectrons as a function of the energy at different angles to determine the electronic structure of the material. They deduced that the stripe phase was a one-dimensional structure, that low-energy spectral weight is confined near the Brillouin zone face and that the nodal state is suppressed. Zhou said they also found that the compound is closely related to genuine superconductors and can be superconducting, though at a lower critical temperature than it would be without neodymium.

A schematic of angle-resolved photoemission spectroscopy (top) shows how linearly or elliptically polarized light kicks out electrons when it is incident on the sample. An energy analyzer measures the intensity of the emitted electrons at a given direction, and the momentum of the photoelectrons is determined by the kinetic energy and the angle.

"It is the most direct and powerful tool to investigate electronic structures of materials," Zhou said of the technique. "The success of this work is a combination of many factors, including the availability of good single-crystal samples; a tunable high-intensity, high-energy resolution synchrotron light source; critical experimental conditions (like ultrahigh vacuum); and high-precision instrumentation."

Zhou said the researchers' next step is to pursue dynamic stripes. "The final goal," he said, "is to provide critical information for establishing a comprehensive theory to understand strongly correlated electron systems in general and high-temperature superconductivity in particular."