Scientists use light to alter properties of high temperature superconductors
By David Szondy
September 2, 2012
When people have a difficult problem they often talk about “shining a light on it.” Creating and controlling high-temperature superconductors has been a problem for scientists and engineers for over two decades. Now, Yoram Dagan, a professor at Tel Aviv University's (TAU) Department of Physics and Center for Nanoscience and Nanotechnology, has made a breakthrough in superconductors by literally shining a light on them. By doing this, he is able to control their properties.
Superconductors are one of those seemingly magical materials. When a conducting material, such as copper or silver, is cooled, its ability to conduct electricity increases. Bring a copper wire down to the temperature of absolute zero (0° K, −273.15° C, −459.67° F) and its electrical resistance will be extremely low, but it will still have some. A superconductor is something very different. If a superconducting material is cooled to below a particular critical temperature, then all of its resistance vanishes. It becomes exactly zero. That means that a loop of superconducting material can conduct an electrical current around and around inside of itself forever.
Needless to say, this makes superconductors very useful to engineers. Since their discovery in the early 20th century, they’ve been used extensively to make super-powerful magnets for maglev trains, particle accelerators and MRI scanners. They’ve also found applications in electronics, computers and many other fields. The problem is that most superconductors operate at the temperature of liquid helium (about five degrees K above absolute zero). This makes them expensive to build and difficult to operate and maintain.
When high-temperature superconductors were discovered in the 1980s, it was hoped that this would lead to making superconductor devices more economical and allow them to find wider applications. Unfortunately, “high-temperature” meant around that of liquid nitrogen (77 K, -196 C, -320.8 F), the materials were difficult to make and they were often quite brittle. Many of these problems remain, but Dagan has found a way of using light to alter the critical temperature of high-temperature superconductors and even control it with the flip of a switch.
The usual way of creating a high-temperature superconductor is by chemical doping, which is a complicated process. Dagan and his team opted for a simpler approach by taking a superconducting film and coating it with an organic material one-molecule thick. When exposed to light, this 50-nanometer thick layer altered the properties of the superconducting film as the molecules in the coating stretched and changed shape.
The team tried three types of molecules for the coating with markedly different results. One raised the critical temperature of the superconductor, while the second raised the critical temperature on exposure to UV light, but lowered it in visible light. The third raised the temperature, but only as long as the light was on. Turning it off caused the critical temperature to revert to normal. In each case, the response was very strong.
The light-altered superconductors are still in the laboratory, so levitating cars won't be hitting showrooms for a while yet, but Dagan does see other potential applications, such as a "non-dissipated memory” that would store data indefinitely without waste or need for additional power.