In a more sophisticated variant of the technique, largely pioneered by Gedik, the standard single pump beam is replaced by two beams hitting the surface from different sides simultaneously. This generates a standing wave of controlled wavelength in the film, but it disappears rapidly as the electrons relax back into their original state.
This technique was applied to the atomically perfect LaSrCuO films synthesized at Brookhaven Lab. In films with a critical temperature of 26 degrees Kelvin (the threshold beyond which the superconductivity breaks down), the researchers discovered two new short-lived excitationsboth caused by fluctuating CDWs.
Gedik's technique even allowed the researchers to record the lifetime of CDW fluctuationsjust 2 picoseconds (a millionth of a millionth of a second) under the coldest conditions and becoming briefer as the temperatures rose. These waves then vanished entirely at about 100 Kelvin, actually surviving at much higher temperatures than superconductivity.
Ruling out a Suspect
The researchers then hunted for those same signatures in cuprate films with slightly different chemical compositions and a greater density of mobile electrons. The results were both unexpected and significant for the future of HTS research.
"Interestingly, the superconducting sample with the highest critical temperature, about 39 Kelvin, showed no CDW signatures at all," Gedik said.
The consistent emergence of CDWs would have bolstered the conjecture that they play an essential role in high-temperature superconductivity. Instead, the new technique's successful detection of such electron waves in one sample but not in another (with even higher critical temperature) indicates that another mechanism must be driving the emergence of HTS.
"Results like this bring us closer to understanding the mystery of HTS, considered by many to be one of the gre
|Contact: Justin Eure|
DOE/Brookhaven National Laboratory