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Laser Spectroscopy Narrows Search for Origins of Superconductivity
Feb 2013
UPTON, N.Y., Feb. 25, 2013 — The mechanism underlying high-temperature superconductivity has remained one of the most important and tantalizing puzzles in physics, but new research that measures fleeting electron waves could help solve the mystery and pave the way for rapid technological advances.

The phenomenon of high-temperature superconductivity (HTS) allows electric current to pass with perfect efficiency through materials chilled to subzero temperatures and plays an essential role in revolutionizing the entire electricity chain, from generation to transmission and grid-scale storage. Pinning down one of the possible explanations for HTS — fleeting fluctuations called charge-density waves (CDWs) — has narrowed the search for superconductivity’s origins.

Researchers at MIT and Brookhaven National Laboratory have combined two state-of-the-art experimental techniques to study those electron waves with unprecedented precision in 2-D, custom-grown materials. They discovered that CDWs cannot be the root cause of the unparalleled power conveyance in HTS materials. In fact, CDW formation is an independent and likely competing instability.

"It has been difficult to determine whether or not dynamic or fluctuating CDWs even exist in HTS materials, much less identify their role," said Brookhaven Lab physicist and study co-author Ivan Bozovic. "Do they compete with the HTS state, or are they perhaps the very essence of the phenomenon? That question has now been answered by targeted experimentation."

Inside a cleanroom, Brookhaven physicists Ivan Bozovic (left) and Anthony Bollinger work on the molecular beam epitaxy system that produced the atomically perfect materials used in the study. Courtesy of Brookhaven National Lab.

Electricity travels imperfectly through traditional metallic conductors, losing energy as heat due to a kind of atomic-scale friction. Impurities in these materials also cause electrons to scatter and stumble, but superconductors can overcome this hurdle — assuming the synthesis process is precise.

For this experiment, a custom-built molecular beam epitaxy system at Brookhaven Lab was used to grow thin films of LaSrCuO, an HTS cuprate (copper-oxide) compound. The metallic cuprates, assembled one atomic layer at a time, are separated by insulating planes of lanthanum and strontium oxides, resulting in what's called a quasi-2-D conductor. When cooled down to a low enough temperature — less than 100 K — strange electron waves began to ripple through that 2-D matrix. At even lower temperatures, these films became superconducting.

"In quasi-two-dimensional metals, low temperatures frequently bring about interesting collective states called charge-density waves," Bozovic said. "They resemble waves rolling across the surface of a lake under a breeze, except that instead of water, here we actually have a sea of mobile electrons."

Once a CDW forms, the electron density loses uniformity as the ripples rise and fall. Detecting CDWs typically requires high-intensity x-rays, such as those provided by synchrotron light sources like Brookhaven's NSLS and, soon, NSLS-II. Even then, the waves are essentially frozen upon formation for the technique to work. If CDWs actually fluctuate rapidly, however, they may escape detection by x-ray diffraction, which typically requires a long exposure time that blurs fast motion.

To catch CDWs in action, Nuh Gedik and colleagues at MIT used an advanced ultrafast spectroscopy technique. Intense laser pulses cause excitations in the superconducting films, which are then probed by measuring the film reflectance with a second light pulse, called a pump-probe process. The second pulse is delayed by precise time intervals, and the measurement series allows the lifetime of the excitation to be determined.

MIT researchers' new method for observing the motion of electron density waves in a superconducting material led to the detection of two kinds of variations in those waves: amplitude (or intensity) changes and phase changes, shifting the relative positions of peaks and troughs of intensity. These new findings could make it easier to search for new kinds of higher-temperature superconductors. Courtesy of the researchers.

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 to generate a standing wave of controlled wavelength in the film. However, 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 K (the threshold beyond which the superconductivity breaks down), the researchers discovered two new short-lived excitations — both caused by fluctuating CDWs.

Gedik's technique even allowed the researchers to record the lifetime of CDW fluctuations — just 2 ps under the coldest conditions and becoming briefer as the temperatures rose. These waves then vanished entirely at about 100 K, surviving at much higher temperatures than superconductivity.

The same signatures were sought 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 K, showed no CDW signatures at all," Gedik said.

CDW’s consistent emergence would have bolstered the conjecture that the waves 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 greatest problems in physics today," Bozovic said. "The source of this extraordinary phenomenon is slowly but surely running out of places to hide."

The study was published in Nature Materials (doi: 10.1038/nmat3571).  

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laser spectroscopy
That part of the science involved in the study of the theory and interpretation of spectra that uses the unique characteristics of the laser as an integral part in the development of information for analysis. Raman spectroscopy and emission spectroscopy are two areas where lasers are used.
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