麻豆传媒

News — UPTON, NY 鈥� Identifying the mysterious mechanism underlying high-temperature superconductivity (HTS) remains one of the most important and tantalizing puzzles in physics. This remarkable phenomenon allows electric current to pass with perfect efficiency through materials chilled to subzero temperatures, and it may play 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鈥攆leeting fluctuations called charge-density waves (CDWs)鈥攃ould help solve the mystery and pave the way for rapid technological advances.

Now, researchers at the Massachusetts Institute of Technology and the U.S. Department of Energy鈥檚 Brookhaven National Laboratory have combined two state-of-the-art experimental techniques to study those electron waves with unprecedented precision in two-dimensional, custom-grown materials. The , published online February 24, 2013, in the journal Nature Materials, reveal 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.

鈥淚t has been difficult to determine whether or not dynamic or fluctuating CDWs even exist in HTS materials, much less identify their role,鈥� said Brookhaven Lab physicist and study coauthor Ivan Bozovic. 鈥淒o 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.鈥�

Custom-grown Superconductors

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鈥攁ssuming the synthesis process is precise.

For this experiment, Bozovic used a custom-built molecular beam epitaxy system at Brookhaven Lab 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鈥檚 called a quasi-two-dimensional conductor. When cooled down to a low enough temperature鈥攍ess than 100 degrees Kelvin鈥攕trange electron waves began to ripple through that 2D matrix. At even lower temperatures, these films became superconducting.

Electron Sea

鈥淚n quasi-two-dimensional metals, low temperatures frequently bring about interesting collective states called charge-density waves,鈥� Bozovic said. 鈥淭hey 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.鈥�

Once a CDW forms, the electron density loses uniformity as the ripples rise and fall. These waves can be described by familiar parameters: amplitude (height of the waves), wavelength (distance between waves), and phase (the wave鈥檚 position on the material). Detecting CDWs typically requires high-intensity x-rays, such as those provided by synchrotron light sources like Brookhaven鈥檚 NSLS and, soon, NSLS-II. And even then, the technique only works if the waves are essentially frozen upon formation. However, if CDWs actually fluctuate rapidly, they may escape detection by x-ray diffraction, which typically requires a long exposure time that blurs fast motion.

Measuring Rolling Waves

To catch CDWs in action, a research group at MIT led by physicist Nuh Gedik used an advanced ultrafast spectroscopy technique. Intense laser pulses called 鈥減umps鈥� cause excitations in the superconducting films, which are then probed by measuring the film reflectance with a second light pulse鈥攖his is called a pump-probe process. The second pulse is delayed by precise time intervals, and the series of measurements allow the lifetime of the excitation to be determined.

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 excitations鈥攂oth caused by fluctuating CDWs.

Gedik鈥檚 technique even allowed the researchers to record the lifetime of CDW fluctuations鈥攋ust 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.

鈥淚nterestingly, the superconducting sample with the highest critical temperature, about 39 Kelvin, showed no CDW signatures at all,鈥� 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鈥檚 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.

鈥淩esults like this bring us closer to understanding the mystery of HTS, considered by many to be one of the greatest problems in physics today,鈥� Bozovic said. 鈥淭he source of this extraordinary phenomenon is slowly but surely running out of places to hide.鈥�

Additional collaborators on this research include Darrius Torchinsky and Fahad Mahmood of the Massachusetts Institute of Technology, and Anthony Bollinger of Brookhaven National Lab.

The work was funded by the National Science Foundation and DOE鈥檚 Office of Science.

DOE鈥檚 Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit .