Researchers have been trying to identify the mechanisms underlying high-temperature superconductivity (HTS) in cuprate superconductors, typically attributed to charge-density waves (CDWs). Researchers at the Massachusetts Institute of Technology (MIT) and Brookhaven National Laboratory (BNL) have revealed that CDWs cannot be the root cause of the unparalleled power conveyance in cuprate HTS materials. In fact, CDW formation is likely an independent and competing instability according to the researchers, as reported in the February 24 online edition of Nature Materials (DOI: 10.1038/NMAT3571).

In describing charge-density waves, co-author Ivan Božovíc of BNL 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, but even then, the technique works only 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.

For their experiment, the researchers grew thin films of La_1.9Sr_0.1CuO₄, a HTS cuprate compound. The metallic cuprates, assembled one atomic layer at a time, are separated by insulating planes of lanthanum and strontium oxides, resulting in a quasi-two-dimensional conductor. When cooled down to less than 100 K, electron waves began to ripple through the 2D matrix. At even lower temperatures, these films became superconducting.

To catch CDWs in action, the research group at MIT, led by Nuh Gedik, used an advanced ultrafast pump-probe spectroscopy technique. Intense laser pulses, “pumps,” cause excitations in the superconducting films, which are then probed by measuring the film reflectance with a second light pulse. The second pulse is delayed by precise time intervals, and the series of measurements allows the lifetime of the excitation to be determined. In a more sophisticated variant of the technique, the researchers replaced the standard single pump beam by two beams hitting the surface from different sides simultaneously. This generated a standing wave of controlled wavelength in the film, but it disappeared rapidly as the electrons relaxed back into their original state.

The researchers applied this technique to the La_1.9Sr_0.1CuO₄ films synthesized at BNL. In films with a critical temperature of 26 K, the researchers discovered two new short-lived excitations—both caused by fluctuating CDWs. The new pulse-pump technique allowed the researchers to record the lifetime of CDW fluctuations, which is just 2 ps under the coldest conditions, becoming briefer as the temperatures rose. These waves then vanished entirely at about 100 K, actually surviving at much higher temperatures than superconductivity.

The researchers then hunted for those same signatures in cuprate films with slightly different chemical compositions and a greater density of mobile electrons. “Interestingly, the superconducting sample with the highest critical temperature, about 39 K, 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 HTS. Instead, the research team’s detection of such electron waves in one sample but not in another (with even higher critical temperature) demonstrates that another mechanism may be driving the emergence of HTS in cuprate superconductors.