It is possible to conduct electricity without loss using copper oxides, but there's a catch. The superconductivity only occurs near absolute zero. Such a cold temperature is impractical for broad use. However, a layered copper oxide conducts electrons along hidden rivers at higher temperatures. An experiment on such a sample, using a high-power pulse of infrared light, reveals a superconductivity throughout the material that's otherwise hidden.
This work offers insights into how electrons move in superconductors at warmer temperatures. It could help us design and manipulate the electronic properties of materials. The impact? These superconductors could improve the performance and reliability of our electrical grid. Also, this approach could let us discover other hidden exotic phases with unexpected properties.
In the 1980s, scientists discovered that copper-oxide compounds exhibit superconductivity at higher temperatures than traditional superconductors, which work only at temperatures near absolute zero. Understanding this high-temperature superconductivity can help scientists design materials that can transmit electricity at even higher temperatures without loss. In general, superconductivity is thought to be an isotropic property throughout a material. However, copper oxides are layered materials, and the superconducting behavior is limited by the weak electronic coupling between layers. This coupling can be probed by shining infrared light from a crystal surface, with the light polarization perpendicular to the layers, and measuring the fraction that is reflected.
One particular copper-oxide compound exhibits a special ordering of electronic charges in the form of regularly spaced stripes within the layers, with the stripe orientation rotating by 90 degrees from one layer to the next. The layers of charge stripes can exhibit the flow of electrical current without resistance as long as the current flows parallel to the layers. However, measurements of light reflectivity indicate that the superconducting coupling between layers cancels out, except at low temperatures. To test this system further, scientists at Brookhaven National Laboratory grew a high-quality crystal and sent it to collaborators at the Max Planck Institute in Hamburg. There, a very intense pulse of infrared light was reflected off the surface of the layered crystals, with the light polarized perpendicular to the copper-oxide layers. The intense light caused a surprising response at a frequency three times that of the incident light. Using theory, scientists determined that this response from the high-intensity light was evidence of hidden 3-D superconductivity between the layers that occurs in a range of surprisingly high temperatures.
Brookhaven National Laboratory
Department of Energy, Office of Science, Basic Energy Sciences (crystal growth) and the European Research Council (terahertz experiments and theory)
S. Rajasekaran, J. Okamoto, L. Mathey, M. Fechner, V. Thampy, G.D. Gu, and A. Cavalleri, "Probing optically silent superfluid stripes in cuprates." Science 359, 575 (2018). [DOI: 10.1126/science.aan3438]
Brookhaven National Laboratory news feature: Bringing a hidden superconducting state to light
Universität Hamburg release: Superconductors earn their stripes
Newswise article: Bringing a hidden superconducting state to light
Science Daily release: Bringing a hidden superconducting state to light
Phys.org news article: Superconductors earn their stripes