Why do some materials carry electric currents without any resistance only when cooled to near absolute zero, while others do so at relatively high temperatures? This key question continues to trouble scientists studying the phenomenon of superconductivity. Now, a team of researchers from Andrea Cavalleri’s group at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg has provided evidence that electron “bands” in certain copper-based compounds can lead to a break in crystal symmetry the material. , which persist even in their superconducting state. Their work was published in PNAS.
Focusing on a series of cuprates, the team investigated the coexistence and competition of their superconducting state with other quantum phases. Such interactions are thought to be crucial for the development of high-temperature superconductivity—a process that remains one of the most important unsolved problems in condensed matter physics today.
The researchers exposed several cuprate crystals, grown and characterized at Brookhaven National Labs, to ultrashort pulses of laser light. They observed how the materials began to emit a specific type of terahertz (THz) light – a technique known as THz emission spectroscopy.
Usually, such emissions appear only in the presence of a magnetic field or a biasing current. However, the MPSD team tested the cuprates without applying any external bias and discovered “anomalous” THz emissions in some of them. These compounds exhibited so-called charge band order – where the electrons arrange themselves in chain patterns rather than moving freely. The charge band order appears to break the crystal symmetry of the material, just as an applied magnetic field or current would, this symmetry breaking persisting in the superconducting state.
“Performing experiments on various compounds,” says Daniele Nicoletti, lead author of the paper, “we were very surprised to find a clear coherent and almost monochromatic THz emission in some superconductors and, on the contrary, a complete lack of response in others. were able to associate the THz emission features with reasonable certainty to the presence of charge band order, a particular ordered phase found in various families of cuprates, which is believed to play a role in the mechanism underlying high-temperature superconductivity. to cause symmetry breaking in the superconductor, the presence of which had not been found by other experimental techniques in the past.”
In collaboration with physicists from Harvard University, ETH Zurich and the theoretical division of the MPSD, the team provided a detailed explanation for this phenomenology. Starting from the observation that coherent THz emission occurs very close to the “Josephson plasma frequency”, which is the tunneling resonance frequency of superconducting electron pairs along the copper-oxygen crystal planes, the researchers identified so-called “Josephson plasmons surface” as the emission source. These are analogs of sound waves that develop at the interface between the superconductor and the external environment. In principle, these are “silent” modes, i.e. they do not directly couple to light and are therefore not expected to radiate. However, it is precisely the presence of the charge modulation introduced by the band order that provides the necessary coupling to the outside world and enables these modes to ignite.
The team’s work provides important new insights into the processes leading to high-temperature superconductivity. It also reveals anomalous coherent THz emission as a sensitive tool to probe the symmetry of superconductors in the presence of other phases. The researchers believe that it should be applied to a wider class of compounds in the future, opening up new possibilities for understanding the physics of the complex interactions in these materials.
A New Insight into Unconventional Superconductivity
D. Nicoletti et al., Coherent emission from Josephson surface plasmons in striped cuprates, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2211670119
Provided by the Max Planck Society
Citation: Terahertz light from superconducting stripes (2022, September 22) Retrieved September 23, 2022, from https://phys.org/news/2022-09-terahertz-superconducting-stripes.html
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