Terahertz light from superconducting strips

High-temperature superconducting cuprates emit THz radiation once their surface is illuminated with ultrashort optical pulses. This effect occurs only in compounds where superconductivity coexists with the order of charge strips. Credit: Jörg Harms, MPSD

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 irritate 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 electronic “streaks” in some copper-based compounds can lead to a break in symmetry. crystalline of the material, which persists even in their superconducting state. Their work was published in PNAS.

Focusing on a range of cuprates, the team investigated the coexistence and competition of their superconducting state with other quantum phases. Such interactions are believed 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, cultured and characterized at Brookhaven National Labs, to ultrashort pulses of laser light. They observed how the materials began to emit a particular type of terahertz (THz) light, a technique known as THz emission spectroscopy.

Usually, such emissions only occur in the presence of a magnetic field or a polarizing current. However, the MPSD team probed the cuprates without applying any external bias and found “anomalous” THz emission in some of them. Those compounds featured the so-called charge strip order, in which electrons arrange themselves in chain patterns rather than moving freely. The order of the charge strips appears to break the crystalline symmetry of the material, just as a magnetic field or applied current would, with this symmetry breaking persisting in the superconducting state.

“By carrying out experiments on various compounds,” says Daniele Nicoletti, the lead author of the article, “we were very surprised to find a clear, coherent and almost one-colored THz emission in some superconductors and, conversely, a total lack of We have been able to associate the characteristics of THz emission with reasonable certainty to the presence of the charge stripe order, a peculiar ordered phase found in various cuprate families, which is believed to take place role in the mechanism underlying high-temperature superconductivity to cause a symmetry breaking of 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 of this phenomenology. Starting from the observation that the coherent emission of THz occurs very close to the “Josephson plasma frequency”, which is the resonant tunneling frequency of the superconducting electron pairs through the copper-oxygen crystalline planes, the researchers identified the so-called “plasmons. Surface Josephson “as the source of the emissions. These are analogues of the sound waves that develop at the interface between the superconductor and the external environment. In principle these are “silent” modes, in the sense that they do not directly couple with light and therefore should not radiate. However, it is precisely the presence of the charge modulation introduced by the strip order that provides the necessary coupling with the outside world and allows these modes to be switched on.

The team’s work provides important new insights into the processes leading to high-temperature superconductivity. It also reveals a consistently anomalous THz emission as a sensitive tool for probing the symmetry of superconductors in the presence of other phases. The researchers believe it should be applied to a wider class of compounds in the future, opening up new possibilities for understanding the physics of complex interactions in these materials.


New information on unconventional superconductivity


More information:
D. Nicoletti et al, Coherent emission from surface Josephson plasmons in striped cuprates, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073 / pnas.2211670119

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Citation: Terahertz light from superconducting strips (2022, September 22) recovered on September 23, 2022 from https://phys.org/news/2022-09-terahertz-superconducting-stripes.html

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