Superconductors are special materials that allow electricity to flow without any resistance, making them essential for advanced technologies such as power transmission, energy storage, magnetic levitation, and quantum computing.
Until recently, this remarkable behavior was only observed at extremely low temperatures, far below what we experience in daily life. That changed with the discovery of superconductivity in hydrogen-rich compounds like hydrogen sulfide (H3S), which becomes superconductive at 203 Kelvin (-70 °C), and lanthanum decahydride (LaH10), which becomes superconductive at 250 Kelvin (-23 °C).
These findings represented a major step toward realizing superconductivity at or near room temperature. Because these materials operate at temperatures far above the boiling point of liquid nitrogen, they are often classified as high temperature superconductors.
At the heart of this phenomenon is the superconducting gap, a crucial feature that reveals how electrons pair together to create the superconducting state. Identifying this gap allows scientists to distinguish superconductors from ordinary metals.
However, studying this gap in hydrogen-rich compounds such as H3S has proven to be a significant challenge. These materials can only be created in situ under immense pressures, over a million times greater than atmospheric pressure, which makes traditional measurement techniques like scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy impossible to use.
To overcome this barrier, researchers at the Max Planck Institute in Mainz developed a planar electron tunneling spectroscopy capable of operating under such extreme conditions. This achievement has enabled them to probe the superconducting gap in H3S for the first time, offering direct insight into the superconducting state of hydrogen-rich compounds.
Using this technique, the researchers discovered that H3S exhibits a fully open superconducting gap with a value of approximately 60 millielectronvolt (meV), while its deuterium analog, D3S, shows a gap of about 44 meV. Deuterium is a hydrogen isotope and has one more neutron. The fact that the gap in D3S is smaller than in H3S confirms that the interaction of electrons with phonons – quantized vibrations of the atomic lattice of a material – causes the superconducting mechanism of H3S, supporting long-standing theoretical predictions.
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