鈥淒ark鈥� electrons within solid materials may help us learn more about the behaviour of high-temperature superconductors, and perhaps solve other mysteries in material science.
Most of a material鈥檚 properties, such as how easily it conducts electricity or reflects light, are dictated by the motion of its electrons. One way of determining these properties is spectroscopy 鈥� shining a light on a material and analysing the spectrum of the light that bounces back in order to reveal which frequencies are absorbed or reflected.
But spectroscopy can鈥檛 tell the whole story. For some simple atoms and molecules, not all of the electrons show up in a spectrum, even though they still influence their physical properties. Researchers have found that these so-called 鈥渄ark states鈥� only occur when the electrons have distinctly different energies, allowing them to interfere and cancel out the signal from each other. But in a solid, where there are many different electrons that can鈥檛 be easily separated in this way, these dark states were thought not to exist.
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Now, at Yonsei University in South Korea and his colleagues have shown that isn鈥檛 the case by detecting these hidden electrons in much more complex materials, including a lead perovskite, which is a crystal used as a solar cell, and a bismuth and copper high-temperature superconductor. 鈥淭his is more than just identifying something undetectable, because dark states, although we cannot see them, are still there, which means that they affect the physics [of a material],鈥� says Kim.
Kim and his team first identified dark states in a crystal called palladium diselenide, which they chose because of its unique structure. The crystal has two repeating patterns of atoms, consisting of a palladium atom surrounded by four selenium atoms, which are slightly rotated from one another throughout the material. When they measured this crystal using spectroscopy, they found missing gaps in the resulting spectrum that weren鈥檛 predicted by standard theories, indicating the presence of dark states.
The team found that the two repeating patterns of atoms meant that the electrons from the different elements become separated enough that they can interfere. The researchers developed a model to predict the electron energies that included this effect, and then used it to predict that they should also show up in similarly structured materials, like a lead perovskite and bismuth and copper superconductor. When they then tested these materials with spectroscopy, they indeed found dark states.
These hidden electrons could help solve a long-standing debate in trying to understand why some materials appear to be superconductors, which allow electricity to flow with no resistance, at much higher temperatures than standard theory predicts. One possible solution has been mysterious missing electron energies found in the superconductors, but there was no obvious reason why. Now that they can be explained, says Kim, it will help us to develop better models that could eventually be used to find new superconducting materials.
Because the effect is quite general, it could be affecting a wide range of material properties that researchers have missed before, says at the Diamond Light Source in Oxfordshire, UK. Finding missing energy levels is relatively common, but it is much harder to explain why, says Cacho 鈥� Kim and his team鈥檚 explanation of interfering lattices could help plug this gap.
Nature Physics
