One of the great successes of 20th-century physics was the quantum mechanical description of solids. This allowed scientists to understand for the first time how and why certain materials conduct electric current and how these properties could be purposefully modified. For instance, semiconductors such as silicon could be used to produce transistors, which revolutionized electronics and made modern computers possible.
To be able to mathematically capture the complex interplay between electrons and atomic nuclei and their motions in a solid, physicists had to make some simplifications. They assumed, for example, that the light electrons in an atom follow the motion of the much heavier atomic nuclei in a crystal lattice without any delay. For several decades, this Born-Oppenheimer approximation worked well.
Now, however, researchers at ETH Zurich and the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg have shown that the electrons in certain materials respond with a delay. Moreover, this delay depends on where the electrons are localized and which energy state they occupy.
Using experiments with attosecond resolution and theoretical calculations, Ursula Keller and Lukas Gallmann at the Department of Physics at ETH could prove that electrons in flat layered materials, so-called MXenes, respond to the motion of atomic nuclei with an appreciable delay.
The researchers published their results in the journal Science. These results could help to develop novel optoelectronic devices in the future.
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