Scientists from ETH Zurich and the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg have observed that electrons in flat layered materials known as MXenes exhibit a measurable delay in response to the motion of atomic nuclei. This discovery challenges the long-standing Born-Oppenheimer approximation, a foundational concept in quantum mechanics that posits electrons instantaneously follow atomic nuclei. The findings were published in the journal Science.
Background on the Born-Oppenheimer Approximation
The 20th-century quantum mechanical description of solids provided a framework for understanding electrical conductivity in materials and how these properties could be modified. This understanding was crucial for the development of semiconductors, such as silicon, which led to the creation of transistors and advanced modern electronics. To simplify the complex mathematical interactions between electrons and atomic nuclei within solids, physicists commonly employed the Born-Oppenheimer approximation. This approximation assumed that the lighter electrons in an atom track the motion of the much heavier atomic nuclei in a crystal lattice without any delay. This simplification has been widely utilized and considered effective for several decades.
Observation of Electron Delay in MXenes
Recent research, however, indicates that electrons in specific materials exhibit a delayed response to atomic motion. This delay was observed to be dependent on the electrons' localization and energy state. Ursula Keller and Lukas Gallmann at ETH Zurich's Department of Physics, along with their colleagues, utilized attosecond resolution experiments and theoretical calculations to confirm this phenomenon.
The electron motion delay was identified by Sergej Neb, a postdoc in Keller's group and the lead author of the paper, during studies of phonons, which are lattice vibrations in MXenes. The MXene material investigated, a two-dimensional material structurally similar to graphene, consisted of multiple layers of titanium, carbon, and oxygen atoms. This material was produced by colleagues at ETH Zurich's Department of Mechanical and Process Engineering.
Experimental Methodology
The research employed attosecond spectroscopy, a technique developed over the past three decades by ETH researchers, capable of resolving physical events at a resolution of 10^-18 seconds. The experimental procedure involved several steps:
- A short infrared laser pulse was used to excite lattice vibrations within the MXene material.
- Subsequently, an attosecond laser pulse in the extreme ultraviolet range irradiated the material.
- The amount of light transmitted through the material was then measured. The wavelength of these pulses allowed for the excitation of electrons to higher energy levels through photon absorption.
- The experiment was repeated without the initial excitation of lattice vibrations.
- The differences between the two sets of results provided data on the motion of electrons and atomic nuclei.
Quantitative Findings and Analysis
By precisely adjusting the time separation between the two laser pulses, ranging from femtoseconds (10^-15 seconds) to picoseconds (10^-12 seconds), the researchers accurately determined the delay in electron response to the lattice vibrations. Observations indicated that electrons lagged behind the atomic nuclei by up to 30 femtoseconds.
This experimental data was compared with a mathematical model developed by their colleagues in Hamburg. The comparison suggested that vibrations of atomic nuclei influence the spatial distribution of electrons, which in turn modifies the electromagnetic field in the vicinity of the atoms in the lattice. Interactions between electrons were also identified as a contributing factor. The attosecond technology facilitated the observation of electron dynamics at the level of individual atoms, allowing for differentiation of behavior based on atom type, bonds, and energy states.
Future Implications
These insights into the interplay between electrons and lattice vibrations are expected to inform the development of more precise mathematical models that extend beyond current approximations. The methodology developed facilitates the measurement of electron-lattice vibration coupling strength, which could predict conditions under which specific electrons contribute to heat conduction. An enhanced understanding of energy and charge transport may lead to improved control over materials, offering new possibilities for nano-scale opto-electronic devices and contributing to the development of more efficient electronic components through microscopic insights into atomic-level heat conduction.