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Physicists witnessed the turning of a dielectric into a conductor

March 27th, 2018
MSU-based physicists witnessed the turning of a dielectric into a conductor
The chart shows the probability of finding out two electrons on a site as a function of time (horizontal axis) and the electron interaction strength (vertical axis). For a moderate interaction, a powerful laser field switches the system from the Mott state (blue area) to the conducting state (yellow area). Credit: ArXiv.org

A scientist from the Faculty of Physics, MSU together with Russian and foreign colleagues studied changes in the behavior of electrons in one of the types of dielectrics with high time resolution and witnessed how the material turned into a conductor under the influence of ultra-short laser impulses. The method may be used to study high-speed processes. The theoretical study was published in Nature Photonics journal.

The authors considered a Mott dielectric formed due to strong electron interaction. The many-body nature of the system complicates its modeling and prevented optical-induced processes in the past. In this work, the physicists used the high harmonics spectroscopy research method. The material was subjected to ultra-short femtosecond-scale laser pulses with known characteristic. When a beam is reflected from the material, a part of the photons acquires 10x and more times higher energy and oscillation frequency compared to the photons of the initial pulse. This is called the generation of high optic harmonics. Changes in beam characteristics provide information about the properties of the material.

"Experiments started about 20 years ago and at that time mainly covered the influence of light on atoms and molecules. In the recent five years scientists have turned their attention to solid state systems, crystals. An important property of crystals is that electrons interact with each other. However, no one has ever described such many-body effects in generation of high harmonics from a solid state. Our theoretical article is the first attempt to study how many-body physics manifests itself in the generation of ultrahigh optical harmonics," said Alexey Rubtsov, a co-author of the article and a Professor of the Faculty of Physics, MSU.

The physicists studied changes in the properties of a Mott dielectric under the influence of laser pulses. Its dielectric state occurs because the energy of electron interaction (repulsion of negatively charged particles) is higher than their average kinetic energy. As a result, electrons cannot move freely within the material, and it fails to conduct electricity.

When a beam hits the surface of a Mott dielectric, the many-body state of electrons changes under the influence of a strong AC field. The kinetic energy of the electrons increases, and the material loses its dielectric properties. This process was studied using high harmonics spectroscopy.

According to the authors of the work, further studies depend on experimental results. "As soon as anything similar is discovered, we'll understand in which direction to move, especially in the description of particular experimental results," added Alexey Rubtsov.

More information:
R. E. F. Silva et al, High-harmonic spectroscopy of ultrafast many-body dynamics in strongly correlated systems, Nature Photonics (2018). DOI: 10.1038/s41566-018-0129-0

Provided by Lomonosov Moscow State University

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