This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Researchers at Friedrich Schiller University Jena and the University of Regensburg have developed a method for determining, using light, a quantum property of electrons that has hitherto been difficult to access. The focus is on what is known as Berry curvature—a quantity that describes certain geometric properties of the quantum mechanical states of electrons.
The researchers measured it in a layer of the semiconductor tungsten diselenide, just one atomic layer thick, using ultrashort laser pulses. The results have been published in the latest issue of the journal Nature Photonics. Electrons in a crystal do not simply move like tiny particles along fixed paths.
Their behavior is determined by quantum mechanical states. Researchers summarize the geometric properties of these states under the term "quantum geometry." An important parameter here is the Berry curvature. Among other things, it influences how electrons react to light or electric fields.
Until now, Berry curvature has mainly been calculated theoretically or investigated using comparatively complex experimental methods. The team from Jena and Regensburg is taking a different approach: it combines optical measurements with an analytical model and is thus able to quantitatively estimate the local Berry curvature of the material. "We wanted to make a quantum property accessible that has so far been difficult to determine experimentally," says lead author Nele Tornow from the Institute of Solid State Physics at the University of Jena.
"What makes our approach special is that we combine an optical experiment with a theoretical model to obtain a specific value for the Berry curvature." For the experiment, the researchers used tungsten diselenide, a semiconductor whose layer under investigation is less than one nanometer thick. Such atomically thin materials react particularly strongly to light and are therefore well suited to nonlinear optical experiments. The team directed two ultrashort laser pulses at the sample.
The first pulse alters the electronic states of the material for a tiny fraction of a second. The second serves to optically detect this change. This produces light at twice the frequency of the incident light.
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