Laser pulses create exotic orders in quantum matter


July 07, 2023

(Nanowerk News) Water flows, ice is rigid – this clear distinction between liquid and solid is part of our everyday experience. This follows from the highly ordered arrangement of atoms and molecules in a crystalline solid, which is lost as it melts. Less clear, however, is the structure of “liquid crystals” – the very interesting state of affairs that combines order and disorder in such a way that important applications such as LCD (liquid crystal display) can be made.

Researchers from the Max Planck Institute (MPI) for Multidisciplinary Sciences in Göttingen, in collaboration with colleagues from the University of Kiel (CAU), have now managed to create a state in a crystalline material which – similar to the structure of a liquid crystal – can be described as not being clearly liquid. or crystal clear. The crystal is plated on the sample carrier made of gold The crystal is plated on the sample carrier made of gold. The material studied consists of thin layers of tantalum and sulfur atoms arranged in loose associations. The image displays an area of ​​approximately 300µm x 200µm. (Image: Till Domröse=, Max Planck Institute for Multidisciplinary Sciences)

The layered crystals studied, developed by Kai Rossnagel’s team in Kiel, are characterized by minimal distortion of the crystal structure at room temperature. This is due to the special structure of the crystal, in which thin layers of metal and sulfur atoms are stacked on top of each other and are only weakly bonded. If this layer is now bombarded with ultra-short laser flashes, the distortions change their orientation in a billionth of a second, suddenly increasing the electrical conductivity of the material. Although both types of distortion have an ordered structure and related crystalline properties, highly disordered states can be observed during the transitions.

“After attracting matter with light, the atoms in the crystal structure have yet to find a new, slightly different position. This shifts the material into a disordered, so-called hexatic state,” said Till Domröse, PhD student at MPI and first author of the study now published in the journal. Natural Ingredients (“Light-induced hexactic states in layered quantum materials”). “The reverse situation is mainly observed in liquid crystals. However, in our experiments, it is very unstable and has disappeared after a fraction of a nanosecond.”

Making the hexatic state visible places high demands on the measurement technology used. On the one hand, for example, it requires very fast temporal resolution to take fairly short snapshots. On the other hand, structural changes in materials are so subtle that they can only be seen with a very high sensitivity to atomic positions. Electron microscopy provides the necessary spatial resolution in principle, but is usually not fast enough.

In recent years, the Göttingen team led by Director Max Planck Claus Ropers has closed this gap by developing an “ultrafast” electron microscope capable of imaging even the unimaginably fast processes in the nanocosm. “This microscope was also used in this experiment and allowed us to capture the unusually ordered phases and their temporal evolution in a series of images,” explained Ropers. “At the same time, we developed a new high-resolution diffraction mode that is important for studying many other functional nanostructures.”


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