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Researchers from Molecular Foundry at the Lawrence Livermore National Laboratory Lawrence Berkeley found that low-temperature quantum phases in crystal lattices make the quasiparticle excitons move even under conditions in which any movement was expected to be impossible.
This discovery adds to fundamental knowledge in the field of materials science and can help improve the stability of quantum technologies, including the use of excitons as qubits. If you stack and rotate two images with the same shapes, such as squares or triangles, you’ll create a moiré pattern. It will be larger a wavy image that appears to create ripples on the surface. This optical effect is achieved by simple repetition and alignment.
A similar effect occurs in nanostructures when researchers impose on top of each other ultra-thin layers of semiconductors called transition metal dichalcogenides, no thicker than an atom. This overlay creates what scientists call a moiré potential. These are repeating energy regions with bulges and depressions between layers of materials. Such patterns can cause unusual electronic and optical behavior of quantum particles.
Until recently, scientists believed that these moiré potentials could not move. However, researchers from the Molecular Foundry has discovered that transition metal dichalcogenides overlapping layers of dichalcogenides move even at extremely low temperatures.
This discovery is promising because managing moiré potentials can help mitigate decoherence in qubits and sensors. Decoherence occurs when the quantum state and information about it is lost due to interference.
Excitation of the layers of these ultrathin materials with a green pulsed laser causes the electrons to enter an excited state. The electrons leave behind a free space with a positive charge. This creates an electron-free space — exciton pair.
Typically, excitons are formed in single-layer materials. However, in two-layer systems, excitons are separated. The electrons move into the tungsten disulfide layer, while the positively charged free sites remain in the tungsten diselenide layer.
Scientists call these special excitons that jump through the layers «interlayer excitons» or IX. According to the leader of the study, Molecular Foundry researcher Antonio Rossi, one would expect these moiré areas to act as traps for the excitons that get trapped there, cannot get out. However, the researchers noticed that these excitons oscillated in the moiré structures despite being blocked in them.
«It takes very little energy to get this moiré potential moving, so the moiré moves just like a stormy sea. We have shown that even at very low temperatures, energy and information are not localized as one might expect There are different ways to transport energy and information at different temperatures. This is a new way to do it», — the researchers explain.
To enable the observation of excitons in motion, Johannes Lieschner and Indrajit Maithi from Imperial College London used simulations to obtain snapshots of the moiré potential. Based on the results of their observations, the researchers came to a single conclusion: the moiré potential itself must be moving.
Scientists believe that low-temperature quasi-particles called phases allow interlayer excitons to move even when they are blocked. Phases are energy quantum in the middle of the crystal lattice, which has its own momentum and position, and generally behaves like a particle.
Antonio Rossi and his colleagues found that the motion of interlayer excitons in the moiré potential depends on the angle and temperature. In particular, they perform maximum motion when the layers of transition metal dichalcogenides are parallel. When the temperature of the system becomes close to zero, the movement of interlayer excitons gradually also approaches zero, but does not stop completely.
The results of the study were published in the journal ACS Nano
Source: SciTechDaily
