Visualizing Quantum States in 2D Materials

Left, ultrafast light pulses excite and probe a tiny sample of WS2 one layer of atoms thick, emitting electrons that are collected by a new detector called a momentum microscope. Right, full 3-D energy-momentum distribution of the emitted electrons. Credit: Stony Brook University

When certain semiconductors absorb light, they can form excitons, which are particle pairs consisting of an electron bound to an electron hole. Two-dimensional tungsten disulfide (WS2) crystals have unique exciton states not found in other materials. However, these states are short-lived and can rapidly transition between each other.

Researchers have developed a novel approach to capture separate images of these individual quantum states. By tracking these states, scientists have discovered that the mechanisms responsible for their mixing may not align completely with current theoretical models.

The scientific community is fascinated by transition metal dichalcogenides, a family of crystals that includes tungsten disulfide, due to their strong light absorption despite being only a few atoms thick. These crystals could be used in the development of nanoscale solar cells and electronic sensors. Using a cutting-edge technique called time-resolved momentum microscopy, researchers can now better monitor the transitions between different exciton quantum states. This versatile technique can also be applied to other next-generation materials and devices for further insight into their functionality.

Under different conditions, various light-induced exciton states can form in monolayer transition metal dichalcogenides (TMDs) like WS2. Modulating the wavelength or power of the incident light, as well as the crystal’s temperature, enables the formation and persistence of different exciton states. By using circularly polarized light, where the electric field’s direction rotates around the direction of the light wave, excitons with specific quantum spin configurations can be selectively created within a particular range of energy bands.

Scientists at Stony Brook University have developed a unique instrument to directly visualize this phenomenon under different ultrafast light excitation conditions and disentangle the complex mixture of quantum states that can emerge.

Published in Physical Review Letters, these groundbreaking findings highlight how the binding force between an electron and electron hole in an exciton contributes to rapid coupling or mixing of different exciton states. The researchers demonstrated that this coupling effect mixes excitons with varying spin configurations while also conserving both energy and momentum.

Surprisingly, the results showed that the rate of exciton mixing did not rely on the exciton energies as previously predicted by researchers. This study provides crucial experimental support for current theories of exciton coupling in TMDs, while also shedding light on important discrepancies. Understanding the interplay between these exciton states is a critical step towards harnessing the potential of TMDs for nanotechnology and quantum sensing.

More information:
Alice Kunin et al, Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS2, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.046202

Provided by
US Department of Energy


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Directly imaging quantum states in two-dimensional materials (2023, June 20)
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