Light-Induced Polaronic Crystals in Single-Layer Transition Metal Dichalcogenides

Light-induced ordered states can emerge in materials
after irradiation
with ultrafast laser pulses. However, their prediction is challenging
because the inverted band occupation confounds our chemical intuition.
Hence, we use a recently developed constrained density functional
perturbation theory approach to systematically screen single-layer
transition metal dichalcogenides (TMDs) for light-induced ordered
states. We demonstrate that all examined single-layer TMDs reveal
similar light-induced charge orderings. The corresponding reconstructions
are periodic arrangements of polarons (polaronic crystals), characterized
by triangular metal clusters and having no equivalent at equilibrium
conditions. The polarons are accompanied by localized midgap states
in the electronic band structure, detectable by experimental methods.
We assess the selenides as the most promising candidates for potential
photoexcitation experiments because they transition at low critical
fluences, have low transition barriers, and maintain an open band
gap under photoexcitation. Our work paves the way for innovative material
design approaches targeting light-induced phases.