Anisotropic effects in two-dimensional materials

Among a huge variety of known two-dimensional (2D) materials, some of them have anisotropic crystal structures; examples include different systems such as a few-layer black phosphorus (phosphorene), beryllium nitride BeN₄, the van der Waals magnet CrSBr, and rhenium dichalcogenides ReX₂. As a consequence, their optical and electronic properties are highly anisotropic as well. In some cases, the anisotropy results in not only smooth renormalization of observable properties in comparison with the isotropic case, but in the appearance of dramatically new physics. The examples are hyperbolic plasmons and excitons, strongly anisotropic ordering of adatoms at the surface of 2D or van der Waals materials, and essential changes in transport and superconducting properties. Here, we present a systematic review of the electronic structure, transport, and optical properties of several representative groups of anisotropic 2D materials, including semiconductors, anisotropic Dirac and semi-Dirac materials, and superconductors.