  Low-symmetry 2D materials---such as ReS$_2$ and ReSe$_2$ monolayers, black
phosphorus monolayers, group-IV monochalcogenide monolayers, borophene, among
others---have more complex atomistic structures than the honeycomb lattices of
graphene, hexagonal boron nitride, and transition metal dichalcogenides. The
reduced symmetries of these emerging materials give rise to inhomogeneous
electron, optical, valley, and spin responses, as well as entirely new
properties such as ferroelasticity, ferroelectricity, magnetism, spin-wave
phenomena, large nonlinear optical properties, photogalvanic effects, and
superconductivity. Novel electronic topological properties, nonlinear elastic
properties, and structural phase transformations can also take place due to low
symmetry. The "Beyond Graphene: Low-Symmetry and Anisotropic 2D Materials"
Special Topic was assembled to highlight recent experimental and theoretical
research on these emerging materials.
