Band unfolding with a general transformation matrix: from code implementation to interpretation of photoemission spectra

Unfolding of a supercell band structure into a primitive Brillouin zone is important for understanding implications of structural distortions, disorder, defects, solid solutions on materials electronic structure. Necessity of the band unfolding is also recognised in interpretation of angle-resolved photoemission spectroscopy (ARPES) measurements. We describe an extension of the fold2Bloch package by implementing an arbitrary transformation matrix used to establish a relation between primitive cell and supercell. This development allows us to overcome limitations of supercells constructed exclusively by scaling of primitive cell lattice vectors. It becomes possible to transform between primitive and conventional cells as well as include rotations. The fold2Bloch is publicaly available from a GitHub repository as a FORTRAN code. It interfaces with the all-electron full-potential WIEN2k and the pseudopotential VASP density functional theory packages. The fold2Bloch is supplemented by additional pre- and post-processing utilities that aid in generating k points in the supercell (such that they later fall onto a desired path in the primitive Brillouin zone after unfolding) and plotting the unfolded band structure. We selected Sr$_2$IrO$_4$ as an illustrative example and, for the first time, present its properly unfolded band structure in direct comparison with ARPES measurements. In addition, critical importance of the band unfolding for interpretation of SrIrO$_3$ ARPES data is illustrated and discussed as a perspective.