Harmonic Decomposition, Irreducible Basis Tensors, and Minimal Representations of Material Tensors and Pseudotensors

We propose a general and efficient method to derive various minimal representations of material tensors or pseudotensors for crystals. By a minimal representation we mean one that pertains to a specific Cartesian coordinate system under which the number of independent components in the representation is the smallest possible. The proposed method is based on the harmonic and Cartan decompositions and, in particular, on the introduction of orthonormal irreducible basis tensors in the chosen harmonic decomposition. For crystals with non-trivial point group symmetry, we demonstrate by examples how deriving restrictions imposed by symmetry groups (e.g., \(C_{2}\), \(C_{s}\), \(C_{3}\), etc.) whose symmetry elements do not completely specify a coordinate system could possibly miss the minimal representations, and how the Cartan decomposition of SO(3)-invariant irreducible tensor spaces could lead to coordinate systems under which the representations are minimal. For triclinic materials, and for material tensors and pseudotensors which observe a sufficient condition given herein, we describe a procedure to obtain a coordinate system under which the explicit minimal representation has its number of independent components reduced by three as compared with the representation with respect to an arbitrary coordinate system.