η η μ μ ν ν is the Minkowski metric with signature (+ - - -) and I d is the d × × d -dimensional unit matrix. In all equations, implicit summation over a and μ μ is used (Einstein convention). Furthermore, Ψ Ψ is the wavefunction. The unifying feature of all Dirac matter is the matrix structure of the equation describing the quasi-particle excitations.