Molecular theory of the constraint-breaking pseudoscalar (axion) in antiferromagnetic Cr2O3

•A theory of the contribution of induced molecular moments to the magnetoelectric tensors of Cr2O3.•These contain a constraint-breaking pseudoscalar X.•X is one-third the trace of the time-odd electric dipole–magnetic dipole polarizability density.•Invariance of observables with respect to molecular coordinate origin is discussed.

We present a semi-classical theory of the contributions of induced molecular moments to the magnetoelectric tensors for Cr2O3. These contain a pseudoscalar (axion) that was extracted by Hehl et al. (2008) [3] from measurements on antiferromagnetic Cr2O3 by Wiegelmann et al. (1994) [7]. This constraint-breaking quantity equals 13G¯kk, where G¯kk is the trace of the time-odd electric dipole–magnetic dipole polarizability density, evaluated relative to crystallographic axes. The calculations employ a new, direct derivation of the response fields D and H; and they show that previous treatments based on transformations overlooked the role of G¯kk (where permitted).