Relativistic analysis of magnetoelectric crystals: Extracting a new 4-dimensional P odd and T odd pseudoscalar from Cr2O3 data

Earlier, the linear magnetoelectric effect of chromium sesquioxide Cr2O3 has been determined experimentally as a function of temperature. One measures the electric field-induced magnetization on Cr2O3 crystals or the magnetic field-induced polarization. From the magnetoelectric moduli of Cr2O3 we extract a 4-dimensional relativistic invariant pseudoscalar α˜. It is temperature dependent and of the order of ∼10−4Y0∼10−4Y0, with Y0Y0 as vacuum admittance. We show that the new pseudoscalar α˜ is odd under parity transformation and odd under time inversion. Moreover, α˜ is for Cr2O3 what Tellegen's gyrator is for two port theory, the axion field for axion electrodynamics, and the PEMC (perfect electromagnetic conductor) for electrical engineering.