Relativistic nature of a magnetoelectric modulus of Cr2O3 crystals:: A four-dimensional pseudoscalar and its measurement

The 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 four-dimensional relativistic invariant pseudoscalar alpha. It is temperature dependent and of the order of similar to 10(-4)/Z(0), with Z(0) as vacuum impedance. We show that the new pseudoscalar is odd under parity transformation and odd under time inversion. Moreover, alpha 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.