On the problem of superdiamagnetism

The possibility of the existence of superdiamagnetism in non-superconducting substances is discussed. Such superdiamagnetic substances may have susceptibility χ ∼− (10−3-10−2) comparable with the ideal one χid = − 14π, whereas in ordinary diamagnetic materials |χ| ⩽ 10−4 ⪡|χidz.sfnc;. It is poined out that under equilibrium superdiamagnetism may occur in toroidal type substances, for which the order parameter T is a toroidal moment density. Toroidal moment is a polar vector which changes sign under time inversion. On a microscopic level toroidal current states arise in the model of excitonic insulator with an imaginary singlet order parameter. Conditions have been found under which superdiamagnetism can be expected in such systems. Anomalous susceptibility, which is probably observed in CuCl and CdS, is possibly of superdiamagnetic nature.