Anomalous enhancement of g-shift in κ-type BEDT-TTF compounds

Systematic investigation of the ESR properties of κ-type BEDT-TTF compounds has been carried out. The salts with crystal symmetry of P21, κ-Cu(NCS)2##Abbreviation used: κ-X = κ-(BEDT-TTF)2X CN)[N(CN)2] and κ-Ag(CN)2-H2O, were found to show an anomalous enhancement of g-shift below about 130K when the static field was applied along b axis. Concomitantly, abrupt broadening of the linewidth has been observed in all principal axes. On the other hand, κ-Cu[N(CN)2]Br (Pnma) and κ1-Cu2(CN)3 (P21/c) have almost temperature independent g-values for all field directions. We discuss the possible explanation of the anomalous enhancement of the compounds with P21 space group.