Anomalous electron-spin-resonance angular intensity variation in the two-chain compound N-ethyl-N-ethyl morpholinium tetracyanoquinodimethane [DEM(TCNQ)2]

The spin susceptibility and electron-spin-resonance (ESR) linewidths of the two inequivalent TCNQ sheets in DEM${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$ (N-ethyl-N-ethyl morpholinium tetracyanoquinodimethane) have been measured as a function of temperature and angle. The chains on one sheet undergo a spin-Peierls transition at 23 K. The other sheet remains paramagnetic down to 1.5 K. An anomalously large angular dependence of the ESR intensity for both sheets is observed. The intensity variation is such that the total susceptibility is isotropic. The $g$ factors behave in a way typical for TCNQ with a small anisotropy. This behavior of the ESR intensity indicates either a large anisotropy of the subchain susceptibilities or a resonance involving a coupled mode in the paramagnetic region. The nature of such a coupled mode has to date eluded explanation.