Dzyaloshinsky-Moriya interaction in theS=12quasi-one-dimensional antiferromagnetCu2Cl4⋅H8C4SO2as determined via high-frequency ESR

Abstract

High-frequency ESR measurements have been performed on the $S=\frac{1}{2}$ quasi-one-dimensional antiferromagnet ${\text{Cu}}_{2}{\text{Cl}}_{4}\ensuremath{\cdot}{\text{H}}_{8}{\text{C}}_{4}{\text{SO}}_{2}$ in the frequency range up to 360 GHz and the field range up to 12 T. Reflecting the small alternation of exchange interaction along the chain direction, very peculiar temperature dependences of $g$ value and linewidth are observed. Moreover, we have observed the direct transition between the singlet ground state and triplet excited states in ${\text{Cu}}_{2}{\text{Cl}}_{4}\ensuremath{\cdot}{\text{H}}_{8}{\text{C}}_{4}{\text{SO}}_{2}$ at $T=1.7\text{ }\text{K}$ and determined the excitation gap (for $c$ axis) $\ensuremath{\Delta}/{k}_{\text{B}}=7.9\ifmmode\pm\else\textpm\fi{}0.4\text{ }\text{K}$. Considering the observed anisotropic nature of the direct transition and applying the selection rules of the direct transition proposed by Sakai [J. Phys. Soc. Jpn. 72, Suppl. B 53 (2003)], we have come to the conclusion that the direct transition is caused by the Dzyaloshinsky-Moriya interaction and are able to determine the $D$ vector in ${\text{Cu}}_{2}{\text{Cl}}_{4}\ensuremath{\cdot}{\text{H}}_{8}{\text{C}}_{4}{\text{SO}}_{2}$ as ${D}_{{a}^{\ensuremath{\ast}}}:{D}_{b}:{D}_{c}=1.0:0.0:2.3\ifmmode\pm\else\textpm\fi{}0.1$.