Optical properties of a monoclinic insulator Cu(H2O)(2)(en)SO4

Cu(H2O)2(en)SO4 was recently investigated especially for its magnetic properties: it is an insulating (with a calculated gap of about 2 eV) magnetically quasi-onedimensional spin S = 1/2 antiferromagnet (with the Neel temperature of 0.9K) [1 3]. In this paper we utilize the theoretical electronic structure obtained in [3] to calculate the macroscopic dielectric tensor and compare it to what can be inferred from Mueller-ellipsometry measurements (of the PCRSCRA type) carried out in the 190 to 1700 nm range (6.5 0.7 eV). The material crystal belongs to the monoclinic system and thus is optically biaxial, i.e. displays anisotropy. Contrary to orthorhombic crystals (biaxial crystals of highest crystallographic symmetry), the crystallographic axes of a monoclinic crystal do not x all the axes of the dielectric frame, de ned as the frame where the real part of the (generally complex) dielectric tensor is diagonal. This makes the situation complex on the one hand, but on the other hand more experimental features, such as the orientation of the dielectric frame, can be checked against theory.