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Anisotropic spin-Peierls state in the inorganic compound CuGeO3

A static susceptibility and susceptibility anisotropy of a single-crystalline CuGeO$_3$ were measured. Combined results of both techniques reveal the following experimental facts: (i) The low temperature susceptibility upturn obeys a power law temperature dependence of the \mth{; ; A \ ; T^{; ; -\alpha}; ; }; ; form with an anisotropic exponent $\alpha$. This is, to our knowledge, the first observation of the anisotropic exponent $\alpha$. (ii) In the paramagnetic region above the spin--Peierls transition temperature the measurements reveal the anisotropic and temperature--dependent exchange interaction, $J$. (iii) The anisotropic exchange interaction has resulted in the anisotropy of the excitation energy gap $\Delta$. (iv) The paramagnetic defects causing a low temperature susceptibility upturn are Cu$^{; ; 2+}; ; $ ions, but of different oxygen surroundings than the rest of the Cu$^{; ; 2+}; ; $ moments.

Magnetic anisotropy; Magnetic impurity interactions; Exchange and superexchange interactions

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