Singlet--Triplet Excitations and High-Field Magnetization in CuTe2O5

By measuring the THz electron spin resonance (ESR) transmission spectra and high-field magnetization on the spin-gapped system CuTe2O5, we identified the singlet--triplet excitations in the dimerized non-magnetic ground state. The determined spin-gap value of h\nu_{0}=4.94 meV at the \Gamma point (\mathbf{Q}\simeq\mathbf{0}) is significantly smaller than the strongest antiferromagnetic exchange interaction between the Cu ions predicted by theoretical investigations. We also observed the critical field H_{\text{c1}}^{a^{*}}=37.6 T for \mathbf{H}\perp\text{bc-plane} and H_{\text{c1}}^{bc}=40.6 T for \mathbf{H}\parallel\text{bc-plane} at the onset of non-zero magnetization, consistent with the gap value and corresponding anisotropic g-factors determined previously. The observed singlet--triplet excitations in Faraday and Voigt configurations suggest a mixing of the singlet state with the S_{z}=0 triplet state and the S_{z}=\pm 1 triplet states, respectively, due to the Dzyaloshinskii--Moriya (DM) interaction with a DM vector perpendicular to the crystalline bc-plane.