Terahertz and Infrared Spectroscopy on Low-Dimensional Quantum Magnets

The field of low-dimensional magnetism is one of the most active research areas in condensed matter physics. Low dimensional quantum magnets provide unique possibilities to study ground and excited states of quantum models, to explore new phases of matter, and to investigate the interplay of quantum and thermal fluctuations. This dissertation focuses on the optical properties of quantum spin dimer and spin chain systems. The spectroscopic techniques, terahertz time-domain spectroscopy, submillimeter spectroscopy, and Fourier transform infrared spectroscopy covering the spectral range from 0.2 meV to 2 eV with and without external magnetic field up to 30 Tesla are used to study the typical spin-dimer systems Sr3Cr2O8, Ba3Cr2O8, and CuTe2O5, and the spin-chain systems SrNi2V2O8 and SrCo2V2O8. Spin singlet-triplet excitations with total spin one are observed in spin-1/2 dimerized antiferromagnets Sr3Cr2O8, Ba3Cr2O8, and CuTe2O5, indicating the existence of Dzyaloshinskii-Moriya interactions in these systems. Polarization and symmetry analysis reveals further that the dynamical Dzyaloshinskii-Moriya interactions with spin-phonon coupling involved are important in Sr3Cr2O8 and Ba3Cr2O8. Phononic excitations associated to the low-temperature monoclinic structure are emergent right below the Jahn-Teller structural transition TJT in Ba3Cr2O8, while in Sr3Cr2O8 they are completely resolved only when the temperature is much lower than TJT. This is ascribed to strong fluctuations persisting in a broad temperature range below TJT in Sr3Cr2O8, which can be attributed to the orbital degrees of freedom according to analysis of temperature-dependent specific heat. Various exotic magnetic excitations and field-induced quantum phase transitions are found in the spin-1 Haldane antiferromagnet SrNi2V2O8 and spin-1/2 XXZ antiferromagnet SrCo2V2O8. In SrNi2V2O8 gapped and gapless magnetic excitations emerge at the quantum phase transition from the Haldane phase to an Ising antiferromagnetic phase induced by a transverse magnetic field. In SrCo2V2O8 confined spinonic excitations are observed in a Néel ordered phase which can be described by a one-dimensional Schrödinger equation with a linear confining potential. Above a field-induced phase transition, deconfined spinonic excitations are observed in a disordered spin-liquid phase