Intermolecular THz Vibrations Relevant to Optically and Thermally Induced Magnetic Phase Transitions in the Strongly Correlated Organic Radical TTTA

Intermolecular vibrations relevant to optically and thermally induced magnetic phase transitions between low temperature (LT) diamagnetic and high temperature (HT) paramagnetic phases in a strongly correlated organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal have been investigated using broadband terahertz (THz) time-domain spectroscopy. Two absorption bands with different polarizations were clearly observed at 1.3 and 4.0 THz in the LT phase, whilst absent in the HT phase. The temperature dependence of the THz absorption peak exhibits hysteretic behavior around room temperature, indicating that the observed THz absorption bands are related to the magnetic phase transition in TTTA. Considering the symmetry of the crystal using group theoretical analysis, the observed absorption bands are attributed to intermolecular vibrations with A(u) symmetry. We found that the absorption band at 4.0 THz disappeared upon irradiation by a laser pulse with a photon density greater than 3.0 x 10(18) photons/cm(2). These results show that probing the intermolecular THz vibrations is indispensable for revealing the photoinduced phase transition from the LT diamagnetic to HT paramagnetic phase.