Boson-Peak-Like Anomaly Induced by Dipole Disordered States in (CH3NH3)4

Boson peaks are observed in glassy materials due to atom, spin, and strain disordered states that provide additional vibration modes at low temperatures. However, Boson peaks have not been observed in pure dipole disordered systems without structural disorder. Here, we report the observation of a Boson-peak-like hump in specific heat near 7 K in organic-inorganic hybrid crystal $M{A}_{4}{\mathrm{InCl}}_{7}(MA={\mathrm{CH}}_{3}{\mathrm{NH}}_{3})$. The energy barrier for unbonded Cl anions migration is extremely low, allowing that the migration persists down to low temperatures. The migration and the subsequent relaxation process induce a random change in its dipole, leading to the dipole disorder states. Our results show that $M{A}_{4}{\mathrm{InCl}}_{7}$ can be regarded as a dipole glass that can give rise to the Boson peak as well.