Soft Phonons and Structural Phase Transitions in La_1.875Ba_0.125CuO₄

Soft phonon behavior associated with a structural phase transition from the low-temperature-orthorhombic (LTO) phase (Bmab symmetry) to the low-temperature-tetragonal (LTT) phase (P4(2)/ncm symmetry) was investigated in La1.875Ba0.125CuO4 using neutron scattering. As temperature decreases, the TO-mode at Z-point softens and approaches to zero energy around T-d2 = 62 K, where the LTO-LTT transition occurs. Below T-d2, the phonon hardens quite rapidly and its energy almost saturates below 50K. At T-d2, the energy dispersion of the soft phonon along in-plane direction significantly changes while the dispersion along out-of-plane direction is almost temperature independent. Coexistence between the LTO phase and the LTT phase, seen in both the soft phonon spectra and the peak profiles of Bragg reflection, is discussed in context of the order of structural phase transitions.