Collective versus local Jahn-Teller distortion inBa3CuSb2O9: Raman scattering study

Abstract

We present temperature dependent Raman spectra of single crystals of two different samples of the spin-orbital liquid candidate ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$. The ``hexagonal'' sample is known to show no magnetic order down to low temperatures, while the ``orthorhombic'' sample undergoes a crossover into an orthorhombic crystal structure below ${T}_{\text{JT}}$ of approximately 200 K and shows spin freezing at 110 $\ensuremath{\mu}\mathrm{K}$. Our Raman scattering results demonstrate a strong influence of disorder in both samples. The frequencies of stretching vibrations of oxygens associated with ${\mathrm{CuSbO}}_{9}$ octahedra indicate a difference in the crystal structure between the hexagonal and orthorhombic samples even at room temperature. On cooling below ${T}_{\text{JT}}$ we observe new bands in the spectra of the orthorhombic sample due to a lowering of symmetry of the unit cell and a collective Jahn-Teller distortion. The spectra of the hexagonal sample show that average hexagonal symmetry is maintained at least down to 20 K. An analysis of the band shape of stretching oxygen vibrations suggests a weak disordered local Jahn-Teller distortion in the hexagonal sample of ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$ which increases slightly on cooling. This Jahn-Teller distortion is either static or dynamic with a frequency below 1.5 THz.