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Spin-phonon coupling probed by infrared transmission spectroscopy in the double perovskite Ba$_2$YMoO$_6$

In this work, we investigate the local structural distortion of the double perovskite Ba$_2$YMoO$_6$ by means of infrared transmission spectroscopy. At 300 K, three bands are observed at $\sim$ 255.1 cm$^{-1}$, $\sim$ 343.4 cm$^{-1}$, and $\sim$ 561.5 cm$^{-1}$, which are related to the motion between the cation Ba$^{2+}$ and the anion YMO$_6^{-2}$, the Y-O stretching motion and the stretching vibration of the MoO$_6$ octahedron, respectively. These modes continue to harden upon cooling owing to the shrink of the lattice constant. When the temperature decreases to $T \leq$ 130 K around which the spin singlet dimer begins to form, an additional phonon mode appears at $\sim$ 611 cm$^{-1}$, suggesting the occurrence of local distortion of MoO$_6$ octahedra. With further decrease of the temperature, its intensity enhances and its peak position keeps unchanged. These results indicate that the formation of the spin singlet dimers is accompanied with the occurrence of the local structure distortion of MoO$_6$ octahedra, providing evidence for the strong spin-phonon coupling in the double perovskite Ba$_2$YMoO$_6$.