Raman scattering in an anisotropic triangular spin lattice system

Spin-disordered quantum phases in an anisotropic triangular spin lattice system, $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$B(CN)$_{\mathrm{4}}$, were recently reported [1]. In this compound, the ratio of the two transfer integrals, ${t}'/t$, reaches 1.44 at 298 K and 1.80 at 100 K. Its optical conductivity in the infrared region is anisotropic. The temperature dependence of the optical anisotropy correlates with that of ${t}'/t$. From the experimentally evaluated optical anisotropy, we expect that the values of ${t}'/t$ are larger than 1.80 in the lower temperature region. For this compound, we observe the polarization-dependent broad Raman scattering signals below 600 cm$^{\mathrm{-1}}$ at 10 K. In such a wavenumber region, we have observed the magnetic Raman signals in triangular spin lattice systems such as $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$X [2] and ${\beta }'$-type Pd(dmit)$_{\mathrm{2}}$ salts [3]. By comparison with them, we discuss the origin of the Raman signals observed for $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$B(CN)$_{\mathrm{4}}$. [1] Y. Yoshida \textit{et al.}, Nat. Phys. \textbf{11}, 679 (2015). [2] Y. Nakamura \textit{et al.}, J. Phys. Soc. Jpn. \textbf{83}, 074708 (2014). [3] Y. Nakamura\textit{ et al.}, J. Phys. Soc. Jpn. \textbf{84}, 044715 (2015).