Magnetic field-induced one-magnon Raman scattering in the magnon Bose-Einstein condensation phase of TlCuCl${}_3$

We report the observation of the $A_{\mathrm{g}}$-symmetric one-magnon Raman peak in the magnon Bose-Einstein condensation phase of TlCuCl${}_3$. Its Raman shift traces the one-magnon energy at the magnetic $\mathrm{\Gamma }$ point, and its intensity is proportional to the squared transverse magnetization. The appearance of the one-magnon Raman scattering originates from the exchange magnon Raman process and reflects the change of the magnetic-state symmetry. Using the bond-operator representation, we theoretically clarify the Raman selection rules, being consistent with the experimental results.