Magnetic field-induced one-magnon Raman scattering in the magnon   Bose-Einstein condensation phase of TlCuCl$_{3}$
  We report the observation of the $A_{\rm g}$-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 $\Gamma$ 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. 
Haruhiko Kuroe, Kouhei Kusakabe, Akira Oosawa, Tomoyuki Sekine, Fumiko   Yamada, Hidekazu Tanaka, and Masashige Matsumoto