THz-Time Domain Spectroscopy Study of spin-frustrated BaCuSi$_{2}$O$_{6}$

A spin 1/2 dimer compound BaCuSi$_{2}$O$_{6}$ realizes a field-induced Bose-Einstein condensation above 23.2 T with a field-induced AF ordering of triplets. To directly observe the low-energy gap, we have employed the THz-Time Domain Spectroscopy down to 350 mK and up to 10 T. Preliminary results indicate two novel optical transitions around 0.36 (1.49) and 0.5 (2.07) THz (meV) below the reported spin gap of 3.21 meV$^{1}$. Temperature and magnetic field (B//c) dependences of the transitions will be discussed. Reference: 1. S Sebastian et al., Phys. Rev. B 74, 180401R (2006).