Raman signatures of strong Kitaev exchange correlations in (Na1−xLix)2IrO3: Experiments and theory

Inelastic light scattering studies on single crystals of (Na1-xLix)(2)IrO3 (x = 0, 0.05 and 0.15) show a polarization-independent broad band at similar to 2750 cm(-1) with a large band-width similar to 1800 cm(-1). For Na2IrO3 the broad band is seen for temperatures <= 200K and persists inside the magnetically ordered state. For Li samples, the intensity of this mode increases, shifts to lower wave numbers, and persists to higher temperatures. Such a mode has recently been predicted (by Knolle et al.) as a signature of the Kitaev spin liquid. We assign the observation of the broad band to be a signature of strong Kitaev exchange correlations. The fact that the broad band persists even inside the magnetically ordered state suggests that dynamically fluctuating moments survive even below T-N. This is further supported by our mean-field calculations. The Raman response calculated in mean-field theory shows that the broad band predicted for the SL state survives in the magnetically ordered state near the zigzag-spin liquid phase boundary. A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter to be J(K) approximate to 57meV. Copyright (C) EPLA, 2016