\textbf{Spin Pumping into Topological Insulator Bi}$_{\mathrm{\mathbf{2}}}$\textbf{Te}$_{\mathrm{\mathbf{3}}}$

A spin chemical potential bias can induce a spin polarized current by the exchange interaction of a ferromagnet with the spin-momentum locking surface states of the topological insulators. We carried out our ferromagnetic resonance experiment in a NiFe/Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ heterostructure. Apart from the enhanced Gilbert damping constant, we observed strong enhancement of the effective magnetic field at low temperatures. The enhanced field decreased exponentially with increasing temperature at an energy scale of 2.5 meV, representing the strength of the exchange coupling. We attribute the enhanced field to the induced spin polarized current in the surface states of Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$.