Novel Spin-Gap Behavior in Layered S=1/2 Quantum Spin System TiOCl

We have investigated the spin fluctuations and their interplay with lattice instabilities in a layered S=1/2 quantum spin system TiOCl. Our Ti and Cl NMR data demonstrate a pseudo spin-gap behavior below T*=135 K, followed by successive phase transitions at T_c1=94K and T_c2=66K into the singlet ground state with an unusually large energy gap E_g=430K. The broad distribution of the local electric field gradient in the intermediate phase between T_c1 and T_c2 suggests unconventional spin and lattice (probably orbital) states with an emerging spin-gap.