Exotic transition in the three-dimensional spin-liquid candidate Tb ₂Ti₂O₇

The insulator pyrochlore Tb₂Ti₂O₇ is a three-dimensional spin liquid candidate: the strength of the magnetic interaction would normally induce order at 20 K, but according to published muon-spin relaxation and neutron diffraction measurements, it is still in a correlated paramagnetic state down to a temperature of 0.05 K. Our detailed experimental investigation shows that an exotic transition takes place at T _t=0.15(2) K. It is characterized by an anomaly in the muon precession frequency shift but not in the specific heat, which nevertheless reveals the signature of degrees of freedom at very low energy. It might be associated with a cooperative Jahn-Teller transition. The robustness of the experimental results is discussed.