Low-temperature states of polycrystalline samples of a frustrated pyrochlore oxide Tb2+xTi2−xO7+y have been investigated by specific heat, magnetic susceptibility, and neutron scattering experiments. We have found that this system can be tuned by a minute change of x from a spin-liquid state (x < xc) to a partly ordered state with a small antiferromagnetic ordering of the order of 0.1µB. Specific heat shows a sharp peak at a phase transition at Tc = 0.5 K for x = 0.005. Magnetic excitation spectra for this sample change from a quasielastic to a gapped type through Tc. The possibility of a Jahn-Teller transition is discussed.PACS numbers: 75.10. Kt, 75.40.Cx, 75.70.Tj, 78.70.Nx Magnetic systems with geometric frustration, a prototype of which is antiferromagnetically coupled Ising spins on a triangle, have been intensively studied experimentally and theoretically for decades 1 . Spin systems on networks of triangles or tetrahedra, such as triangular 2 , kagomé 3 , and pyrochlore 4 lattices, play major roles in these studies. Subjects that have fascinated many investigators in recent years are classical and quantum spin-liquid states 5-8 , where conventional long-range order (LRO) is suppressed to very low temperatures. Quantum spin-liquids 6,7 in particular have been challenging both theoretically and experimentally since the proposal of the resonating valence-bond state 9 . The spin ice materials R 2 Ti 2 O 7 (R = Dy, Ho) are the well-known classical examples 5 , while other experimental candidates found recently have been studied 10-14 .Among frustrated pyrochlore oxides 4 , Tb 2 Ti 2 O 7 has attracted much attention because it does not show any conventional LRO down to 50 mK and remains in a dynamic spin-liquid state [15][16][17] . Theoretical considerations of the crystal-field (CF) states of Tb 3+ and exchange and dipolar interactions of the system [18][19][20] showed that it should undergo a transition into a magnetic LRO state at about 1.8 K within a random phase approximation 20 . The puzzling origin of the spin-liquid state of Tb 2 Ti 2 O 7 is a subject of hot debate 4,21-28 . An interesting scenario for the spin-liquid state is the theoretical proposal of a quantum spin-ice state 22 . More recently, another scenario of a two-singlet spin-liquid state was proposed to explain why inelastic neutron spectra in a low energy range are observed despite the fact that Tb 3+ is a nonKramers ion 23,24 .Several experimental puzzles of Tb 2 Ti 2 O 7 originate from the difficulty of controlling the quality of single crystalline samples, resulting in strongly sampledependent specific-heat anomalies at temperatures below 2 K 18,26,[29][30][31][32][33] . In contrast, experimental results on polycrystalline samples are more consistent 15,16,26 . Among the experimental results reported to date, an important clue to solve the puzzles of Tb 2 Ti 2 O 7 seems to be a change of state at about 0.4 K suggested by specific heat 26 , inelastic neutron scattering 26 , and neutron spin echo 16 measurements on polycrystalline samp...
