Power-Law Spin Correlations in the Pyrochlore Antiferromagnet Tb₂Ti₂O₇

Spin correlations with power-law decay are usually associated with a critical point, but stable phases with power-law correlations may exist in frustrated magnets. Such phases are interesting, because they represent model materials where short-range interactions and local constraints lead to emergent symmetries and fractional quasiparticles. For example, in a spin ice, spin configurations that respect a local ice-rule constraint give rise to dipolar (i.e., 1/r³) spin correlations and emergent magnetic monopoles. Dipolar correlations are identified in scattering experiments by the existence of distinctive sharp and anisotropic features known as pinch points. We investigated the low-temperature state of the rare-earth pyrochlore Tb₂Ti₂O₇ using polarized neutron scattering experiments performed on TASP (SINQ) and D7 (ILL). Tb₂Ti₂O₇ is often described as an antiferromagnetic spin liquid with isotropic spin correlations extending over lengths comparable to individual tetrahedra of the pyrochlore lattice. We confirmed this picture at 20 K but find that at 0.05 K the data contain evidence of pinch-point scattering, suggesting that the low temperature state of Tb₂Ti₂O₇ has power-law spin correlations and may be described by an as yet unknown emergent gauge theory.