Field-induced magnetic structures in Tb2Ti2O7 spin liquid under field H || [111]

We have studied the field-induced magnetic structures of Tb2Ti2O7 pyrochlore by single-crystal neutron diffraction under a field applied along the [111] local anisotropy axis, up to H=12T and down to T=40mK. We collected a hundred magnetic Bragg peaks for each field and temperature value and refined the magnetic structures with k=0 propagation vector by performing a symmetry analysis in the space group R-3m, reducing the number of free parameters to three only. We observe a gradual reorientation of the Tb magnetic moments towards the field direction, close to a "3-in, 1-out / 1-in, 3-out" spin structure in the whole field range 0.05-12T. We perform a quantitative comparison with mean-field calculations and we propose the presence of a low-temperature dynamic symmetry breaking of the local trigonal symmetry, akin to a dynamic Jahn-Teller effect, i.e. preserving the overall cubic symmetry. We discuss the possible origin of this off-diagonal mixing term in the crystal field hamiltonian in terms of quadrupole-quadrupole interaction or magneto-elastic effects.