  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.
