Crystal field excitations from Yb3+ ions at defective sites in highly stuffed Yb2Ti2O7

The pyrochlore magnet Yb2Ti2O7 has been proposed as a quantum spin ice candidate, a spin liquid state expected to display emergent quantum electrodynamics with gauge photons among its elementary excitations. However, Yb2Ti2O7's ground state is known to be very sensitive to its precise stoichiometry. Powder samples, produced by solid-state synthesis at relatively low temperatures, tend to be stoichiometric, while single crystals grown from the melt tend to display weak “stuffing” wherein ∼2% of the Yb3+, normally at the A site of the A2B2O7 pyrochlore structure, reside as well at the B site. In such samples Yb3+ ions should exist in defective environments at low levels and be subjected to crystalline electric fields very different from those at the stoichiometric A sites. Neutron scattering measurements of Yb3+ in four compositions of Yb2+xTi2−xO7−y show the spectroscopic signatures for these defective Yb3+ ions and explicitly demonstrate that the spin anisotropy of the Yb3+ moment changes from XY-like for stoichiometric Yb3+ to Ising-like for “stuffed” B site Yb3+ or for A site Yb3+ in the presence of oxygen vacancies.