Neutron spectroscopic study of crystalline electric field excitations in stoichiometric and lightly stuffed Yb2Ti2O7
Résumé
Time-of-flight neutron spectroscopy has been used to determine the crystalline electric field (CEF) Hamiltonian, eigenvalues and eigenvectors appropriate to the J = 7/2 Yb 3+ ion in the candidate quantum spin ice pyrochlore magnet Yb2Ti2O7. The precise ground state (GS) of this exotic, geometrically-frustrated magnet is known to be sensitive to weak disorder associated with the growth of single crystals from the melt. Such materials display weak "stuffing" wherein a small proportion, ≈ 2%, of the non-magnetic Ti 4+ sites are occupied by excess Yb 3+. We have carried out neutron spectroscopic measurements on a stoichiometric powder sample of Yb2Ti2O7, as well as a crushed single crystal with weak stuffing and an approximate composition of Yb2+xTi2−xO7+y with x = 0.046. All samples display three CEF transitions out of the GS, and the GS doublet itself is identified as primarily composed of mJ = ±1/2, as expected. However,"stuffing" at low temperatures in Yb2+xTi2−xO7+y induces a similar finite CEF lifetime as is induced in stoichiometric Yb2Ti2O7 by elevated temperature. We conclude that an extended strain field exists about each local "stuffed" site, which produces a distribution of random CEF environments in the lightly stuffed Yb2+xTi2−xO7+y, in addition to producing a small fraction of Yb-ions in defective environments with grossly different CEF eigenvalues and eigenvectors.
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