Crystal field excitations from $\mathrm{Yb^{3+}}$ ions at defective sites in highly stuffed $ m Yb_2Ti_2O_7$

摘要

The pyrochlore magnet $\rm Yb_2Ti_2O_7$ has been proposed as a quantum spinice candidate, a spin liquid state expected to display emergent quantumelectrodynamics with gauge photons among its elementary excitations. However,$\rm Yb_2Ti_2O_7$'s ground state is known to be very sensitive to its precisestoichiometry. Powder samples, produced by solid state synthesis at relativelylow temperatures, tend to be stoichiometric, while single crystals grown fromthe melt tend to display weak "stuffing" wherein $\mathrm{\sim 2\%}$ of the$\mathrm{Yb^{3+}}$, normally at the $A$ site of the $A_2B_2O_7$ pyrochlorestructure, reside as well at the $B$ site. In such samples $\mathrm{Yb^{3+}}$ions should exist in defective environments at low levels, and be subjected tocrystalline electric fields (CEFs) very different from those at thestoichiometric $A$ sites. New neutron scattering measurements of$\mathrm{Yb^{3+}}$ in four compositions of $\rm Yb_{2+x}Ti_{2-x}O_{7-y}$, showthe spectroscopic signatures for these defective $\mathrm{Yb^{3+}}$ ions andexplicitly demonstrate that the spin anisotropy of the $\mathrm{Yb^{3+}}$moment changes from XY-like for stoichiometric $\mathrm{Yb^{3+}}$, toIsing-like for "stuffed" $B$ site $\mathrm{Yb^{3+}}$, or for $A$ site$\mathrm{Yb^{3+}}$ in the presence of an oxygen vacancy.