Neutron spectroscopic study of Crystal-field excitation in $Yb_{2}(Ti_{2-x}Yb_{x})O_{7-\frac{x}{2}}$

Among the rare-earth titanate pyrochlores, $Yb_2Ti_2O_7$ has attracted much attention as a potential realization of a quantum spin ice [1]. While strong quantum effects are absent in classical spin ice compounds, they are thought to be significant in $Yb_2Ti_2O_7$ because of its effective spin S=1/2 and its XY spin anisotropy, quantities both determined by the Crystal-Electric Field (CEF) levels. However, a thorough neutron spectroscopy study of the CEF levels is still lacking. Here, we report time-of-flight inelastic neutron scattering measurements on $Yb_2Ti_2O_7$. Our results lead to the unambiguous determination of the CEF levels, the ground-state wavefunction and therefore the nature of the spin anisotropy of the J=7/2 $Yb^{3+}$. A significant sample dependence in the low temperature heat capacity has been reported and attributed to an excess of $Yb^{3+}$ (''stuffing'') in the structure [2]. Our measurements, carried out on two well-characterized samples with different levels of stuffing, allow us to discuss the impact of such disorder on the CEF levels. [1] K.A. Ross, L. Savary, B.D. Gaulin, L. Balents,Physical Review X, 1(2), 021002, (2011). [2] K.A. Ross, Th. Proffen, H.A. Dabkowska, J.A. Quilliam, L.R. Yaraskavitch, J.B. Kycia and B.D. Gaulin, Phys. Rev. B 86, 17442