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

TL;DR

This study investigates how defective environments in Yb2Ti2O7 affect the crystal field excitations of Yb3+ ions, revealing a transition from XY to Ising spin anisotropy due to stuffing and vacancies, with implications for quantum spin ice behavior.

Contribution

It provides the first spectroscopic evidence of how stuffing and vacancies alter the crystal field and spin anisotropy of Yb3+ ions in Yb2Ti2O7, highlighting the impact of defects on magnetic properties.

Findings

Defective Yb3+ ions exhibit different crystal field signatures.

Spin anisotropy shifts from XY to Ising in defective sites.

Neutron scattering reveals environment-dependent Yb3+ behavior.

Abstract

The pyrochlore magnet $\rm Yb_2Ti_2O_7$ 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, $\rm Yb_2Ti_2O_7$'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 $\mathrm{\sim 2\%}$ of the $\mathrm{Yb^{3+}}$, normally at the $A$ site of the $A_2B_2O_7$ pyrochlore structure, 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 to crystalline electric fields (CEFs) very different from those at the stoichiometric $A$ sites. New neutron scattering measurements of $\mathrm{Yb^{3+}}$ in four compositions of $\rm Yb_{2+x}Ti_{2-x}O_{7-y}$, show the spectroscopic signatures for these defective $\mathrm{Yb^{3+}}$ ions and explicitly demonstrate that the spin anisotropy of the $\mathrm{Yb^{3+}}$ moment changes from XY-like for stoichiometric $\mathrm{Yb^{3+}}$, to Ising-like for "stuffed" $B$ site $\mathrm{Yb^{3+}}$, or for $A$ site $\mathrm{Yb^{3+}}$ in the presence of an oxygen vacancy.