Neutron Spectroscopic Study of Crystalline Electric Field Excitations in Stochiometric and Lightly Stuffed Yb2Ti2O7

TL;DR

This study uses neutron spectroscopy to analyze the crystalline electric field excitations in Yb2Ti2O7, revealing how weak stuffing and disorder influence the ground state and local environments of Yb ions in this quantum spin ice candidate.

Contribution

It provides detailed CEF Hamiltonian parameters for both stoichiometric and lightly stuffed Yb2Ti2O7, highlighting the effects of weak disorder on the local electronic environments.

Findings

All samples show three CEF transitions and a ground state doublet mainly of m_J = ±1/2.

Stuffing induces a finite CEF lifetime similar to that caused by elevated temperature.

Light stuffing creates a distribution of random CEF environments and local strain fields.

Abstract

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 $\rm Yb_2Ti_2O_7$. 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, $\approx$ 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 Yb$_2$Ti$_2$O$_7$, as well as a crushed single crystal with weak stuffing and an approximate composition of Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ 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 m$_J$ = $\pm$1/2, as expected. However,"stuffing" at low temperatures in Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ induces a similar finite CEF lifetime as is induced in stoichiometric Yb$_2$Ti$_2$O$_7$ 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 Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$, in addition to producing a small fraction of Yb-ions in defective environments with grossly different CEF eigenvalues and eigenvectors.