Importance of dynamic lattice effects for crystal field excitations in the quantum spin ice candidate Pr2Zr2O7

We explore dynamic interactions between the crystal lattice and magnetic degrees of freedom in a frustrated magnetic system using the example of a pyrochlore quantum spin-ice candidate ${\mathrm{Pr}}₂{\mathrm{Zr}}₂{\mathrm{O}}₇$. Using Raman scattering spectroscopy we demonstrate that crystal electric field excitations of Pr3+, which define the magnetic properties of ${\mathrm{Pr}}₂{\mathrm{Zr}}₂{\mathrm{O}}₇$, cannot be understood within a model of a static lattice. We identify vibronic interactions with a phonon which lead to a splitting of a doublet crystal field excitation at around 55 meV. We also observe an unconventional behavior of a splitting of the non-Kramers ground state doublet of ${\mathrm{Pr}}³⁺$, revealed by observing excitations to the first excited singlet state ${E}_g⁰\ensuremath{\rightarrow}{A}_1g$ at around 10 meV. The splitting has a strong temperature dependence, where the doublet structure is most prominent between 50 and 100 K, and the weight of one of the components strongly decreases on cooling contrary to simple thermal population tendency. We suggest a static or dynamic deviation of ${\mathrm{Pr}}³⁺$ from the position in the ideal crystal structure can be the origin of the effect, with the deviation strongly decreasing at low temperatures.