Kramers doublets, phonons, crystal-field excitations, and their coupling in Nd2ZnIrO6

Abstract

We report comprehensive Raman-scattering measurements on a single crystal of double-perovskite Nd2ZnIrO6 in a temperature range of 4–330 K, spanning a broad spectral range from 20 to 5500cm−1. The paper focuses on lattice vibrations and electronic transitions involving Kramers doublets of the rare-earth Nd3+ ion with local C1 site symmetry. Temperature evolution of these quasiparticle excitations has allowed us to ascertain the intricate coupling between lattice and electronic degrees of freedom in Nd2ZnIrO6. Strong coupling between phonons and crystal-field excitation is observed via renormalization of the self-energy parameter of the phonons, i.e., peak frequency and linewidth. The phonon frequency shows abrupt hardening and linewidth narrowing below ∼100 K for the majority of the observed first-order phonons. We observed splitting of the lowest Kramers doublets of ground state (4I9/2) multiplets, i.e., lifting of the Kramers degeneracy, prominently at low temperature (below ∼100 K), attributed to the Nd-Nd/Ir exchange interactions and the intricate coupling with the lattice degrees of freedom. The observed splitting is of the order of ∼2–3 meV and is consistent with the estimated value. We also observed a large number of high-energy modes, 46 in total, attributed to the intraconfigurational transitions between 4f3 levels of Nd3+ coupled to the phonons reflected in their anomalous temperature evolution.