Abstract
We report comprehensive Raman-scattering measurements on a single crystal of double-perovskite Nd2ZnIrO6 in temperature range of 4-330 K, and spanning a broad spectral range from 20 cm-1 to 5500 cm-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 quasi-particle excitations have 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 line-width. The phonon frequency shows abrupt hardening and line-width 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 intra-configurational transitions between 4f3 levels of Nd3+ coupled to the phonons reflected in their anomalous temperature evolution.
Highlights
The physics of correlated electron systems associated with 5d transition-metal oxides, especially Ir oxides, has drawn considerable research interest in recent years owing to the possible formation of quantum spin liquids, Mott insulators, unconventional superconductors, and Weyl semimetals [1,2,3,4,5,6,7,8,9,10]
Another important aspect of the presence of rare-earth ion in these systems is the crystal-field excitations (CFEs) originating from the splitting of rare-earth ion multiplets with specific local site symmetry owing to the surrounding ions static field
We found signature of strong coupling of lattice with crystal-field excitations arising from ground state (4I9/2) multiplets of Nd3+ ion at low temperature because of their same energy and symmetry, led to the strong renormalization of the phonon self energy parameters
Summary
The physics of correlated electron systems associated with 5d transition-metal oxides, especially Ir oxides, has drawn considerable research interest in recent years owing to the possible formation of quantum spin liquids, Mott insulators, unconventional superconductors, and Weyl semimetals [1,2,3,4,5,6,7,8,9,10]. Depending on the ionic radius of A-site ions, different quantum magnetic spin interactions such as Heisenberg and Kitaev type may be realized in doublepervoskite iridates [23,24,25] Another important aspect of the presence of rare-earth ion in these systems is the crystal-field excitations (CFEs) originating from the splitting of rare-earth ion multiplets with specific local site symmetry owing to the surrounding ions static field. Probing these electronic excitations is very important because CFE as a function of temperature may provide crucial information about the nature of underlying ground state ordering and provide the avenues to probe the coupling with lattice degrees of freedom [26,27,28,29]. Our measurements reveal strong coupling between phonons and CFE, and evidence the lifting of Kramers degeneracy of Nd3+ ground state multiplets at low temperature attributed to the Nd-Nd and Nd-Ir exchange interactions
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