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Author: Büchner, B. × Type: article × search.filters.applied.f.series: Physical Review Research 2 (2020) ×

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    Coupling of lattice, spin, and intraconfigurational excitations of Eu3+ in Eu2ZnIrO6
    (Washington, DC : American Association for the Advancement of Science, 2020) Singh, Birender; Vogl, M.; Wurmehl, S.; Aswartham, S.; Büchner, B.; Kumar, Pradeep
    In Eu2ZnIrO6, effectively two atoms are active; i.e., Ir is magnetically active, which results in complex magnetic ordering within the Ir sublattice at low temperature. On the other hand, although Eu is a Van Vleck paramagnet, it is active in the electronic channels involving 4f6 crystal-field split levels. Phonons, quanta of lattice vibration involving vibration of atoms in the unit cell, are intimately coupled with both magnetic and electronic degrees of freedom (DOF). Here, we report a comprehensive study focusing on the phonons as well as intraconfigurational excitations in double-perovskite Eu2ZnIrO6. Our studies reveal strong coupling of phonons with the underlying magnetic DOF reflected in the renormalization of the phonon self-energy parameters well above the spin-solid phase (TN∼12K) until temperature as high as ∼3TN evidences broken spin rotational symmetry deep into the paramagnetic phase. In particular, all the observed first-order phonon modes show softening of varying degree below ∼3TN, and low-frequency phonons become sharper, while the high-frequency phonons show broadening attributed to the additional available magnetic damping channels. We also observed a large number of high-energy modes, 39 in total, attributed to the electronic transitions between 4f levels of the rare-earth Eu3+ ion and these modes shows anomalous temperature evolution as well as mixing of the crystal-field split levels attributed to the strong coupling of electronic and lattice DOF.
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    Kitaev magnetism and fractionalized excitations in double perovskite Sm2ZnIrO6
    (College Park, ML : American Physical Society, 2020) Singh, Birender; Vogl, M.; Wurmehl, S.; Aswartham, S.; Büchner, B.; Kumar, Pradeep
    The quest for Kitaev spin liquids in particular three-dimensional solids is a long sought goal in condensed matter physics, as these states may give rise to exotic new types of quasiparticle excitations carrying fractional quantum numbers, namely Majorana fermionic excitations. Here we report the experimental signature of this characteristic feature of the Kitaev spin liquid via Raman measurements. Sm2ZnIrO6 is a strongly spin-orbit-coupled Mott insulator where Jeff=1/2 controls the physics, which provides striking evidence for this characteristic feature of the Kitaev spin liquid. As the temperature is lowered, we find that the spin excitations form a continuum in contrast to the conventional sharp modes expected in ordered antiferromagnets. Our observation of a broad magnetic continuum and anomalous renormalization of the phonon self-energy parameters shows the existence of fractionalization excitations in the double-perovskite structure, as theoretically conjectured in a Kitaev-Heisenberg geometrically frustrated double-perovskite system.
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    Kramers doublets, phonons, crystal-field excitations, and their coupling in Nd2ZnIrO6
    (College Park, ML : American Physical Society, 2020) Singh, Birender; Vogl, M.; Wurmehl, S.; Aswartham, S.; Büchner, B.; Kumar, Pradeep
    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.