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Coupling among charge, orbital, spin, and lattice degrees of freedom (d.o.f.) is essential for realizing
various functional properties, such as ferroelectricity, the magnetoelectric effect, the metal-insulator
transition, superconductivity, and charge-density-wave (CDW) order. However, the relative contributions
of the competing orders in controlling the desired behavior are difficult to discern. A detailed
experimental and theoretical mapping of electron, electron-spin, and lattice susceptibility is necessary to
decipher the governing mechanism(s). In this talk, I will describe our recent efforts to identify the origin
of CDW instability in a-Uranium and ��′-U 2 Mo [1,2], and spin-orbital-phonon coupling in
antiferromagnetic oxides CrVO 4 , NiO, and YCrO 3 [3,4,5] by experimentally and theoretically mapping
the phonon, electron, and magnon quasiparticles.
Several mechanisms have been invoked to explain the origin of the CDW in various classes of materials,
including Fermi-surface and hidden-nesting effects, wavevector-dependent electron-phonon coupling, and
strong electron correlations. In a-Uranium [1], we probed the lattice susceptibility (i.e., phonon
dispersions and linewidth) in the entire reciprocal space using inelastic neutron scattering measurements
and phonon simulations. We observed, for the first time, Kohn anomalies in multiple phonon branches at
the CDW wavevector q CDW , highlighting the role of the lattice degree of freedom in the CDW instability.
The observation of multiple Kohn anomalies at q CDW is rationalized by electron-susceptibility simulations
that show a divergence at q CDW . The divergence is insensitive to the Fermi level or to external
perturbations, suggesting that the combined effects of Fermi surface nesting and hidden nesting drive it.
The same observations now apply to the recently synthesized single-phase ��′-U 2 Mo [2].
In the second part, I will describe our recent efforts to experimentally and theoretically identify spin-
orbital-phonon coupling in the prototypical three antiferromagnetic (AFM) oxides CrVO 4 , NiO, and
YCrO 3 [3,4,5], which exhibit contributions from different partially occupied Ni and Cr d orbitals. In
CrVO 4 [3], measurements showed anomalous responses in the lattice parameters, electronic states, spin
dynamics, and phonons at four times the Neel transition temperature (T N ), which is significantly higher
than that of other d-orbital compounds, such as manganites and vanadates, where effects are limited to
near or below T N . We rationalized the above findings using first-principles and Green’s function-based
phonon and spin simulations that showed unprecedentedly strong orbital-selective spin-phonon coupling.
Furthermore, we applied the same simulation approach to quantify the phonon-driven effects on spin and
orbital d.o.f. in the NiO and YCrO 3 . We find that selective, narrow- and finite-bandwidth phonon
excitation perturbs orbital overlaps and their relative contributions to ferromagnetic (FM) and AFM
interactions, further lifting degeneracies, renormalizing magnetic exchange interactions (J), and
significantly altering spin waves. The renormalized Js further modify the interatomic force constant and
the phonons via a coupled spin-phonon Hamiltonian. Our quantitative analysis provides a deeper
understanding of exchange pathways and spin dynamics under phonon excitation, offering opportunities
to tailor magnetic functionalities in spintronics and quantum materials. |