Unmasking of the high temperature orbital precursors in quantum materials

Emil S. Bozin
Brookhaven National Laboratory, ZDA


The rich physics associated with the emergence of technologically relevant quantum orders in materials stems from complex interaction of electronic charge, spin, and orbitals, and their coupling to the host lattice. In transition metal systems with partial filling of d-manifolds novel properties often engage the orbital sector. Systems exhibiting orbital degeneracy and/or electronic frustration imposed by their lattice topology are particularly interesting, as orbitals couple both to the spin, via electronic interactions, and to the lattice, via Jahn-Teller mechanisms. The removal of this orbital degeneracy and the subsequent relief of frustration then impact symmetry lowering and material properties. The electronic complexity of the low temperature ordered symmetrybroken states has been thoroughly studied in systems displaying diverse emergent behaviors such as frustrated magnetism, colossal magnetoresistivity, charge and orbital order, metalinsulator transition, pseudogap, and high temperature superconductivity. Their understanding employs Fermi surface nesting, Peierls, and band Jahn-Teller mechanisms, among others.

In systems where orbital degeneracies are anticipated, crystallographic symmetry lowering at the temperature driven structural phase transitions is often assumed to imply simultaneous orbital degeneracy lifting (ODL) by engaging some cooperative mechanism. Consequently, seemingly mundane high temperature regimes possessing high crystallographic symmetry remain much less explored. In contrast to this concept, recent utilization of probes
sensitive to local symmetry qualified the ODL as a
local electronic effect existing at temperature well above the global symmetry breaking transitions.

This presentation will showcase a sensitive local structural technique, x-ray atomic pair distribution function analysis, as it reveals the presence of fluctuating local-structural distortions at high temperature of several transition metal-based quantum materials exhibiting orbitalselective ground states. We argue that this hitherto overlooked fluctuating symmetry-lowering is electronic in nature, thereby modifying the energy-level spectrum and electronic and magnetic properties. The origin is a local, spatio-temporally fluctuating, orbital degeneracy lifted state, that acts as a precursor to electronic phenomena observed at low temperature. It will be demonstrated that such local orbital states come in many flavors (e.g. engaging single [1,2] or multiple transition metal d orbitals [3]), and that they are likely to exist both in the proximity to itinerant-to-localized crossover [1,3] and deep in the Mott insulating regime where charge fluctuations are suppressed [4]. These observations suggest that such precursor states are likely to be widespread amongst
diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties.

[1] E.S. Bozin et al., Nature Comms. 10, 3638 (2019).
[2] R.J. Koch
et al., Phys. Rev. B 100, 020501(R) (2019).
[3] L. Yang
et al., Phys. Rev. B 102, 235128 (2020).
[4] R.J. Koch
et al., Phys. Rev. Lett. 126, 186402 (2021).

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