Local orbital degeneracy lifting as a precursor to an orbital-selective Peierls transition
Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the d-electron orbitals and the influence of this filling on structural configurations and responses. Curiously, some of the transition metal systems feature a large discrepancy between the long-range ordering temperatures (tens to hundreds of Kelvin) and the energy scales of the underlying electronic phenomena involved (hundreds to thousands of meV). In this presentation I will address this often ignored and largely unexplained disparity through a study of one such compound, CuIr2S4 (CIS) spinel, where the orbital degrees of freedom play crucial role.
CuIr2S4 displays temperature driven metal to insulator transition (MIT), where the low temperature insulating state consists of long range ordered Ir3+ (5d6) and Ir4+ (5d5) ions, with a four-fold periodicity, an example of tetrameric charge ordering . Concurrently, spin dimerization of Ir4+ pairs occurs within the tetramer, with large associated structural distortions (0.5 Å) as they move towards each other, making this charge-order particularly amenable to detection using structural probes . Notwithstanding the complexities of the insulating state, including formation of remarkable three-dimensional Ir3+8S24 and Ir4+8S24 molecule-like assemblies embedded in the lattice, its quasi-one-dimensional character was unmasked, and MIT attributed to an orbital-selective Peierls mechanism, postulated from topological considerations . By utilizing a sensitive local structural technique, x-ray atomic pair distribution function analysis, we reveal the presence of fluctuating local-structural distortions deep in the high temperature metallic regime of CuIr2S4 . The distortions are the fingerprints of a precursor high temperature state that enables the rich phenomenology observed at low temperature. Through judicious chemical substitutions, we show that this hitherto overlooked fluctuating symmetry lowering has electronic origin that can be understood as a local, fluctuating, orbital-degeneracy-lifted (ODL) state. This is related to, but qualitatively different from, the dimer-state observed in the insulating phase. Observation of the ODL state provides a natural way to understand the observed energy-scale discrepancy in a range of transition metal systems. Our study also presents a very new view on MIT and related phenomena in the material studied – CIS, and CIS-derived spinel systems – and experimentally verifies that the orbital sector indeed drives the physics in this material class.
While the electronic driving force for the formation is ubiquitous, the mechanisms of achieving the ODL state may be diverse (e.g. Jahn-Teller, local crystal field, covalency, molecular orbital formation, relativistic spin-orbit coupling, etc.). Our study exemplifies that such states exist but are difficult to detect and should be studied in a more systematic manner. The ODL state, characteristic of the high temperature regime, could be a critical ingredient and a missing link enabling more comprehensive understanding of phenomena as widespread as nematicity, pseudogaps, metal insulator transitions, spin glass behavior etc. Time permitting, the presentation will also spotlight a few other ODL systems such as perovskites, pyroxenes, and delafossites.
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