Orbital degeneracy lifted ‘liquid’ state—precursor to ordered quantum matter

August 13, 2019

In a study published in Nature Communications, researchers explained a local, fluctuating, orbital-degeneracy-lifted state. A phenomenon likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems.

Observation of high-temperature fluctuating ODL state in CuIr2S4. a Temperature waterfall stack of xPDFs measured on warming from 10 K (bottom) to 500 K (top) in 2 K increments. TMI is the MIT transition temperature (226 K). The dimer peak at ~3 Å (marked by arrow) is only seen in the insulating phase, and disappears above TMI. b Fit of the undistorted cubic Fd3¯¯¯m model (red line) to the 500 K data (blue open circles) and their difference (green line, offset for clarity) unmask the footprint of the localized ODL state at ~3.5 Å. c Simulated Fd3¯¯¯m total xPDF of CuIr2S4 (blue line), decomposed into Ir–Ir partial xPDF (green line) and its complement (red line). Shaded peaks in total xPDF are sensitive to t2g orbital overlaps (sketched) and their spatial correlations. Inset: t2g-derived molecular orbitals discussed in the main text. d, e Section of Ir pyrochlore sublattice of corner-shared Ir4 tetrahedra for undistorted (cubic) and distorted (tetragonal) spinel structure, respectively. The strongest t2g orbital overlaps (e.g. xy with xy, etc.) are along the chains formed by the tetrahedral edges of the Ir sublattice.

Scientific Achievement

Using the thiospinel CuIr₂S₄ as a prototype, we have identified a new framework for understanding lattice-charge coupling in quantum materials: orbital degeneracy lifting (ODL), an effect in which electronic degeneracies generate fluctuating local symmetry-broken states precursory to long range order.

Significance and Impact

Analogous ODL states are expected to be widespread among quantum materials with partially filled d-bands, providing a unifying construct to understand phenomena in a range of technologically relevant systems.

Research Details

X-ray atomic pair distribution functions were measured on a fine temperature and composition grids on CuIr₂S₄ powder, and on Cr and Zn doped versions, at 28-ID-2@NSLS-II.
The local structure requires a broken symmetry model to fit the PDF even when the average structure is cubic.

Sample syntheses and characterizations were performed at Argonne and Brookhaven national laboratories.

DOI: 10.1038/s41467-019-11372-w

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