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arXiv · 2509.20337

Spin-polaron fingerprints in the optical conductivity of iridates

Francesco Cassol·Léo Gaspard·Cyril Martins·Michele Casula·Benjamin Lenz
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Abstract

As a consequence of their spin-orbit entangled ground state, many $5d^{5}$ iridate materials display a peculiar double peak structure in optical transport quantities, such as absorption and conductivity. Their common interpretation is based on the presence of Hubbard subbands in the half-filled $j_{\mathrm{eff}}=1/2$ manifold. Herein, we challenge this picture, proposing a scenario based on the presence of spin-polaron (SP) quasiparticles, and assigning a dominant SP character to the first peak. We illustrate it by taking the materials Ba$_2$IrO$_4$ and Sr$_2$IrO$_4$ as paradigmatic examples, which we investigate within the dynamical mean-field theory and the self-consistent Born approximation. Both theories reproduce nontrivial features revealed by angle-resolved photoemission spectroscopy and optical transport measurements, supporting our interpretation. In the case of Sr$_2$IrO$_4$, we show how the SP scenario survives in the low-doped regime. Similar optical transport fingerprints are expected to be found in the wider class of $5d^5$ iridates and more generally in strongly correlated antiferromagnetic regimes, such as those found in cuprates.

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Francesco Cassol, Léo Gaspard, Cyril Martins, Michele Casula, Benjamin Lenz. 2026-04-15. Spin-polaron fingerprints in the optical conductivity of iridates. https://arxiv.org/abs/2509.20337

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