Strong Pseudospin-Lattice Coupling in Sr3Ir2O7: Coherent Phonon Anomaly and Negative Thermal Expansion

Abstract

The similarities to cuprates make iridates an interesting potential platform for investigating superconductivity. Equally attractive are their puzzling complex intrinsic interactions. Here, we report an ultrafast optical spectroscopy investigation of a coherent phonon mode in Sr3Ir2O7, a bilayer Ruddlesden-Popper perovskite iridate. An anomaly in the A1g optical phonon (ν = 4.4 THz) is unambiguously observed below the Néel temperature (TN), which we attribute to pseudospin-lattice coupling (PLC). Significantly, we find that PLC is the dominant interaction at low temperature, and we directly measure the PLC coefficient to be λ = 150 +/- 20 cm-1, which is two orders of magnitude higher than that in manganites (< 2.4 cm-1) and comparable to that in CuO (50 cm-1, the strongest PLC or spin-lattice coupling (SLC) previously known). Moreover, we find that the strong PLC induces an anisotropic negative thermal expansion. Our findings highlight the key role of PLC in iridates and uncovers another intriguing similarity to cuprates.

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• Sr${}_2$IrO${}_4$/Sr${}_3$Ir${}_2$O${}_7$ superlattice for a model 2D quantum Heisenberg antiferromagnet
• Non-thermal breaking of magnetic order via photo-generated spin defects
• Reconciling monolayer and bilayer $J_{\mathrm{eff}}=1/2$ square lattices in hybrid oxide superlattice
• Lattice dynamics and spin-phonon coupling in the kagome spin ice HoAgGe
• Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7
• Phase coexistence and negative thermal expansion in the triple perovskite iridate Ba${}_3$CoIr${}_2$O${}_9$