Magnetic order, disorder, and excitations under pressure in the Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$

Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at high pressure is a rare example of a Mott insulator with a paramagnetic ground state. Here, using optical Raman scattering, we measure both the phonon and magnon evolution in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ under pressure and identify three different magnetically-ordered phases, culminating in a spin-disordered state beyond 18 GPa. A strong first-order structural phase transition drives the magnetic evolution at $\sim 10$ GPa with reduced structural anisotropy in the ${\mathrm{IrO}}_6$ cages, leading to increasingly isotropic exchange interactions between the Heisenberg spins and a spin-flip transition to $c$-axis-aligned antiferromagnetic order. In the disordered phase of Heisenberg $J_{\mathrm{eff}}=1/2$ pseudospins, the spin excitations are quasielastic and continuous to 10 meV, potentially hosting a gapless quantum spin liquid in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

Figure 1

(a) Schematics of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ lattice structure, emphasizing both ${\mathrm{Ir}}^{4+}$ ions (red) and octahedral ${\mathrm{IrO}}_6$ cages (grey). [(b), (c)] Schematics of two Raman scattering geometries (laser wave vectors $k_{\mathrm{i}}$, $k_{\mathrm{o}}$, and polarizations $P$, $P^{\prime }$) relative to single-crystal lattice coordinates $a$, $b$, $c$ inside the high-pressure diamond anvil cell, and photographs of real assemblies. (d) A summary of magnetic evolution, including schematics of four different spin arrangements under pressure.

Figure 2

Magnetic Raman spectra, measured in the $\overline{c}\left(ab\right)c$ and $\overline{a}\left(cb\right)a$ configurations, are shown for [(a)–(d)] the AFM-111 phase below 3 GPa, [(e), (f)] the mixed $c$-axis stacking AFM between 3-10 GPa, [(g), (h)] spins aligned parallel to the $c$ axis between 10–20 GPa, and [(i), (j)] the disordered spin phase above 18.5 GPa. The measured Raman spectra inside the (–9, 9) ${\mathrm{cm}}^{-1}$ region are dominated by the laser line and the notch filters. All spectra are normalized to a total incident laser exposure of 0.9 J (e.g., 0.1 mW over 150 mins). Spectra are presented with measurements either [(a)–(f)] at several different spots, or [(g)–(j)] at one spot but with different laser powers. The spots in different panels are not correlated. The sample temperature is kept at $4.8\pm 0.3$ K, but the local temperature within the illuminated $5\mu \text{m}$ focus-spot-size surface region is estimated to be $\sim 8$, 17, and 25 K for 0.1, 1 and 2 mW laser powers, respectively, based on ratios of Stokes and anti-Stokes line intensities. [Inset of (i)] The difference of QES Raman spectra between 0.1 and 2 mW power settings (dots) is fit to a Lorentzian form.

Figure 3

(a) Phonon Raman spectra in $\overline{c}\left(CU\right)c$ and $\overline{a}\left(CU\right)a$ configurations at ambient pressure and 4.8 K, with modes marked with arrows and the type. [(b), (c)] Correlated pressure evolution of magnon and phonon behavior.