Persistent insulating state at megabar pressures in strongly spin-orbit coupled $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$

It is commonly anticipated that an insulating state will collapse in favor of an emergent metallic state at high pressures: The average electron density must increase with pressure, while the electronic bandwidth is expected to broaden and fill the insulating energy band gap. Here we report an unusually stable insulating state that persists up to at least 185 GPa in $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$, the archetypical spin-orbit-driven $J_{\mathrm{eff}}=1/2$ insulator. This study shows that the electrical resistance R of single-crystal $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ initially decreases with applied pressure P, reaches a minimum in the range 32–38 GPa, then abruptly rises to recover the insulating state with increasing P up to 185 GPa. However, evidence of a saturation of R below 80 K for $P\ge 124\mathrm{GPa}$ GPa raises the possibility of a low-temperature exotic state. Our synchrotron x-ray diffraction and Raman scattering data show the emergence of the rapid increase in R is accompanied by a structural phase transition from the native tetragonal $I{4}_1/acd$ phase to an orthorhombic $Pbca$ phase (with much reduced symmetry) at 40.6 GPa. The clear correspondence of the onset pressures of these two anomalies is key to understanding the stability of the insulating state at megabar pressures: Pressure-induced, structural distortions prevent the expected onset of metallization, despite the sizable volume compression attained at the highest pressure accessed in this study.

Figure 1

The temperature dependence of the basal-plane resistance R over pressures ranging from (a1) 0.6–32.1 GPa and (a2) 36.7–54.2 GPa for run 1, and (c) 24.7–185 GPa for run 2. Note the gray arrows that indicate the increase or decrease in R with increasing P. Corresponding contour plots are shown in (b) for the pressure range 0.6–54.2 GPa for run 1 and in (d) for the pressure range 24.7–185 GPa for run 2. The colors red and blue represent the highest and lowest resistance R, respectively, and other colors indicate intermediate resistance values. The white dashed lines mark the pressure regime of $32.1-37.9$ where R reaches its minimum. (e) Data marked by the green oval circle in (c) is shown in an expanded plot of the near-saturated regime for R (i.e., $P\approx 185\mathrm{GPa}$ and $T<80\mathrm{K}$). Inset: A snapshot of the diamond anvil cell with a sample at 27 GPa.

Figure 2

(a) Representative synchrotron x-ray diffraction patterns at room temperature spanning $P=0.9$ to 73.7 GPa. The black curves represent the native $I{4}_1/acd$ phase $\left(Z=8\right)$, red patterns the pressure-induced Pbca phase $\left(Z=4\right)$. Note that the green XRD spectrum in the top of (a) marked by a green arrow indicates that the crystal structure of the sample recovers its ambient structure after decompression (marked dp). (b)–(d) The structural transition at 40.6 GPa is marked by red rhombi. The refinement results for 0.9 GPa are shown in (a). The experimental data are the solid circles, and the calculated data are red lines. Bragg peaks are represented by black vertical sticks.

Figure 3

Pressure dependence of the lattice parameters of (a) the $a$ axis and $b$ axis, (b) the $c$ axis, and (c) the unit cell volume. The black squares and red circles represent the lattice parameters of the native $I{4}_1/acd$ phase $\left(Z=8\right)$ and the pressure-induced Pbca phase $\left(Z=4\right)$, respectively. Note that the gray dashed lines mark the critical pressure $P_c=40.6\mathrm{GPa}$. For comparison and contrast, the lattice parameters for the native phase marked by the faint gray squares are plotted above $P_c$. The black and red solid lines represent the fits for the phases with the Birch-Murnaghan equation of states.

Figure 4

(a) Selected room-temperature Raman spectra of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ for applied pressures extending from 1.5 to 65.6 GPa. The black patterns include ${\mathrm{M}}_{1\text{−}4}$; the red curves cover a series of new peaks marked $P_{1\text{−}4}$ after the structural transition. The uppermost green line is the spectrum after decompression. (b) Raman frequencies as a function of pressure for phonon modes. Note that the red solid line marks the critical pressure of 40.6 GPa, consistent with that obtained in the XRD measurements. Insets: The rotation of $\mathrm{Ir}{\mathrm{O}}_6$ below $P_c$ and the rotation and tilt of $\mathrm{Ir}{\mathrm{O}}_6$ above $P_c$. Note that the spectrum marked by a green arrow in (a) recovers its low-pressure form after decompression back to 3.6 GPa, suggesting that the sample integrity is preserved at 65.6 GPa.

Figure 5

The pressure dependence of the lattice parameter ratio of the $c$ axis to the a axis,$c/a$, for the tetragonal phase (blue), and c/a and c/b for the orthorhombic phase (red).