Pressure Tuning of the Spin-Orbit Coupled Ground State in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$

X-ray absorption spectroscopy studies of the magnetic-insulating ground state of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at ambient pressure show a clear deviation from a strong spin-orbit (SO) limit $J_{\mathrm{eff}}=\frac{1}{2}$ state, a result of local exchange interactions and a nonzero tetragonal crystal field mixing SO split $J_{\mathrm{eff}}=\frac{1}{2}$, $\frac{3}{2}$ states. X-ray magnetic circular dichroism measurements in a diamond anvil cell show a magnetic transition at a pressure of $\sim 17\mathrm{GPa}$, where the “weak” ferromagnetic moment is quenched despite transport measurements showing insulating behavior to at least 40 GPa. The magnetic transition has implications for the origin of the insulating gap and the nature of exchange interactions in this SO coupled system. The expectation value of the angular part of the SO interaction, $⟨\mathbf{L}·\mathbf{S}⟩$, extrapolates to zero at $\sim 80–90\mathrm{GPa}$ where an increased bandwidth strongly mixes $J_{\mathrm{eff}}=\frac{1}{2}$, $\frac{3}{2}$ states and SO interactions no longer dominate the electronic ground state of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

Figure 1

(Top left) Ir $L_{2,3}$-edge XANES and XMCD data collected at $T=6\mathrm{K}$, $H=0.8\mathrm{T}$, and ambient pressure. (Top right) CI calculations of XANES and XMCD intensities. All models (1–5) include SO ${\zeta }_{5d}=0.22\mathrm{eV}$ and CEF $10Dq=1.8\mathrm{eV}$ interactions. Model 1 (solid line) forces a pure $J_{\mathrm{eff}}=\frac{1}{2}$ state; model 2 (dotted line) adds an exchange field acting on the spin alone $H_{\mathrm{exch}}=\beta S_z$ with $\beta =230\mathrm{meV}$; model 3 (short dashed line) adds a tetragonal CEF $\Delta =75\mathrm{meV}$; model 4 (long dashed line) includes both $\Delta =75\mathrm{meV}$ and $H_{\mathrm{exch}}=\beta S_z$ with $\beta =230\mathrm{meV}$; model 5 (dotted-dashed line) reproduces the data and includes $\Delta =75\mathrm{meV}$ and $H_{\mathrm{exch}}=\alpha L_z+\beta S_z$ with $\alpha =-22\mathrm{meV}$ and $\beta =230\mathrm{meV}$. (Bottom) Field- and temperature-dependent $L_3$-edge ($E=11.2106\mathrm{keV}$) XMCD peak intensity at ambient pressure.

Figure 2

(Top) Pressure dependence of the Ir $L_3$-edge XMCD signal. The inset displays the data in log scale to highlight the sharpness of the magnetic transition. (Bottom) Raw XMCD data (left) and field-dependent XMCD signal (right) normalized to its value at 0.5 T, under applied pressure.

Figure 3

Resistance versus temperature at various pressures from four-probe measurements in the DAC (main panel). Estimates of the insulating gap (inset) are obtained using $\ln R\propto \frac{E_g}{2k_{B}T}$ in the 50–100 K range.

Figure 4

Ir $L_{2,3}$ XANES data at $T=300\mathrm{K}$ as a function of pressure to 70 GPa (top), together with the derived branching ratio (bottom).