Pressure-Induced Confined Metal from the Mott Insulator ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$

The spin-orbit Mott insulator ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ provides a fascinating playground to explore insulator-metal transition driven by intertwined charge, spin, and lattice degrees of freedom. Here, we report high-pressure electric resistance and resonant inelastic x-ray scattering measurements on single-crystal ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ up to 63–65 GPa at 300 K. The material becomes a confined metal at 59.5 GPa, showing metallicity in the $ab$ plane but an insulating behavior along the $c$ axis. Such an unusual phenomenon resembles the strange metal phase in cuprate superconductors. Since there is no sign of the collapse of spin-orbit or Coulomb interactions in x-ray measurements, this novel insulator-metal transition is potentially driven by a first-order structural change at nearby pressures. Our discovery points to a new approach for synthesizing functional materials.

Figure 1

Electric resistance measurements of single-crystal ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ at high pressure (a) within the $ab$ plane and (b) along the crystal $c$ axis. (c) Four gold electric leads and single-crsytal ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ loaded into a symmetric diamond anvil cell. The inset shows two of the leads attached to the top of the single crystal, and the other two attached to the bottom. Each diamond culet is approximately $300\mu \mathrm{m}$ wide. (d) Temperature dependences of the resistances at 59.5 and 63.0 GPa. The system shows metallic behavior in the $ab$ plane, whereas it still exhibits an insulating state along the $c$ axis.

Figure 2

(a) Ir $L_3$-edge RIXS map collected at 1.86 GPa. (b) Atomic energy-level diagram for an ${\mathrm{Ir}}^{4+}$ ion in an octahedral crystal field. (c) and (d) RIXS data collected at high pressures between an energy loss of $-1.0$ and 6.0 eV, and between 0.0 and 1.0 eV, respectively. The incident energy is 11.216 KeV.

Figure 3

(a) Energies of crystal-field excitation $10Dq$ measured from peak $D$ and the spin-orbit exciton peak $C$ in RIXS as a function of pressure. (b) Pressure dependences of the RIXS spectral line widths of peaks $D$ and $C$. (c) Crystal-field excitation energy $10Dq$ versus the lattice parameter $a$ from diffraction measurements. Areas of different color represent phases with different structures. (d) The fitted $ab$ plane Ir-O bond length and the Ir-O-Ir bond angle. The inset displays the orthorhombic structure of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ viewed along the $c$ axis in the low-pressure, insulating phase.