Heavy carrier doping by hydrogen in the spin-orbit coupled Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$

Highly efficient carrier doping into the spin-orbit coupled Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is achieved by low-energy hydrogen ion beam irradiation at low temperature. We demonstrate that heavy doping of hydrogen into a ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ epitaxial thin film induces a large increase in conductivity by band-filling control via electron doping, which is confirmed by Hall effect measurements. The introduction of a large amount of hydrogen and its distribution along the depth direction are clarified by nuclear reaction analysis. The doped interstitial and substitutional hydrogens act as electron donors with minimum perturbation to the lattice, as evidenced by crystal structural analysis and first-principles calculations of the defect formation energy for doped hydrogen. The hydrogen-doping method offers a strategy toward realization of novel quantum phases in strongly correlated spin-orbit entangled systems.

Figure 1

X-ray diffraction $2\theta \text{−}\theta$ scan of the ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin film before (a) and after (b) hydrogen ion beam irradiation. The diffraction was measured at room temperature. The asterisks denote the $\left(00n\right)$ reflections from the LSAT substrate. (c) and (d) Reciprocal space mapping around the $\left(11\underset{-}{18}\right)$ reflection of the ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and the $\left(103\right)$ reflection of the LSAT substrate.

Figure 2

(a) Temperature dependence of resistivity of pristine (top) and hydrogen-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin films, in which hydrogen with an acceleration voltage of 2.5 kV was implanted in incremental doses at 100 K. The resistivity showed reversible temperature dependences below 100 K after the irradiations. A subsequent heating to 300 K after the final irradiation induced irreversible change in resistivity. (b) Dose dependence of resistivity under the hydrogen ion beam irradiation (Irra.) with an acceleration voltage of 2.5 kV at 100 K.

Figure 3

(a) Carrier density and (b) mobility of the pristine (triangles) and hydrogen-doped (squares) ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin film obtained via ex situ Hall effect measurements. The magnetic field was applied along the $c$ axis. The total hydrogen dose of $2.5\times {10}^{16}{\mathrm{H}}_2\mathrm{ions}/{\mathrm{cm}}^2$ was applied to the hydrogen-dosed ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin film as shown in Fig. 2.

Figure 4

Depth profile of hydrogen in the hydrogen-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin film. The hydrogen was implanted with an acceleration voltage of 2.5 kV at 100 K until the resistivity reached saturation. conc., concentration.

Figure 5

Absorption coefficient spectra of the pristine and hydrogen-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin films.

Figure 6

Optimized crystal structures in first-principles calculations of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ with several types of defects: (a) hydrogen replacing the equatorial oxygen, (b) interstitial hydrogen near the apical oxygen, and (c) interstitial hydrogen in the ${\mathrm{IrO}}_2$ plane. The lines show the periodic cell in the calculation. These pictures were drawn using vesta software [44].