Spin-Orbit Semimetal ${\mathrm{SrIrO}}_3$ in the Two-Dimensional Limit

We investigate the thickness-dependent electronic properties of ultrathin ${\mathrm{SrIrO}}_3$ and discover a transition from a semimetallic to a correlated insulating state below 4 unit cells. Low-temperature magnetoconductance measurements show that spin fluctuations in the semimetallic state are significantly enhanced while approaching the transition point. The electronic properties are further studied by scanning tunneling spectroscopy, showing that 4 unit cell ${\mathrm{SrIrO}}_3$ is on the verge of a gap opening. Our density functional theory calculations reproduce the critical thickness of the transition and show that the opening of a gap in ultrathin ${\mathrm{SrIrO}}_3$ requires antiferromagnetic order.

Figure 1

(a) Optical image of a Hall bar used for transport measurements. (b) HAADF-STEM image of a ${\mathrm{SrTiO}}_3/10\mathrm{u}\mathrm{.}\mathrm{c}\mathrm{.}$ ${\mathrm{SrIrO}}_3/{\mathrm{SrTiO}}_3$ heterostructure. (c) $R(T)$ and (d) $\rho (T)$ curves for films of different thicknesses. The inset shows the temperature of the resistance minimum ($T_{\min }$) as a function of thickness.

Figure 2

(a) Magnetoconductance $\mathrm{\Delta }\sigma =\sigma (B)-\sigma (0)$ in units of $e^2/\pi h$ measured in an out-of-plane magnetic field for films of different thicknesses. (b) $\mathrm{\Delta }\sigma$ fitted by the Maekawa-Fukuyama formula (solid red lines). (c) $l_{so}$ and $l_{\phi }$ extracted from the fits. (d) Relative susceptibility $\chi (t)/\chi (30\mathrm{u}\mathrm{.}\mathrm{c}\mathrm{.})$ versus thickness.

Figure 3

(a) Differential conductance $(dI/dV)$ spectra acquired on three different samples with film thicknesses of 4, 6, and 10 u.c.. Inset: STM topographic image of the surface of a 10 u.c. ${\mathrm{SrIrO}}_3$ film. (b) $dI/dV$ spectra measured in a smaller energy range.

Figure 4

Calculated electronic structure for (a) 4 and (b) 3 u.c. ${\mathrm{SrIrO}}_3$ films on tetragonal ${\mathrm{SrTiO}}_3$ with $U=1.50\mathrm{eV}$. Right: the corresponding DOS per formula unit as a function of energy.