Correlated vs. conventional insulating behavior in the ${\mathrm{J}}_{\mathrm{eff}}=\frac{1}{2}$ vs. $\frac{3}{2}$ bands in the layered iridate ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$

We employ molecular beam epitaxy to stabilize ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ thin films and utilize in situ angle-resolved photoemission spectroscopy to investigate the evolution of its electronic structure through the Néel temperature $T_{\mathrm{N}}$. Our measurements indicate that dispersions of the relativistic $J_{\mathrm{eff}}=1/2$ and 3/2 bands exhibit an unusual dichotomy in their behavior through the Néel transition. Although the charge gap survives into the paramagnetic state, only the $J_{\mathrm{eff}}=1/2$ state exhibits a strong temperature dependence and its gap softens with increasing temperature approaching $T_{\mathrm{N}}$, while the nearly fully occupied $J_{\mathrm{eff}}=3/2$ state which remains nearby in energy exhibits negligible changes with temperature.

Figure 1

(a) Structure view of ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. (b) Temperature dependence of the longitudinal in-plane resistivity of a 16-nm ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ film grown on ${\mathrm{PrScO}}_3$.

Figure 2

RHEED images of (a) the bare ${\mathrm{PrScO}}_3$ substrate and (b) after the growth of a 16-nm-thick ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ film, taken along the [110]${}_{\left(p\right)}$ azimuthal direction. (c) RHEED intensity curves integrated within each rectangle window. (d)–(f) The results for the [100]${}_{\left(p\right)}$ direction.

Figure 3

LEED images of a ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ film, taken along the [001] direction with an electron energy of (a) 150 and (b) 200 eV. (c) XRD $\theta -2\theta$ scan of a 16-nm-thick ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ film grown on ${\mathrm{PrScO}}_3$ substrate. Substrate peaks are marked with an asterisk.

Figure 4

Valence-band photoemission spectra at (a) $(0,0)$, (b) $(\pi ,0)$, and (c) $(\pi ,\pi )$ points for ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$, respectively, taken at 100 K to prevent charging, showing Ir $5d$ $t_{2\mathrm{g}}$ bands and O $2p$ bonding and nonbonding states at higher binding energies. (d) Magnified low-energy electronic structures along $(0,0)-(\pi ,0)-(\pi ,\pi )-(0,0)$ high-symmetry lines of the tetragonal Brillouin zone.

Figure 5

(a) Near-$E_{\mathrm{F}}$ ARPES spectral intensity plot along high-symmetry directions, taken at 100 K. Extracted EDC and MDC peak positions are overlaid with circles with error bars. DFT calculations including the spin-orbit coupling with $U$ of (b) 0 and (c) 2.5 eV. The $J_{\mathrm{eff}}$ character is calculated by projecting the eigenstates onto the $J_{\mathrm{eff}}=1/2$ and 3/2 bases in the atomic limit.

Figure 6

(a) Unsymmetrized isoenergy map at $0.4\pm 0.005$ eV binding energy, (b) crystal structure projected on (001), and (c) $E$ versus $k$ spectra along high-symmetry lines for ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. Dashed squares in (a) and (b) indicate the in-plane Brillouin zone and unit cell, respectively. (d)–(f) The corresponding data from a ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ thin film grown on LSAT are shown for comparison.

Figure 7

(a) Temperature-dependent EDCs at momentum positions A $\sim$ $(\pi /2,\pi /2)$, B $\sim$ $(\pi ,0)$, and C $\sim$ $(\pi ,\pi )$ as sketched in (c). The spectra were measured by cycling samples from 112 to 299 K and then back to 103 K. Fitting results (one Gaussian curve and a background) for 103-K data are also represented by black curves. (b) Temperature evolution of the peak position (upper panel) and width (lower panel) for A–C. (c) Symmetrized isoenergy map at 0.4 eV in the full Brillouin zone, integrated within a $\pm 5$-meV window.

Figure 8

Schematic illustration of the temperature dependence of (a) the near-$E_{\mathrm{F}}$ band dispersion and (b) the in-plane spin state in ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$. Dashed curves represent the original branch without including the electron correlation. Energy dispersions affected by (c) weak or (d) strong correlation are also shown.