Fingerprints of Mott and Slater gaps in the core-level photoemission spectra of antiferromagnetic iridates

Abstract

We present Ir $4f$ core-level hard-x-ray photoemission spectroscopy (HAXPES) experiments conducted across antiferromagnetic (AFM) ordering transition in Ruddlesden-Popper iridates Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$. The Ir $4f$ spectra exhibit distinct changes between the AFM and paramagnetic (PM) phases, with the spectral difference $I_\text{PM}-I_\text{AFM}$ showing a contrasting behavior in the two compounds. By employing computational simulations using the local-density approximation combined with the dynamical mean-field theory method, we elucidate that $I_\text{PM}-I_\text{AFM}$ primary reflects the Slater or Mott-Hubbard character of the AFM insulating state rather than material specific details. This sensitivity to fine low-energy electronic structure arises from the dependence of charge-transfer responses to the sudden creation of a localized core hole on both metal-insulator transitions and long-range AFM ordering. Our result broadens the applications of core-level HAXPES as a tool for characterization of electronic structure.

Figures (8)

• Figure 1: (a) HAXPES spectrum of Ir core levels in Sr$_{2}$IrO$_{4}$. (b) Temperature dependence of Ir $4f$ spectra for Sr$_{2}$IrO$_{4}$ and Sr$_{3}$Ir$_{2}$O$_{7}$. The spectra are normalized by the area under the curves after subtraction of the Shirley-type background Shirley72. The difference spectra are also shown.
• Figure 2: Ir core-level spectra calculated by the LDA+DMFT AIM for Sr$_{2}$IrO$_{4}$ (top) and Sr$_{3}$Ir$_{2}$O$_{7}$ (bottom) in the PM (red) and AFM (blue) solutions. The IrO$_6$ cluster-model results for the Ir $4f_{7/2,5/2}$ spectra are shown in the dashed lines. The inset shows the calculated spectra (Sr$_{2}$IrO$_{4}$) in a wide $E_B$ window.
• Figure 3: LDA+DMFT density of states of the Ir 5$d$$j_{\rm eff}=1/2$ state and core-level XPS spectra for (a--d, i--l) Sr$_{2}$IrO$_{4}$ and (e--h, m--p) Sr$_{3}$Ir$_{2}$O$_{7}$ calculated for different $\mu_{\rm dc}$ values (from top to bottom panels). The spectra in the PM (red curve) and AFM (blue curve) solutions for each $\mu_{\rm dc}$ value are shown. The results in blue (red) frames exhibit the Mott-Hubbard (Slater) type electronic structure with (without) charge gap in the PM solution.
• Figure 4: Experimental valence-band spectra of Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$ below and above $T_N$. (a) Total and (b) enlarged valence-band HAXPES spectra. SX-ARPES spectra reported in the previous study Yamasaki16 are also shown in (c).
• Figure 5: HAXPES spectra of Ir core levels at 100 and 300 K in Sr$_{3}$Ir$_{2}$O$_{7}$, shown before background subtraction. The difference spectrum is also shown. The inset shows the enlarged spectra around the Ir 4$f$ and 5$p_{1/2}$ core levels.