Origin of Charge Density Wave Formation in Insulators from a High Resolution Photoemission Study of ${\mathrm{BaIrO}}_3$

We investigate the origin of charge density wave (CDW) formation in insulators by studying ${\mathrm{BaIrO}}_3$ using high-resolution (1.4 meV) photoemission spectroscopy. The spectra reveal the existence of localized density of states at the Fermi level, $E_F$, in the vicinity of room temperature. These localized states are found to vanish as the temperature is lowered, thereby, opening a soft gap at $E_F$, as a consequence of CDW transition. In addition, the energy dependence of the spectral density of states reveals the importance of magnetic interactions, rather than well-known Coulomb repulsion effect, in determining the electronic structure thereby implying a close relationship between ferromagnetism and CDW observed in this compound. Also, Ba core level spectra surprisingly exhibit an unusual behavior prior to CDW transition.

Figure 1

Valence band spectra at different excitation energies. The line represents the simulated O $2p$ contributions. The inset shows two ${\mathrm{Ir}}_3{\mathrm{O}}_{12}$ clusters with the long axis tilted by 12°.

Figure 2

(a) Ir $5d$ band at different excitation energies. The red line represents the resolution broadened 40.8 eV spectrum. (b) The temperature evolution of the Ir $5d$ band at different excitation energies. (c) Expanded 40.8 eV spectra close to $E_F$.

Figure 3

(a) High-resolution He i spectra near $E_F$ for ${\mathrm{BaIrO}}_3$ and Ag. (b) SDOS of ${\mathrm{BaIrO}}_3$ at different temperatures. SDOS for Ag at 300 K is shown by stars. (c) SDOS plotted as a function of $(\mathrm{BE}{)}^{1.5}$.

Figure 4

(a) Ba $3d_{5/2}$ and (b) Ba $4d_{5/2}$ spectra at different temperatures. The dashed lines are the subspectra as described in the text. The solid lines passing through 300 K spectra are the sum of the subspectra. (b) also shows Ir $4f$ spectra collected in the same scan with $4d$ spectra.