Phonon-assisted optical excitation in the narrow bandgap Mott insulatorSr3Ir2O7

Abstract

We examined the temperature (T ) evolution of the optical conductivity spectra of Sr 3 Ir 2 O 7 over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of 0.1 eV. The low-energy features of the optical d-d excitation ( ω < 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge x-ray-absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr 3 Ir 2 O 7 .

Strong pseudospin-lattice coupling in Sr3Ir2O7 : Coherent phonon anomaly and negative thermal expansion

“…Ultrafast laser pulses of 800 nm central wavelength, 70 fs pulse duration, and 250 kHz repetition rate are used to generate and detect coherent phonons in Sr3Ir2O7. The 1.55 eV photons excite the transitions from the Jeff = 3/2 valence band to the Jeff = 1/2 conduction band [20] (see Appendix A). The polarizations of the pump and probe laser beams are chosen to be perpendicular to each other to minimize the noise.…”

Spin–lattice and electron–phonon coupling in 3d/5d hybrid Sr3NiIrO6

“…This model can be used to determine which phonon activates the electronic transition. While phonon-assisted d-to-d excitations have been reported in Sr 3 Ir 2 O 7 [46], quantitative vibronic coupling analyses such as we do here are rare in 4-and 5d-systems and particularly so in 3d/5d hybrids. Figure 2(b) displays the oscillator strength analysis of the Ir 4+ on-site excitations in Sr 3 NiIrO 6 .…”

Infrared Spectroscopic Evidences of Strong Electronic Correlations in (Sr1−xLax)3Ir2O7

“…2(a,b), the conductivity spectra at high temperatures are significantly different from the data at 10 K. The increase in temperature leads to the filling of the optical gap as well as the redshift of the peak α . This behavior was found not to be associated with the changes in the bare electronic structure but with the variation of the contribution from the thermal carriers23.…”

“…1(a). As temperature increases, R ( ω ) below 0.3 eV is enhanced accompanying a change in its slope; at higher temperatures R ( ω ) rises toward lower frequency implying the onset of an electronic contribution due to thermally-induced carriers23. Two broad peaks at about 0.4 and 0.8 eV represent the optical excitations between the J eff bands, which will be discussed in detail later.…”

“…The optical gap decreases progressively with increasing temperature accompanying the spectral weight transfer from the energy region above 0.3 eV to lower energies. This temperature dependence was attributed to the enhancement of the phonon-assisted optical excitations23.…”