Excitonic quenching of the oxygen-chain phonon in the photoinduced metal-to-insulator transition of photoexcited ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ studied by ultraviolet-resonance Raman scattering

The quasi-one-dimensional metal ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ shows a metal-to-insulator transition upon laser-induced pumping of the d-d exciton at 1.1 eV. The oxygen-chain-related phonon modes are highly susceptible to this metal-to-insulator transition due to their sensitivity to structural and electronic changes. We employ angle-dependent UV-resonance Raman spectroscopy to probe the nonequilibrium oxygen-chain-related phonon dynamics and the concomitant changes in the angle-dependent midinfrared reflectance. The latter shows the expected reduction of charge carrier density upon pumping, whereas the central chain mode at 680 $\mathrm{cm}{}^{-1}$ can be selectively quenched upon pumping the d-d excitons. We find that the underlying quenching mechanism for this phonon is electronically driven and related to the photo-induced bleaching of the charge-transfer transition. First-principles calculations show the connection between phonon dispersion and electronic band structure leading to the selective quenching of the oxygen-chain mode. Our results contribute to a deeper understanding of the interplay between lattice and charge degrees of freedom for exciton-based optoelectronics.

Figure 1

(a) Crystal structure of ($n=5$)-type ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$. Along the $c$ axis, networks of ${\mathrm{NbO}}_6$ octahedra are periodically grouped into 5-octahedra-thick slabs, while along the $a$ and $b$ axes, the Nb-O-Nb linkages are chainlike and zigzaglike, respectively. (b) Refractive index $n$ and extinction coefficient $k$ of ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ [18]. The extinction coefficient $k$ shows a strong Drude response at low energies for the $a$ axis, showing its metallic character. The energies of the MIR-Drude probe, excitonic pump, and Raman probe beams are marked with yellow, gray, and pink lines, respectively.

Figure 2

Nonequilibrium MIR-reflectance measurements and angle dependence of the UV-resonance Raman spectra taken at room temperature. (a) Absolute power during a pump-probe cycle for different polarization angles relative to the metallic $a$ axis of the crystal. The residual power when the pump is on is a stray light effect caused by the pump beam. (b) Reflectance of the unpumped system and the reflectance change upon pumping as a function of the angle between the MIR polarization and metallic $a$ axis fitted with a sinusoidal function. (c) Raman spectra of ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ at 260 nm with different angles between the metallic $a$ axis and the beam polarization. Note that the frequency analysis is shown in the Supplemental Material [23]. (d) Relative peak intensity changes as a function of the polarization of the incident beam and the metallic $a$ axis for the 640 $\mathrm{cm}{}^{-1}$ and 840 $\mathrm{cm}{}^{-1}$ modes, respectively.

Figure 3

Simulation of the phonon dispersion in ${\mathrm{SrNbO}}_{3.4}$ (a) Phonon dispersion for the investigated Raman active modes at 680 $\mathrm{cm}{}^{-1}$ and 840 $\mathrm{cm}{}^{-1}$ highlighted in red and blue, respectively. (b)–(d) Visualization of the relevant oxygen-related vibrations.

Figure 4

Resonant and nonequilibrium Raman spectroscopy taken at room temperature and band structure calculation. Raman spectrum of ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ at different incident photon energies with polarization along the (a) metallic and (b) insulating axes. (c), (d) Fit of the relevant peak intensities as a function of the incident photon energy. Dashed lines show the derivative of $k$ for the (c) metallic and (d) insulating directions. For $k$ itself, see Fig. 1. (e) Raman spectroscopy data under photoexcitation (reprinted from Ref. [18]) with the pump beam parallel to the $b$ axis (${90}^{\circ }$) and (f) fit of the corresponding peak intensities. (g) Band structure calculation of the electronic levels for resonance Raman scattering of ${\mathrm{SrNbO}}_{3.4}$ with the color-coded density of states of ${\mathrm{O}}_{2p}$ and ${\mathrm{Nb}}_{4d}$ orbitals in the central oxygen chain. White arrows indicate the electronic transitions for the Raman resonance and the excitonic pump.