Experimental manifestations of the Nb${}^{4+}$-O${}^{-}$ polaronic excitons in KTa${}_{0.988}$Nb${}_{0.012}$O${}_3$

The formation of the photopolaronic excitons in ABO${}_3$ perovskite-type oxides has been detected experimentally by means of the photoinduced electron paramagnetic resonance (EPR) studies of KTa${}_{0.988}$Nb${}_{0.012}$O${}_3$ crystals. The corresponding microwave x-band spectrum at $T<$ 10 K consists of a narrow, nearly isotropic signal located at $g$ ∼ 2 and a strongly anisotropic component. The first signal, which has a rich structure due to hyperfine interactions with the lattice nuclei, is attributed to the single trapped charge carriers: the electrons and/or the holes. The anisotropic spectrum is caused by the axial centers oriented along the C${}_4$ pseudocubic principal crystalline axes. The spectrum angular dependence can be described well by an axial center with $S$ $=$ 1, $g_{\parallel }$ $=$ 0.82, $g_{\perp }$ $=$ 0.52, and $D$ $=$ 0.44 cm${}^{-1}$. The anisotropic spectrum is attributed to the Nb${}^{4+}$-O${}^{-}$ polaronic excitons. The temperature dependence of the anisotropic component is characterized by two activation energies: the internal dynamics activation $E_a$${}_1$ $=$ 3.7 $\pm$ 0.5 meV, which makes the EPR spectrum unobservable above 10 K, and the destruction energy $E_a$${}_2$ $=$ 52 $\pm$ 4 meV. By comparing the anisotropic photo-EPR spectrum and the photoinduced optical absorption temperature dependencies, we found that the Nb${}^{4+}$-O${}^{-}$ polaronic excitons also manifested themselves via the wide absorption band at ∼0.7 eV arising under ultraviolet light excitation in the weakly concentrated KTaO${}_3$:Nb crystals.

Figure 1

EPR spectra of the KTN-1.2 crystal at $T$ $=$ 4.0 K before UV illumination and under stationary illumination. $B_0$ forms an angle of θ ≈ 40° with the C${}_4$ axis in the C${}_4$-C${}_2$-C${}_4$ plane; $B_1$ ∥ C${}_4$.

Figure 2

Angular dependence of the photo-EPR spectrum at $T$ $=$ 4.0 K with $B_0$ in C${}_4$-C${}_2$-C${}_4$ plane. (a) θ $=$ 0° corresponds to $B_0$ ∥ C${}_4$. (b) The two magnetically nonequivalent centers are observed simultaneously where θ ≈ 45°.

Figure 3

EPR spectrum of the KTN-1.2 crystal at $T$ $=$ 4.0 K, $\Delta B_0$ ≈ 0.3 mT, and $B_0$ ∥ C${}_4$.

Figure 4

(a) Angular dependence of the resonance field $B_{\mathrm{res}}$(θ) for photo-EPR signal II. The dashed curves are the angular dependencies $B_{\mathrm{res}}=B_{\mathrm{res}}^0/\cos \theta$, with $B_{\mathrm{res}}^0=312.4$ mT. The solid lines indicate the angular dependence fits with the parameters given in the text. (b) Energy level scheme for the center, with $S$ $=$ 1 (θ $=$ 25°).

Figure 5

(a) Temperature dependence of signal II in the photo-EPR spectrum of KTN-1.2 at θ $=$ 30°. (b) Photo-EPR spectra observed after blocking the illumination at the temperatures shown in the graph and fast cooling of the sample in the dark down to $T$ $=$ 4.2 K.

Figure 6

Temperature dependencies of the photo-EPR signal intensity under cw illumination (squares) and after cooling from the given temperature to $T$ $=$ 4.2 K with the illumination switched off (circles; see text). Intensity of the photoinduced IR absorption of ∼0.7 eV is shown with triangles (Ref. 6). Solid curves are the fits of the data to Eq. (2).