Ultrafast optical spectroscopy of the lowest energy excitations in the Mott insulator compound YVO${}_3$: Evidence for Hubbard-type excitons

Revealing the nature of charge excitations in strongly correlated electron systems is crucial to understanding their exotic properties. Here we use broadband ultrafast pump-probe spectroscopy in the visible range to study low-energy transitions across the Mott-Hubbard gap in the orbitally ordered insulator YVO${}_3$. Separating thermal and nonthermal contributions to the optical transients, we show that the total spectral weight of the two lowest peaks is conserved, demonstrating that both excitations correspond to the same multiplet. The pump-induced transfer of spectral weight between the two peaks reveals that the low-energy one is a Hubbard exciton, i.e., a resonance or a nearly bound state between a doublon and a holon. Finally, we speculate that the pump-driven spin disorder can be used to quantify the kinetic energy gain of the excitons in a ferromagnetic environment.

Figure 1

(a) Reflectivity for $E\left|\right|c$ as a function of temperature of YVO${}_3$ obtained by ellipsometry data. (b) The ratio between the spectral weight of the excitonic and single-particle bands increases rapidly while entering the G-OO phase and increases further at the spin ordering temperature. (c) Both the SP and the HE spectral weights increase upon cooling. In (b) and (c) dashed lines represent the transition temperatures towards the orbital ordering ($T_{\mathrm{OO}}=200$ K) and the additional spin ordering ($T_{\mathrm{SO}}=116$ K). In (b) the ratio is normalized to the value at 300 K. Note that the reflectivity measurements in (a) are displaced for clarity from the measurement at 80 K.

Figure 2

Dielectric constant of YVO${}_3$ for $E\left|\right|c$ at 80 and 300 K. (a) Imaginary and (b) real parts of the dielectric constant of YVO${}_3$ measured by ellipsometry[22] along the $c$ axis at 80 and 300 K. Dashed vertical lines mark the positions of the central frequencies of the oscillators used to perform the static fitting procedure (see text).

Figure 3

Broadband transient reflectivity spectra. Transient reflectivity as a function of energy and pump-probe delay for the three phases: (a) disordered, $T=300$ K; (b) G-type orbital order, $T=140$ K; and (c) C-type spin order, $T=80$ K. Gray lines represent the transient reflectivity at the fixed energy of 2.33 eV as retrieved in standard single-color pump-probe measurements. The oscillatory trend was assigned to acoustic vibrations[48] and is ignored. For (a), (b), and (c), the respective ordering patterns are sketched in (d), (e), and (f).

Figure 4

From transient reflectivity to spectral weight. The transient reflectance was fitted using a variational approach on the static fit. (a, b, c) Transient reflectance for two characteristic times, typical of the “fast” and “slow” dynamics, at 300, 140, and 80 K, respectively, and the relative variational fit. Insets: Static reflectance for the different phases.[22] (d, e, f) Time evolution of the SW variations in the different phases (see text).

Figure 5

Long-time-scale pump-probe measurement at 80 K. (a) Time-domain reflectivity data at 80 K. Note that the fast response visible in Fig. 2 is absent because of the coarse temporal step used. (b) Transient reflectance at $t=40$ ps (red curve) and 900 ps (black curve). The expected thermal response, [$R^{\mathrm{static}}\left(85\mathrm{K}\right)-R^{\mathrm{static}}\left(80\mathrm{K}\right)]/R^{\mathrm{static}}\left(80\mathrm{K}\right)$, is shown for comparison [dotted (green) curve].

Figure 6

Nonthermal spectral weight (SW) changes of the HE and SP peaks. The excitonic nature of the low-energy transition is revealed by the direct SW exchange between the two excitations (see text). The error bars were estimated from the fitting procedure.

Figure 7

Nonthermal spin disorder. YVO${}_3$ is in the G-OO/C-SO phase; $t$ indicates the pump-probe delay ($t<0$ represents the unperturbed state). The photoexcited holons and doublons perturb the magnetic coupling locally along the FM chain $J_c^{*}$ ($J_c^{{}^{\prime },*}$) ($t=0$, central site). The photoexcited electrons relax within a few picoseconds ($t\approx t_1$), leaving a local perturbation on the spin system. The spin disorder diffuses along the FM chains on a longer time scale ($t=t_2$), resulting in SW transfer between the HE and the SP peaks (see text).