Spin-Orbital Excitation Continuum and Anomalous Electron-Phonon Interaction in the Mott Insulator ${\mathrm{LaTiO}}_3$

Raman scattering experiments on stoichiometric, Mott-insulating ${\mathrm{LaTiO}}_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800{\mathrm{cm}}^{-1}$ upon cooling below the Néel temperature $T_N=146\mathrm{K}$. In the antiferromagnetic state, the spectral weight below the gap is transferred to well-defined spectral features due to spin and orbital excitations. Low-energy phonons exhibit pronounced Fano anomalies indicative of strong interaction with the electron system for $T>T_N$, but become sharp and symmetric for $T<T_N$. The electronic continuum and the marked renormalization of the phonon lifetime by the onset of magnetic order are highly unusual for Mott insulators and indicate liquidlike correlations between spins and orbitals.

Figure 1

Raman spectrum of ${\mathrm{LaTiO}}_3$ at various temperatures measured in $zz$ polarization. Note the logarithmic scale of the energy axis.

Figure 2

Temperature dependence of the intensity of the electronic Raman signal recorded (a) in the phonon range at $155{\mathrm{cm}}^{-1}$ and (b) at the orbital excitation energy $2000{\mathrm{cm}}^{-1}$. The background was determined at $4000{\mathrm{cm}}^{-1}$. Pronounced changes are observed for both energies at $T_N$ (vertical dashed lines). The red solid lines are guides to the eye.

Figure 3

(a) Low-energy phonon part of the Raman spectrum of ${\mathrm{LaTiO}}_3$ at various temperatures. See the Supplemental Material for the mode assignment [18]. (b) Low-energy phonons above and below $T_N$. The lines are the results of fits to Fano profiles. (c) Temperature dependence of the Fano asymmetry parameter. The vertical line indicates $T_N$. The solid lines are guides to the eye.

Figure 4

Temperature dependence of (a),(c),(e) peak energy and (b),(d),(f) linewidth (FWHM) of selected $A_g$ phonon modes and (g),(h) the first-order magnon at $300{\mathrm{cm}}^{-1}$ (37.2 meV). The red lines correspond to the temperature dependence in case of anharmonic phonon-phonon interactions. The linewidths of all observed phonons decrease below $T_N$ (vertical dashed lines), and the phonon energies deviate strongly from the behavior expected from pure phonon-phonon interactions below this temperature. The red solid lines for the magnon serve as guides to the eye.