Optical study of phonons and electronic excitations in tetragonal Sr${}_2$VO${}_4$

We report on the optical excitation spectra in Sr${}_2$VO${}_4$. The phonon modes are assigned and their evolution with temperature is discussed in the frame of the different phase transitions crossed upon cooling. Besides the expected infrared-active phonons, we observe two additional excitations at about 290 and 840 cm${}^{-1}$, which could correspond to electronic transitions of the V${}^{4+}$ ions. Our experimental results are discussed in the context of recent experimental and theoretical studies of this material with a unique spin-orbital ground state.

Figure 1

(a) Specific heat divided by temperature. (b) and (c) Magnetic susceptibility measured in $0.2$ T.

Figure 2

(a) Reflectivity measured in far and near infrared. (b) Real (${\epsilon }_1$) and imaginary (${\epsilon }_2$) part of the dielectric function measured by ellipsometry. (c) Real part of the optical conductivity (${\sigma }_1$) for selected temperatures.

Figure 3

Low-energy part of the (a) reflectivity and (b) optical conductivity for different temperatures. The optical-phonon frequencies are indicated by filled circles. Interorbital optical excitations ${\omega }_1$, ${\omega }_2$, and ${\omega }_3$ are marked by arrows. The dotted line corresponds to the Lorentz fit at 13 K (dotted curves were shifted for reasons of clarity).

Figure 4

Left panels: Temperature dependence of the energy of phonon modes as numbered in Table . Right panels: Associated ionic displacement and naming.

Figure 5

Color map of the experimental optical conductivity in the energy range corresponding to the low-energy set of transitions. The open symbols represent the temperature dependence of the central frequency of the Lorentz oscillator used to fit the reflectivity data. $T_1$ and $T_2$ correspond to the maxima of the two pronounced peaks in the specific heat.

Figure 6

$\eta$ dependence of electron level splitting due to the effects of the tetragonal crystal field, spin-orbit interaction, and antiferromagnetic and ferromagnetic superexchange. Optical transitions are indicated as arrows.