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

Abstract

We report on the optical excitation spectra in Sr2VO4. 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 cm −1 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.

Alternating spin-orbital order in tetragonal Sr2VO4

“…11 Specific heat data revealed two distinct broad maxima occurring at 98 and 127 K mirroring the borders of the two-phase regime. 18,19 The disappearance of the high-temperature phase is accompanied by a significant drop in the susceptibility at about 100 K, which has been attributed to the onset of long-range AFM and orbital order. 11 Theoretically, the ground state of Sr 2 VO 4 has been interpreted in terms of stripe-like orbital and collinear AFM spin order, 10 or an ordering of magnetic octupoles.…”

“…13 Recent optical experiments reported excitations at 31 meV (visible for T > 80 K), 36 meV (visible for T < 120 K) and a two-peak structure at 100 meV and 108 meV, which remains visible from 13 K to room temperature. 19 To elucidate the nature of the ground state and to understand the observed excitation spectrum we consider the effects of spin-orbit coupling, crystal-field, and superexchange on the energy levels of the vanadium ions. The resulting free energy points towards a novel alternating spin-orbital order in the ground state.…”

“…However, this choice of ϑ = ±π/2 cannot explain the observed splitting of the highest-lying doublet ε 4,6 which was observed by neutron scattering and optical spectroscopy. 13,19 Now let us turn to the ground state ε 1 . A minimum in energy of this level will occur at ϑ = ±π/2 only if 3|J…”

“…2(a). 13,19 In Fig. 2(b) we compare the free energy per vanadium site at 10 and 100 K (close to T N ) as a function of the parameter ϑ using the values for the exchange constants estimated above: There is a minimum at ϑ = 0 and ϑ = ±π in both cases and the energy difference with respect to ϑ = ±π/2 decreases from 1.5 meV to 0.5 meV, respectively.…”