Low-energy excitations in the magneto-optical response ofα′−NaV2O5

Abstract

We employ the sensitivity of high-energy sidebands on the 1 eV absorption band to temperature and applied magnetic field to investigate the phase transition in ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}.$ Data were collected in steady (45 T) and pulsed (60 T) fields between 4 and 45 K. We use the spectral changes to map the $H\ensuremath{-}T$ phase diagram and find a temperature-independent phase boundary at $\ensuremath{\sim}27 \mathrm{T}$ ${(H}_{c})$ which persists above 34 K $[{T}_{c}(0)],$ a kink in ${T}_{c}(H)$ near 27 T, and ${H}^{2}$ behavior of ${T}_{c}(H)$ in the high-field phase with prefactor $\ensuremath{\alpha}=0.09.$ It is notable that the energy scale of ${H}_{c}$ is identical to that of ${T}_{c}(0).$ To understand the high-energy sidebands on the 1 eV band in the optical conductivity, we invoke the idea of low-energy excitations coupled with the optically induced charge transfer excitation. The 34 K phase transition is assigned as a crossover between thermal excitation over the on-rung potential barrier above ${T}_{c}$ and tunneling through the barrier below ${T}_{c},$ and ${H}_{c}$ is associated with a change in the on-rung potential, which modifies the low-energy excitation dynamics and may lead to the formation of field-induced ferromagnetic domains above 27 T.