Low-energy excitations in the magneto-optical response of α^′-NaV₂O₅

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 α^′-NaV₂O₅. 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-T phase diagram and find a temperature-independent phase boundary at ∼27 T (H_c) which persists above 34 K [T_c(0)], a kink in T_c(H) near 27 T, and H² behavior of T_c(H) in the high-field phase with prefactor α=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.