Spin-gap mode in the charge-ordered phase of NaV2O5 studied by Raman scattering under high pressures

By means of Raman-scattering measurement under pressures at low temperatures we study the ``devil's staircase''-type phase of ${\text{NaV}}_{2}{\text{O}}_{5}$. The spin-gap mode shows a drastic softening with increasing pressure up to 0.9 GPa in ${\text{C}}_{1/4}$ phase and disappears between 0.9 and 1 GPa. It appears again between 1 and 2.3 GPa in ${\text{C}}_{0}$ phase, indicating that this phase is also a spin-gap state. Taking the charge ordering into consideration, we discuss the spin-gap states and clarify that the spin-gap mode is created by the exchange-interaction Raman-scattering mechanism and it comes from a gap between the spin-singlet ground state and the ${S}_{x}=0$ spin-triplet excited one. This model explains that the spin gap is almost independent of the applied magnetic field and it vanishes at about $10\ifmmode^\circ\else\textdegree\fi{}$ lower than the critical temperature in ${\text{C}}_{1/4}$ phase.