  In this study, we performed high-magnetic-field magnetization, dielectric,
and ultrasound measurements on an organic salt showing a ferroelectric
spin-Peierls (FSP) state, which is in close proximity to a quantum critical
point. In contrast to the sparsely distributed gas-like spin solitons typically
observed in conventional spin-Peierls (SP) states, the FSP state exhibits dense
liquid-like spin solitons resulting from strong quantum fluctuations, even at
low fields. Nevertheless, akin to conventional SP systems, a
magnetic-field-induced transition is observed in the FSP state. In conventional
high-field SP states, an emergent wave vector results in the formation of a
spin-soliton lattice. However, in the present high-field FSP state, the strong
quantum fluctuations preclude the formation of such a soliton lattice, causing
the dense solitons to remain in a quantum-mechanically melted state. This
observation implies the realization of a quantum liquid--liquid transition of
topological particles carrying spin and charge in a ferroelectric insulator.
