Hidden quantum spin gap state in the static stripe phase of La₂₋ₓSrₓCuO₄

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Low energy spin excitations were investigated in the static stripe phase of La₂₋ₓSrₓCuO₄ using elastic and inelastic neutron scattering on single crystals. For x = 1/8 in which long-range static stripe order exists, an energy gap of Eg = 4 meV exists in the excitation spectrum in addition to strong quasi-elastic, incommensurate spin fluctuations associated with the static stripes. When x increases, the spectral weight of the spin fluctuations shifts from the quasi-elastic continuum to the excitation spectrum above Eg. The dynamic correlation length as a function of energy and the temperature evolution of the energy spectrum suggest a phase separation of two distinct magnetic phases in real space.