Manifestation of spin degrees of freedom in the double fractional quantum Hall system

The double fractional quantum Hall system of spin-1/2 electrons is studied numerically. We predict that the inclusion of spin degrees of freedom gives rise to a spin-unpolarized quantum liquid with the simplest example being at =4/7. The state, which emerges from an intimate link between the spin state and the interlayer electron correlation, and also survives the Zeeman effect up to a critical g factor, illustrates that the fractional quantum Hall liquid is versatile enough to accommodate both the doubled fermion species and spin. Even when the ground state is spin polarized as in =1, the lowest charge-excitation mode can involve the spin when the interlayer tunneling is considered.