  Nambu-Goldstone modes in immiscible two-component Bose-Einstein condensates
are studied theoretically. In a uniform system, a flat domain wall is
stabilized and then the translational invariance normal to the wall is
spontaneously broken in addition to the breaking of two U(1) symmetries in the
presence of two complex order parameters. We clarify properties of the
low-energy excitations and identify that there exist two Nambu-Goldstone modes:
in-phase phonon with a linear dispersion and ripplon with a fractional
dispersion. The signature of the characteristic dispersion can be verified in
segregated condensates in a harmonic potential.
