COLLECTIVE MODES OF SPIN, DENSITY, PHASE, AND AMPLITUDE IN EXOTIC SUPERCONDUCTORS

The equations of motion of pairlike excitations in the superconducting state are studied for various types of pairing using the random-phase approximation. The collective modes are computed of a layered electron gas described by a t-t' tight-binding band, where the electrons experience besides the long-range Coulomb repulsion an on-site Hubbard U repulsion and a nearest-neighbor attractive interaction. From numerical calculations we see that the collective-mode spectrum now becomes particularly rich. Several branches can occur below the continuum of quasiparticle excitations, corresponding to order-parameter fluctuations of various symmetries of pairing, and collective spin-density fluctuations. From the collective-mode softening near the nesting vectors it is concluded that in the d-wave paired state an instability occurs toward the formation of a spin-density wave. © 1995 The American Physical Society.