Collective modes of spin, density, phase, and amplitude in exotic superconductors

D. van der Marel

doi:10.1103/PhysRevB.51.1147

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Collective modes of spin, density, phase, and amplitude in exotic superconductors

D. van der Marel

DOI 10.1103/PhysRevB.51.1147 · Physical Review B

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Abstract

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.