Possible Superconductive Pairing Symmetries On Honeycomb Lattice

Since superconductivity was found in 1911, it has always been the hotspot issue in condensed matter physics, and many materials have been recognized as superconductors, some of which are mediated by phonons, known as conventional superconductors. But the mechanism of unconventional superconductors, such as cuprate superconductors, are still in debate. Recently, researches of superconductivity on honeycomb-lattice draw much attention. Using random phase approximation (RPA), basing on spin-fluctuation mechanism, we research the possible superconductive pairing symmetries of correlated electron system on honeycomb lattice. Our calculation bases on the extended Hubbard model, with onsite Coulomb interaction U, nearest-neighbor (NN) Coulomb interaction V and ionic potential △ under different doping concentration δ, to study the superconductive pairing symmetry mediated by electronic spin and charge fluctuation. The results are as follows:A). t-U model:By changing δ∈ [5%,15%] and U∈ [1.0f,2.6t], we find that a spin-singlet dx2-y2+idxy-wave is shown to be the leading superconducting pairing symmetry when only the on-site Coulomb interaction V is considered, with the gap function being a mixture of the nearest-neighbor and next-nearest-neighbor pairings.B). t-U-V model:For fixed U, no matter δ=5% or 15%,the maximum eigenvalue of gap’s equation in spin-triplet channel can be larger than in spin-singlet channel when V exceeds a critical value, which means that the spin-triplet pairing channel is dominant. Meanwhile, the gap function shows f-wave symmetry, which is mainly composed of the next-nearest-neighbor pairing. We conclude that the NN Coulomb interaction V is against singlet pairing but beneficial to triplet pairing, and the main reasons are:V suppresses the electrons’pairing from two NN lattice points, while f-wave pairing here mainly comes from NNN pairing, so V affects little to the triplet pairing; However d+id-wave pairing owns much to NN pairing, which explains V’s suppression to singlet superconductivity.C). When the offset of the energy level(ionic potential) between the two sub-lattices exceeds a critical value, the most favorable pairing also changes into a spin-triplet f-wave pairing which is mainly composed of NNN pairing, in other words, pairing from the same sub-lattice. With introducing ionic potential, the balance of two sub-lattices’weight in each energy band of honeycomb lattice system is broken, one of which increases, and that makes for f-wave pairing from the same sub-lattice. So, the ionic potential between two sub-lattices is in favor of the spin-triplet f-wave pairing.