Attractive interaction between electrons (or neutral fermions) is responsible for superconductivity (or superfluidity).In condensed matter systems,attractive interaction is usually induced by the boson-exchange mechanism [1].Indeed,in the celebrated Bardeen-Cooper-Schreiffer (BCS) theory,electrons develop attractive interaction by exchanging phonons [2].Subsequent studies show that other collective excitations such as charge density waves [3] and spin fluctuations [4] can also induce attractive interaction.The bosonexchange mechanism,together with the concept of Cooperpair,is considered to be the cornerstone of the modern theory of superconductivity.