Physical interpretation of the quantum two-stream instability

The unexpected features of the two-stream instability in electrostatic quantum plasmas are interpreted in terms of the coupling of approximate fast and slow waves. This is accomplished through the factorization of the dispersion relation into different sectors having positive or negative energy. Therefore, the concept of negative and positive energy waves is useful not only for classical, but for quantum plasmas as well. We discuss the limitations of the quantum two-stream model in view of the weak coupling assumption.