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      • Notes
      24 posts
    Manybody: Introduction to Green's Function
    • Mar 19, 2019
    • 15 minutes read
    Classnote for the class in PKU: 'Quantum Theory for Manybody System'. This note gives a review for theory of Green's function. Covers the definition of real-time Green's function of the many body theory at temperature is zero, and the thermal Green's function , or temperature Green's function. And discussion how to extract information from Green's function.
    Manybody: Coherent State
    • Feb 25, 2019
    • 18 minutes read
    This is the first classnote for the class in PKU: 'Quantum Theory for Manybody System'. This note gives a brief introduction to Grassmann numbers and then define the generic coherent states of Boson and Fermion, and discuss their properties. The coherent state is widely used in the path integral expression for many body system, or the field theory.
    Interacting Field Theory: Scattering Theory
    • Jan 12, 2019
    • 12 minutes read
    We will review the scattering theory which has been studied in quantum-mechanics. The main character is the S-matrix and how to compute cross section by it. Then we will introduce the Lehmann-Symanzik-Zimmermann reduction fomula, which will show the connection between time-ordered Green's function and S-matrix in scattering theory. The former can be computed by the Feynman Diagram and Feynman Rule which we have studied in the last section, and the latter is the only thing we can measure in experiments.