A path-integral formulation of the Pauli equation using two real spin coordinates

Abstract

Feynman's path-integral quantum-mechanical formulation is generalised for particles of spin 1/2. In the one-particle case, the path-integral formulation uses paths in a Euclidean real five-dimensional space, two coordinates (u, v) being reserved for spin. The path integral is proven to correspond exactly to the Pauli equation. A canonical density-matrix formulation is also dealt with. Basic ideas are to start with differential spin operators instead of the Pauli matrices and apply them to functions ψ=ψ ₁(r,t)u +gy ₂(r,t)v whereψ ₁,ψ ₂ are the Pauli wave functions. Then a ‘nilpotent’ spin ‘kinetic-energy’ term is added to the Hamiltonian. This enables us to find a non-matrix spin-dependent Lagrangian which is used as usual in the action of a path integral of the Feynman type. Integral relations are derived from which the path integral can be transformed into components of the Pauli matrix Green's function (propagator) or the canonical density matrix. As an example, a path-integral calculation of the normal Zeeman splitting is carried out.