The Kubo formula is an equation which expresses the linear response of an observable quantity due to a time-dependent perturbation. Among the numerous applications of linear response formula, one can mention charge and spin susceptibilities of, for instance, electron systems due to external electric or magnetic fields. Responses to external mechanical forces or vibrations can also be calculated using the very same formula.
Consider a quantum system described by the (time independent) Hamiltonian . The expectation value of a physical quantity, described by the operator , can be evaluated as:
where is the partition function. Suppose now that just above some time an external perturbation is applied to the system. The perturbation is described by an additional time dependence in the Hamiltonian: where is the Heaviside function ( = 1 for positive times, =0 otherwise) and is hermitian and defined for all t, so that has for positive again a complete set of real eigenvalues But these eigenvalues may change with time.
However, one can again find the time evolution of the density matrix rsp. of the partition function to evaluate the expectation value of
The time dependence of the states is governed by the Schrödinger equation which thus determines everything, corresponding of course to the Schrödinger picture. But since is to be regarded as a small perturbation, it is convenient to now use instead the interaction picture representation, in lowest nontrivial order. The time dependence in this representation is given by where by definition for all t and it is:
To linear order in , we have . Thus one obtains the expectation value of up to linear order in the perturbation.
The brackets mean an equilibrium average with respect to the Hamiltonian Therefore, although the result is of first order in the perturbation, it involves only the zeroth-order eigenfunctions, which is usually the case in perturbation theory and moves away all complications which otherwise might arise for .