  In the rough phase, the width of interfaces separating different phases of
statistical systems increases logarithmically with the system size. This
phenomenon is commonly described in terms of the capillary wave model, which
deals with fluctuating, infinitely thin membranes, requiring ad hoc cut-offs in
momentum space. We investigate the interface roughening from first principles
in the framework of the Landau-Ginzburg model, that is renormalized field
theory, in the one-loop approximation. The interface profile and width are
calculated analytically, resulting in finite expressions with definite
coefficients. They are valid in the scaling region and depend on the known
renormalized coupling constant.
