SUM-RULE FOR THE OPTICAL HALL ANGLE

We consider the optical Hall conductivity of a general electronic medi um and prove that the optical Hall angle obeys a new sum rule. This su m rule governs the response of an electronic fluid to a Lorentz force and can be thought of as a counterpart to the f-sum rule in optical co nductivity. The physical meaning of this sum rule is discussed, giving a number of examples of its application to a variety of electronic me dia.