Abstract
This paper is a review of the present status of the problem of the properties of the dielectric function of matter. It is shown that the dielectric function obeys the Kramers-Kronig relations only when momentum q is identically equal to zero. For an arbitrary momentum the dielectric function does not obey the Kramers-Kronig relations, and the static dielectric function at may have a negative sign []. It is shown that the appearance of a negative sign for the static dielectric function does not contradict the requirements of system stability. Special attention is paid to the possible existence of a negative sign for the static dielectric function in real physical systems. It is shown that the inequality at is valid for a rather wide class of condensed media (simple metals, nonideal plasma, etc.). The interrelation between the existence of a negative sign for the dielectric function and the problem of high-temperature superconductivity is briefly discussed.
DOI:https://doi.org/10.1103/RevModPhys.53.81
©1981 American Physical Society