Chain formation in a monolayer of dipolar hard spheres under an external field

Abstract

The phase behavior of a monolayer of dipolar hard spheres under an external field, which makes all dipoles of the monolayer orientate along its direction, is investigated. Using integral equation theory in the reference hypernetted chain (RHNC) approximation we calculate the correlation functions, which are used to obtain the response matrix of grand potential with respect to density fluctuations. The smallest eigenvalue of this response matrix determines the stability of the monolayer. When the smallest eigenvalue approaches zero, the monolayer becomes unstable and the corresponding eigenvector characterizes this instability. At dilute densities, with decreasing temperature the dipoles of the monolayer begin to form chains and simultaneously condensate. At medium and high densities, however, the dipoles of the monolayer have a stronger tendency to form dipolar chains with decreasing temperature and there is no condensation. The part of specific heat related to potential energy is investigated and found to increase sharply near the temperature of dipolar chain formation. This is in accordance with a sharp decrease in potential energy induced by the formation of a dipolar chain.