Phase diagram of colloidal spheres in a biaxial electric or magnetic field.

Colloidal particles with a dielectric constant mismatch with the surrounding solvent in an external biaxial magnetic or electric field experience an "inverted" dipolar interaction. We determine the phase behavior of such a system using Helmholtz free energy calculations in Monte Carlo simulations for colloidal hard spheres as well as for charged hard spheres interacting with a repulsive Yukawa potential. The phase diagram of colloidal hard spheres with inverted dipolar interactions shows a gas-liquid transition, a hexagonal ABC stacked crystal phase, and a stretched hexagonal-close-packed crystal. The phase diagram for charged spheres is very similar, but displays an additional layered-fluid phase. We compare our results with recent experimental observations.