Negative permittivity in bubble and stripe phases

The physics of itinerant electrons in condensed matter is by and large governed by repulsive Coulomb forces. However, attractive interactions may emerge and prevail in determining the ground state despite the pervasive Coulomb repulsion. A notable example is electron pairing and superconductivity. The interplay of attractive and repulsive interactions may also instigate spontaneous symmetry lowering and clustering of charges in geometric patterns even without net attraction. Both types of attractive interaction triggered physics-pairing and charge ordering-are at play in two-dimensional electron systems exposed to a quantizing magnetic field. The charge ordering has been concluded indirectly from transport behaviour. Here we report the observation of negative permittivity present solely when bubble and stripe phases form. In conjunction with a theoretical model, the negative permittivity provides evidence for the underlying attractive exchange-correlation energy which suffciently countervails Coulomb repulsion at small distances to enable and mediate charge clustering.