  Changing the interactions between particles in an ensemble-by varying the
temperature or pressure, for example-can lead to phase transitions whose
critical behaviour depends on the collective nature of the many-body system.
Despite the diversity of ingredients, which include atoms, molecules, electrons
and their spins, the collective behaviour can be grouped into several families
(called 'universality classes') represented by canonical spin models1. One kind
of transition, the Mott transition2, occurs when the repulsive Coulomb
interaction between electrons is increased, causing wave-like electrons to
behave as particles. In two dimensions, the attractive behaviour responsible
for the superconductivity in high-transition temperature copper oxide3,4 and
organic5-7 compounds appears near the Mott transition, but the universality
class to which two-dimensional, repulsive electronic systems belongs remains
unknown. Here we present an observation of the critical phenomena at the
pressure-induced Mott transition in a quasi-two-dimensional organic conductor
using conductance measurements as a probe. We find that the Mott transition in
two dimensions is not consistent with known universality classes, as the
observed collective behaviour has previously not been seen. This peculiarity
must be involved in any emergent behaviour near the Mott transition in two
dimensions.
