Phase transitions, such as the transition between a solid and a liquid, play an important role in condensed matter physics. Since the number of participating particles is huge, it is practically impossible to predict the exact behaviour of a material near a phase transition: this is what makes them so intriguing.
An extra fascinating class of phase transitions consists of those transitions in which not thermal fluctuations, but quantum fluctuations drive the change in the material. These quantum phase transitions are maintained even at zero temperature. In general it is very difficult to probe a quantum phase transition experimentally, as in practice it is often overshadowed by more mundane effects.
Ranko Toskovic and coworkers now report in Nature Physics that they have succeeded in designing and building tiny ‘materials’ consisting of only a few atoms, exactly in such a way that they display the beginnings of quantum criticality. The materials consist of magnetic atoms that prefer to align in an alternating fashion. Only when a magnetic field of 6 Tesla is applied, they collectively surrender and point in the same direction. This transition is not sudden, but consists of a number of discrete quantum jumps, which the researchers could observe in detail. Together, these jumps constitute the start of a quantum phase transition.
Scanning tunnelling spectroscopy measurements taken on each atom of chains with lengths ranging from one (left) to six atoms (right). At predicted magnetic field strengths (red dashed lines) the spectroscopic features show sudden jumps. These jumps get closer as the critical field value of 6 Tesla is reached, after which the transition to the paramagnetic phase is complete.