The pairing habits of electrons

Researchers at RIKEN’s Advanced Science Institute in Wako, formerly the Discovery Research Institute, have discovered a novel mechanism by which an anion radical salt becomes superconductive at high pressures. Their discovery may help physicists to understand the transition between different quantum states in a broad class of materials.

Mott insulators are materials where constituent electrons cannot move due to the strong electronic repulsion between them, thereby impeding the flow of electrical current. Under external pressure, the electrons can be ‘set free’ and these insulators become conductive through a process known as Mott transition.

Reporting in the journal Physical Review Letters¹, the researchers have studied the Mott transition in the anion radical salt EtMe₃P[Pd(dmit)₂]₂. The anions of this salt form a triangular lattice, where each anion dimer occupies one of the corner positions (inset of Fig. 1). At low pressures and low temperatures, the electrons of the Mott insulator pair with a neighboring electron to form a rare quantum state: the valence-bond solid (VBS) state. This state contrasts with the antiferromagnetic state of conventional Mott insulators, where pairing does not occur. The researchers have now studied how this VBS state evolves at higher pressures.

In measuring the electrical resistance of the salt, the researchers mapped its phase diagram (Fig. 1). Importantly, they observed that at higher pressures, a Mott transition occurs from the insulating VBS state to either a superconducting or a metallic state. The transition from a VBS to a superconducting state, in particular, is of fundamental interest, and physicists have sought to observe it for decades, according to Yasuhiro Shimizu from the RIKEN team.

Owing to the rarity of VBS states, previous Mott transitions to superconducting states had been found only in materials where electrons do not form a VBS. Further, this first observation of a transition from the VBS state by the researchers is significant because electrons not only pair up to form the VBS state, but also the superconducting state. However, there are crucial differences between the electron pairs: in a VBS they sit tight at the anions, whereas in a superconductor they are free to roam the crystal.

In future work, Shimizu suggests that it will be important to study this transition in electron pairing in further detail, as “this will help to understand the electron pairing mechanism of superconductivity.” The unique properties of this anion radical salt certainly provide an exceptional platform from which to study the physics of quantum states like VBS and superconductivity.