In the laboratory, at temperatures close to absolute zero, the electrons in a CeRu₄Sn₆ crystal behave as if they were ignoring all the old rules. An international team of researchers has recorded a topological semimetallic phase there — a state that until now was considered incompatible with the regime of quantum criticality.
Quantum criticality arises when a material sits at the boundary of a phase transition. Instead of familiar particles, the electrons are described by collective fluctuations resembling a turbulent sea of waves. It was previously believed that in such a "sea" a stable topological structure of electronic states simply would not survive.
The experiment refuted this idea. The scientists cooled a sample of CeRu₄Sn₆ almost to zero and passed a current through it. With no external magnetic field whatsoever, an anomalous Hall effect arose — a deflection of the charge carriers to one side. This effect serves as direct proof of the topological nature of the state.
The physicist Qimiao Si of Rice University noted that strong quantum fluctuations do not destroy topology but, on the contrary, help it to form and take hold. The maximum topological response was observed precisely where the instability of the electrons was highest.
The discovery opens the way to new materials that combine sensitivity to external influences with the robustness of quantum properties. Such compounds could prove useful in quantum computers, precision sensors and energy-efficient electronics.
The researchers are now checking whether a similar phase occurs in other compounds.
