Different kinds of phase transitions depend on parameters such as a magnetic field. These phase transitions are particularly essential to understand the quantum properties of materials, mainly when they directly occur at the absolute zero points of temperature.

These transitions are called “quantum phase transitions” or a “quantum critical points.” Now, scientists from the Vienna University of Technology have discovered an unusually pristine form of quantum-critical point in a novel material. Scientists are now studying the properties of this material.

According to scientists, the material could be a so-called Weyl-Kondo semimetal.

Prof. Silke BÃ¼hler-Paschen from the Institute of Solid State Physics at TU Wien said,Â *“Usually quantum critical behavior is studied in metals or insulators. But we have now looked at a semimetal. The material is a compound of cerium, ruthenium, and tinâ€”with properties that lie between those of metals and semiconductors.”*

Wesley Fuhrman, a Ph.D. student in Prof. Collin Broholm’s team at Johns Hopkins University, who made an essential contribution to the result with neutron scattering measurements, said, *“Usually, quantum criticality can only be created under particular environmental conditionsâ€”a certain pressure or an electromagnetic field. Surprisingly, however, our semimetal turned out to be quantum critical without any external influences at all.”*

*“Normally, you have to work hard to produce the appropriate laboratory conditions, but this semimetal provides the quantum criticality all by itself.”*

BÃ¼hler-Paschen said, *“This surprising result is probably related to the fact that the behavior of electrons in this material has some special features. It is a highly correlated electron system. This means that the electrons interact strongly with each other and cannot explain their behavior by looking at the electrons individually. This electron interaction leads to the so-called Kondo effect. Here, a quantum spin in the material is shielded by electrons surrounding it, so that the spin no longer has any effect on the rest of the material.”*

If there are only relatively few free electrons, as is the case in a semimetal, then the Kondo effect is unstable. This could be the reason for the quantum critical behavior of the material: the system fluctuates between a state with and a state without the Kondo effect, and this has the effect of a phase transition at zero temperature.

Silke BÃ¼hler-Paschen said, *“We suspect that the quantum criticality we observed favors the occurrence of such Weyl fermions. Quantum critical fluctuations could, therefore, stabilize Weyl fermions in a similar way to critical quantum fluctuations in high-temperature superconductors holding superconducting Cooper pairs together. This is a very fundamental question that is the subject of a lot of research around the world, and we’ve discovered a hot new lead here.”*

Scientists specifically found that the newly discovered material has tightly intertwined specific quantum effectsâ€”namely quantum critical fluctuations, the Kondo effect, and Weyl fermions.

BÃ¼hler-Paschen said, *“This could lead to the establishment of a design concept with which such materials can be specifically improved, tailored, and used for concrete applications.”*

##### Journal Reference:

- Wesley T. Fuhrman et al., Pristine quantum criticality in a Kondo semimetal, Science Advances (2021). DOI: 10.1126/sciadv.abf9134