April 23, 2024

A research team of US researchers and TU Wien discovered a surprising form of ‘quantum criticism.’ This could be used to design new materials.

Phase transitions are usually related to temperature changes in everyday life. For example, an ice cube melts when it gets warmer. Many types of phase transitions depend on other parameters, such as the magnetic field. Phase transitions are beneficial for understanding the quantum properties and materials. They can occur at absolute zero temperature, which is crucial to understand their quantum properties. These transitions are known as “quantum phase changes” or “quantum critical points.”

An Austrian-American research group discovered such a critical point in a novel material. It was also found in an unorthodoxly pure form. Further research is underway to determine the material’s properties. The material could be a Weyl-Kondo-semimetal. This is because of its potential for quantum technology, as it has particular quantum states (so-called topological states). If this is true, it would be possible to identify a key for topologically-developed quantum materials. These results were obtained in cooperation between TU Wien and Johns Hopkins University. They have been published in the journal Science Advances.

Quantum criticality — more accessible and more apparent than ever

“Quantum critical behavior is usually studied in metals and insulators. Professor Silke Buhler Paschen, Institute of Solid State Physics at TU Wien, says we now have looked at a semimetal. This material comprises cerium, ruthenium, and tin and has properties between metals and semiconductors.

Quantum criticality is usually only possible under certain environmental conditions, such as a specific pressure or electromagnetic field. “Surprisingly, however, our semimetal turned to be quantum critical regardless of any external influences at all,” Wesley Fuhrman, a Ph.D. candidate in Professor Collin Broholm’s Johns Hopkins University team, said. He made a significant contribution to the result through neutron scattering measurements. “Normally, you must work hard to create the right laboratory conditions. But this semimetal provides quantum criticality all on its own.”

This unexpected result may be due to the unique characteristics of the electron behavior in this material. It is a highly correlated electron network. Buhler-Paschen explains that electrons interact strongly with one another and that it is impossible to explain their behavior by simply looking at each electron individually. This electron interaction is what leads to the “Kondo effect.” This is where a quantum spin in a material is shielded from electrons surrounding it. The reel does not affect the rest.

The Kondo effect can be unstable if there are few free electrons (as with a semimetal). This could explain the quantum critical behavior of this material. The system oscillates between a state with and without the Kondo effect. This has the effect that a phase transition occurs at zero temperature.

Quantum fluctuations could result in Weyl particles.

The result is believed to be closely related to “Weyl fermions” in solids. Weyl fermions may appear as quasiparticles (i.e., collective excitations, such as waves in the pond. Theoretical predictions suggest that such Weyl fermions would exist in this material,” Qimiao Si, Rice University theoretical physicist, says. However, experimental proof is still to be found. Silke Buhler–Paschen says, “We suspect the quantum criticality that we observed favors such Weyl fermions.” “Quantum-critical fluctuations could stabilize Weyl fermions similarly to quantum critical changes found in superconducting Cooper pairs. This fundamental question has been the focus of much research all over the globe. We have a new hot lead.

We believe that quantum effects, such as the Kondo effect and critical quantum fluctuations, are closely intertwined within the newly discovered material. Together, they give rise to the exotic Weyl Kondo states. Unlike other quantum states, these “topological” states are stable and can’t be destroyed by external disturbances. These states are beautiful for quantum computers.

Further measurements will be taken under various external conditions to confirm this. They expect to find a similar interaction of the different quantum effects in other materials. Buhler-Paschen says this could result in a design idea with which these materials can be improved, tailored, and used for concrete applications.

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