A research team at the Vienna University of Technology and a US research institute have come across a surprising form of “quantum criticality.” This can lead to new material design concepts.
In everyday life, phase transitions are usually associated with temperature changes. For example, when the ice cubes get warm and melt. However, there are also different types of phase transitions, depending on other parameters such as the magnetic field. To understand the quantum properties of materials, phase transitions are of particular interest when they occur directly. absolute temperature Temperature point. These transitions are called “quantum phase transitions” or “quantum critical points.”
Such a quantum critical point was discovered by an Austrian-American research team in a new material, in a very pristine form. The properties of this material are currently being investigated further. The material may be a so-called Weyl-Kondo semimetal, which is thought to have great potential for quantum technology due to its special quantum state (so-called topological state). If this proves to be true, the key to targeted development of topological quantum materials will be found. The results were discovered in collaboration with TU Vienna, Johns Hopkins University, the National Institute of Standards and Technology (NIST), and Rice University and are now published in the journal. Science Advances..
The Importance of Quantum — Simpler and Clearer Than Ever
“Usually, quantum critical behavior has been studied in metals or insulators, but now we have considered semimetals,” said Silke Bühler-Paschen, a professor at the Institute of Solid State Physics at the Vienna University of Technology. This material is a compound of cerium, ruthenium and tin, with properties between metals and semiconductors.
Generally, quantum critical points can only be created under specific environmental conditions (specific pressures or electromagnetic fields). “But surprisingly, our metalloids turned out to be quantum critical with no external influence,” said a PhD student on the team of Professor Collimbroholm at Johns Hopkins University. Wesley Farmer says. Measurement. “Usually, you have to work hard to create the right laboratory conditions, but this metalloid provides quantum criticality on its own.”
This amazing result is probably related to the fact that the electronic behavior of this material has some special characteristics. “This is a highly correlated electronic system, which means that the electrons interact strongly with each other and cannot look at the electrons individually to explain their behavior,” says Buehler Paschen. “This electron interaction leads to the so-called Kondo effect. Here, the quantum spins in the material are shielded by the surrounding electrons, so the spins do not affect the rest of the material.”
The Kondo effect becomes unstable when there are relatively few free electrons, as in the case of semimetals. This may be the reason for the quantum critical behavior of the material. The system fluctuates between with and without the Kondo effect, which has the effect of a phase transition at zero temperature.
Quantum fluctuations can lead to Weil particles
The main reason the results are so centrally important is that they are suspected to be closely related to the “Weilfermion” phenomenon. In solids, Weilfermions can appear in the form of quasiparticles, that is, as collective excitations such as pond waves. According to theoretical predictions, such Weilfermions should be present in this material, “said Qimiao Si, a theoretical physicist at Rice University. However, no experimental evidence has yet been found. “The quantum criticality we have observed appears to favor the generation of such Weilfermions,” says Silke Bühler-Paschen. “Therefore, quantum critical fluctuations can have a stabilizing effect on Weilfermions, as well as quantum critical fluctuations in high-temperature superconductors that hold superconducting Cooper pairs together. It’s a very basic question that has been the subject of much research in, and I found a hot new lead here. “
Certain quantum effects, namely quantum critical variation, Kondo effect, and Weil fermions, are intimately intertwined with newly discovered matter and together appear to cause the exotic Weil Kondo state. These are very stable “topological” states, and unlike other quantum states, they cannot be easily destroyed by disturbances. This makes them particularly interesting for quantum computers.
Further measurements need to be performed under various external conditions to verify all of this. The team hopes that similar interactions of various quantum effects can be seen in other sources. “This could lead to the establishment of a design concept that could specifically improve, adjust and use such materials for concrete applications,” said Bühler-Paschen.
Reference: Wesley T. Farman, Andrey Sidrenco, Jonathan Hennell, Hannes Winkler, Andrey Prokofyev, Jose A. Rodriguez Rivera, Imin Chiu, Peter Braha, Kimiao Si, Colin L. Broholm , Sirke Paschen, May 19, 2021 Science Advances..
DOI: 10.1126 / sciadv.abf9134
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