26-May-2021 - Technische Universität Wien

New quantum material discovered

A research team came across a surprising form of "quantum criticality". This could lead to a design concept for new materials

In everyday life, phase transitions usually have to do with temperature changes – for example, when an ice cube gets warmer and melts. But there are also different kinds of phase transitions, depending on other parameters such as magnetic field. In order to understand the quantum properties of materials, phase transitions are particularly interesting when they occur directly at the absolute zero point of temperature. These transitions are called "quantum phase transitions" or a "quantum critical points".

Such a quantum critical point has now been discovered by an Austrian-American research team in a novel material, and in an unusually pristine form. The properties of this material are now being further investigated. It is suspected that the material could be a so-called Weyl-Kondo semimetal, which is considered to have great potential for quantum technology due to special quantum states (so-called topological states). If this proves to be true, a key for the targeted development of topological quantum materials would have been found. The results were found in a cooperation between TU Wien, Johns Hopkins University, the National Institute of Standards and Technology (NIST) and Rice University and has now been published in the journal "Science Advances".

Quantum criticality – simpler and clearer than ever before

"Usually quantum critical behaviour is studied in metals or insulators. But we have now looked at a semimetal," says Prof. Silke Bühler-Paschen from the Institute of Solid State Physics at TU Wien. The material is a compound of cerium, ruthenium and tin – with properties that lie between those of metals and semiconductors.

Usually, quantum criticality can only be created under very specific environmental conditions – a certain pressure or an electromagnetic field. "Surprisingly, however, our semimetal turned out to be quantum critical without any external influences at all," says Wesley Fuhrman, a PhD student in Prof. Collin Broholm's team at Johns Hopkins University, who made an important contribution to the result with neutron scattering measurements. "Normally you have to work hard to produce the appropriate laboratory conditions, but this semimetal provides the quantum criticality all by itself."

This surprising result is probably related to the fact that the behaviour 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 that you cannot explain their behaviour by looking at the electrons individually," says Bühler-Paschen. "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.

Quantum fluctuations could lead to Weyl particles

The main reason why the result is of such central importance is that it is suspected to be closely connected to the phenomenon of "Weyl fermions". In solids, Weyl fermions can appear in the form of quasiparticles – i.e. as collective excitations such as waves in a pond. According to theoretical predictions, such Weyl fermions should exist in this material," says theoretical physicist Qimiao Si of Rice University. Experimental proof, however, is yet to be found. "We suspect that the quantum criticality we observed favours the occurrence of such Weyl fermions," says Silke Bühler-Paschen. "Quantum critical fluctuations could therefore have a stabilising effect on Weyl fermions, in a similar way to quantum critical 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."

It seems to us that certain quantum effects – namely quantum critical fluctuations, the Kondo effect and Weyl fermions – are tightly intertwined in the newly discovered material and, together, give rise to exotic Weyl-Kondo states. These are "topological" states of great stability that, unlike other quantum states, cannot be easily destroyed by external disturbances. This makes them particularly interesting for quantum computers.

To verify all this, further measurements under different external conditions are to be carried out. The team expects that a similar interplay of the various quantum effects should also be found in other materials. "This could lead to the establishment of a design concept with which such materials can be specifically improved, tailored and used for concrete applications," says Bühler-Paschen.

Facts, background information, dossiers
  • phase transitions
  • quantum phase transitions
  • quantum criticality
More about TU Wien
  • News

    A Sandblaster at the Atomic Level

    From semiconductors to moon rocks: Many materials are treated with ion beams. A research group at TU Wien has now been able to explain how this process depends on the roughness of the surface. If you want to remove a layer of paint from a metal surface, you can use a sandblaster: Countless ... more

    Anchoring single atoms

    There is a dictum to “never change a running system”. New methods can however be far superior to older ones. While to date chemical reactions are mainly accelerated by catalytic materials that comprise several hundreds of atoms, the use of single atoms could provide a new approach for catal ... more

    How ions get their electrons back

    Very unusual atomic states are produced at TU Wien: Ions are created by removing not just one but 20 to 40 electrons from each atom. These “highly charged ions” play an important role in current research. For a long time, people have been investigating what happens when such highly charged ... more

  • Videos

    Epoxy Resin

    A flash of ultraviolet light sets off a chain reaction which hardens the whole object. more

    Noreia

    The coating machine Noreia was built at TU Wien. This time-lapse video shows the construction process. more

    Shaping Drops: Control over Stiction and Wetting

    Some surfaces are wetted by water, others are water-repellent. TU Wien (Vienna), KU Leuven and the University of Zürich have discovered a robust surface whose adhesive and wetting properties can be switched using electricity. This remarkable result is featured on the cover of Nature magazin ... more

More about Johns Hopkins University
More about National Institute of Standards and Technology
More about Rice University
  • News

    Reversal speeds creation of important molecule

    The story of halichondrin B, an inspirational molecule obtained from a marine creature, goes back to the molecule's discovery in an ocean sponge in 1986. Though it has been replicated in the laboratory several times before, new work by Rice University chemists could make halichondrin B and ... more

    Cerium sidelines silver to make drug precursor

    Save your silver! It's better used for jewelry than as a catalyst for drugs. Rice University scientists have developed a greatly simplified method to make fluoroketones, precursors for drug design and manufacture that typically require a silver catalyst. Rice chemist Julian West and graduat ... more

    A little soap simplifies making 2D nanoflakes

    Just a little soap helps clean up the challenging process of preparing two-dimensional hexagonal boron nitride (hBN). Rice University chemists have found a way to get the maximum amount of quality 2D hBN nanosheets from its natural bulk form by processing it with surfactant (aka soap) and w ... more