Amazing stuff!
"At TU Wien, researchers have discovered a state in a quantum material that had previously been considered impossible. The definition of topological states should be generalized. ...
Now, a research team at TU Wien has shown that such materials can nevertheless exhibit topological properties—even though these have so far been explained using particle-like behavior. This demonstrates that topological states are more general than previously thought: two seemingly contradictory concepts turn out to be compatible. ...
However, there are also situations in which this picture appears to break down completely and the charge carriers lose their particle-like character. This seems to happen in the material composed of cerium, ruthenium and tin (CeRu₄Sn₆), which has now been investigated at TU Wien at extremely low temperatures. “Near absolute zero, it exhibits a specific type of quantum-critical behavior,” ... “The material fluctuates between two different states, as if it cannot decide which one it wants to adopt. In this fluctuating regime, the quasiparticle picture is thought to lose its meaning.” ...
Indeed, at extremely low temperatures—less than one degree above absolute zero. .. observed behavior that clearly indicates the presence of topological states: a spontaneous (anomalous) Hall effect. In the Hall effect, charge carriers are normally deflected by a magnetic field. However, this deflection can also arise from topological effects, even in the absence of any external magnetic field. What is particularly remarkable is that the charge carriers behave as if they were particles, even though the particle picture seems to fail in this material. ...
The team refers to the newly discovered state as an emergent topological semimetal ... a new theoretical model capable of combining the phenomena of quantum criticality and topology.
“In fact, it turns out that a particle picture is not required to generate topological properties,” ... “The concept can indeed be generalized—the topological distinctions then emerge in a more abstract, mathematical way. And more than that: our experiments suggest that topological properties can even arise because particle-like states are absent.” ..."
From the abstract:
"The electronic topology of a material is generally described by its Bloch states and the associated band structure, and can be altered by electron–electron interactions.
In metallic systems, the interactions are usually treated through the concept of quasiparticles. Here we investigate what happens if no well-defined quasiparticles are present and show that a topological semimetal phase can emerge from the material’s quantum critical state.
Using the non-centrosymmetric heavy-fermion compound CeRu4Sn6, which is intrinsically quantum critical, we show that the topological phase exhibits a dome structure as a function of the magnetic field and pressure. To understand these results, we study a Weyl–Kondo semimetal model at a Kondo destruction quantum critical point.
Indeed, it exhibits features in the spectral function that can define topological crossings beyond the quasiparticle picture. Our results outline the importance of the interplay of quantum critical fluctuations and symmetry to search for other emergent topological phases."
New state of matter discovered in a quantum material
Quantum Physics: New State of Matter Discovered (original news release) "At TU Wien, researchers have discovered a state in a quantum material that had previously been considered impossible. The definition of topological states should be generalized."
Fig. 5: Kondo destruction quantum criticality nucleating a Weyl–Kondo semimetal.
