Where Quasiparticles Vanish, Topology Emerges: An 'Impossible' New State at a Quantum Critical Point
*English adaptation of a zhichai.net forum post discussing Kirschbaum et al., Nature Physics (2026).*
The experiment
In Silke Bühler-Paschen's lab at TU Wien, a dilution refrigerator ran for three months at 30 millikelvin - a hundred times colder than the cosmic microwave background. The sample: a single crystal of CeRu₄Sn₆ (cerium-ruthenium-tin, 1:4:6). Graduate student David Kirschbaum, performing Hall measurements across fields from 0 to 30 tesla, saw something that should not exist: an anomalous Hall step that persisted at zero magnetic field - in a material where the electron-as-quasiparticle picture has completely collapsed.
Background: when electrons stop being electrons
- In ordinary metals, electrons behave as well-defined quasiparticles - the foundation of Landau Fermi-liquid theory for most of a century.
- In heavy fermion materials, localized 4f spins (e.g., cerium) entangle with conduction electrons via the Kondo effect (explained by Jun Kondo in 1964), amplifying electron effective mass by hundreds of times. The quasiparticle picture survives, though distorted.
- Qimiao Si (Rice University) predicted in 2001 that in some heavy fermions the Kondo effect does not grow smoothly but abruptly collapses at a Kondo-destruction quantum critical point (QCP). The Fermi surface reconstructs, and the quasiparticle picture fails entirely - not a large correction, but a breakdown of the whole framework.
- The dome's peak coincides with the strongest failure of the quasiparticle picture - effective mass diverges at its center, a clear signature of Fermi-liquid breakdown.
- Like superconducting domes in the cuprate phase diagrams (strange metal + quantum critical fluctuations), this suggests quantum critical fluctuations can nucleate new phases - not only superconductivity, but also topological semimetallicity.
- Title: Emergent topological semimetal from quantum criticality
- Journal: Nature Physics, 2026-01-14
- First author: D. M. Kirschbaum (TU Wien)
- Corresponding authors: Silke Bühler-Paschen (TU Wien), Qimiao Si (Rice University)
- Material: CeRu₄Sn₆ (Ce-Ru-Sn heavy-fermion semimetal)
- Key phenomenon: anomalous Hall effect (topological signal) emerges as a magnetic-field dome precisely where quasiparticle physics fails at the Kondo-destruction QCP
- Framework: Weyl-Kondo semimetal at a Kondo-destruction quantum critical point
Why this challenges topological physics
Topological materials derive their protected edge states and anomalous responses from band-structure topology - the 2016 Nobel Prize honored Thouless, Haldane, and Kosterlitz for introducing topology into condensed matter. But band topology presupposes quasiparticles. If quasiparticles cease to exist near a QCP, where does the topology come from?
The dome: topology as an emergent phase
The anomalous Hall signal did not appear everywhere. It formed a dome in magnetic field: present within a window, absent outside it. Crucially:
"Emergent" here has a precise meaning: a collective property that exists in none of the microscopic constituents, arising from collective behavior at the critical point.
Theory: the Weyl-Kondo semimetal
Si's group provides the framework: the Weyl-Kondo semimetal. Unlike conventional Weyl semimetals (e.g., TaAs), where band crossings involve bare quasiparticles, here the crossings form near the Kondo gap, and the Kondo screening cloud itself participates in building the topology. At the Kondo-destruction QCP the cloud collapses - but its topological imprint survives, sharpened by enhanced quantum fluctuations. As the post puts it: the bridge's structure is destroyed, yet the arch survives in the turbulence of the river.
Why CeRu₄Sn₆
CeRu₄Sn₆ (space group Ī42m) is "born critical": it naturally breaks spatial inversion symmetry while preserving time-reversal symmetry (a Weyl-friendly combination), and its Kondo temperature nearly matches its magnetic ordering temperature, so Kondo screening and RKKY interactions are inherently in competition.
A design principle: from finding materials to designing critical points
The traditional route to topological materials is search: scan crystal libraries for the right symmetries. The new principle is to engineer a quantum critical point and let topology nucleate from critical fluctuations. As Si writes, "quantum phase transitions can nucleate emergent topological phases."
The larger question
If topology persists without quasiparticles, perhaps quasiparticles are just one carrier of topology, not topology itself - implying topological behavior may be far more universal in strongly correlated systems than previously assumed. The author draws a structural analogy to emergent capabilities in large language models: the carrier can collapse while the structure persists.