Where Quasiparticles Die, Topology Emerges: An 'Impossible' New State at a Quantum Critical Point

Key points

• The experiment: At TU Wien, Silke Bühler-Paschen's group cooled a single crystal of the heavy-fermion semimetal CeRu₄Sn₆ (cerium-ruthenium-tin, 1:4:6) to 30 millikelvin and measured Hall resistivity across magnetic fields from 0 to 30 tesla. First author: D. M. Kirschbaum.
• The anomaly: An anomalous Hall effect—a topological signal—was observed even at zero magnetic field, and it forms a dome as a function of field and temperature. It vanishes outside a specific window and peaks exactly where quasiparticle physics fails most severely.
• Why this is 'impossible': In conventional topological materials (e.g., Weyl semimetals like TaAs), topology is read off from band structure, which presupposes that electrons behave as quasiparticles. CeRu₄Sn₆ sits at a Kondo-destruction quantum critical point (QCP), where the Kondo screening cloud collapses, the Fermi surface abruptly reconstructs, and the effective mass diverges—meaning Landau's Fermi-liquid theory breaks down entirely.
• Theoretical framework — Weyl-Kondo semimetal: Qimiao Si's team (Rice University) provides the conceptual model: the band crossing occurs near a Kondo-induced gap, and although the screening cloud collapses at the QCP, its topological imprint survives and even sharpens amid enhanced quantum fluctuations. The paper's phrasing: quantum phase transitions can "nucleate" emergent topological phases.
• Why CeRu₄Sn₆ is special: The crystal (space group Ī42m) naturally breaks inversion symmetry while preserving time-reversal symmetry—a Weyl-friendly setup—and its Kondo temperature nearly coincides with its magnetic ordering temperature, placing it intrinsically at criticality without tuning by doping or pressure.
• Echo of high-Tc superconductivity: The dome shape mirrors the superconducting domes of cuprates, where superconductivity emerges from a strange-metal phase with broken quasiparticles. The common mechanism: near criticality, a system releases accumulated entropy by forming a new phase—superconducting or topological.
• Design principle: The field may shift from *finding* topological materials to *engineering* quantum critical points in strongly correlated systems, letting topology emerge from critical fluctuations.
• Bigger question: If topology can survive the collapse of its quasiparticle carrier, quasiparticles may be just one carrier of topology—not topology itself. Topological order may be far more widespread in strongly correlated matter than previously recognized.

Paper information

• 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 (cerium-ruthenium-tin heavy-fermion semimetal)
• Key phenomenon: At a Kondo-destruction QCP, the quasiparticle picture fails while an anomalous Hall effect (topological signature) emerges in a dome-shaped distribution
• Framework: Weyl-Kondo semimetal at a Kondo-destruction QCP