Spin‑triplet, odd‑parity superconductivity uncovered in the heavy-fermion metal YbRh₂Si₂

20. Mai 2026

Farbcodiertes Phasendiagramm, welches das Magnetfeld in Abhängigkeit von der Temperatur mit Konturlinien darstellt. — Figure 1: This is a color plot of the sample resistance, as measured on a single crystal of YbRh₂Si₂ with a magnetic field (H) applied within the basal plane of its tetragonal crystalline structure. Blue indicates zero resistance, while red indicates normal state resistance. The parabolic contours emphasized by the continuous lines are the signature of Pauli-limited superconductivity. Notably, when the field is applied along the c-axis, superconductivity exceeds the Pauli limit and these contours are absent.

© MPI CPfS

Figure 1: This is a color plot of the sample resistance, as measured on a single crystal of YbRh₂Si₂ with a magnetic field (H) applied within the basal plane of its tetragonal crystalline structure. Blue indicates zero resistance, while red indicates normal state resistance. The parabolic contours emphasized by the continuous lines are the signature of Pauli-limited superconductivity. Notably, when the field is applied along the c-axis, superconductivity exceeds the Pauli limit and these contours are absent.

© MPI CPfS

To the point:

Unconventional Superconductivity: Researchers discovered a rare odd- parity, spin triplet superconducting state in the heavy fermion metal YbRh2Si2.
Advanced measurement techniques: The team developed a novel ultrasensitive SQUID-based ohmmeter to measure resistance down to microkelvin temperatures.
Research collaboration and impact: This breakthrough involved international collaboration between Royal Holloway University of London, Goethe University Frankfurt, and the Max Planck Institute for Chemical Physics of Solids Dresden and sets a benchmark for future studies of triplet superconductivity in similar materials.

Ultra–low-temperature transport and thermodynamic measurements down to 200 microkelvin reveal a rare, spin-triplet odd-parity superconducting state underpinned by antiferromagnetism in the heavy-fermion metal YbRh₂Si₂.

An international team from Royal Holloway University of London (RHUL, UK), Goethe University Frankfurt (Germany), and the Max Planck Institute for Chemical Physics of Solids (MPI-CPfS, Dresden) reports evidence for odd-parity, spin-triplet superconductivity in the heavy-fermion compound YbRh₂Si₂. Interestingly, the results, published in Nature Physics, indicate that one of the superconducting phases in this material is a topological helical state.

YbRh₂Si₂, discovered at MPI-CPfS in 2000, is a model system for quantum criticality and strong electronic correlations. Superconductivity below 10 millikelvin was first observed in 2016 by scientists at MPI-CPfS and collaborators, but the pairing symmetry and its relationship to the magnetic phases of the material remained open questions. In fact, this material hosts two antiferromagnetic phases of unknown ordering wave vectors with the lower phase being the result of a striking electro‑nuclear phase transition at the extremely low temperature T_A = 1.5 millikelvin.

In this new study, the team at RHUL combined the precision thermodynamic measurements with new high-sensitivity complex impedance (resistance) measurements under carefully controlled magnetic fields applied along different crystallographic directions. The orientation dependence of the superconducting response, together with the robustness of superconductivity against paramagnetic pair breaking, is inconsistent with conventional spin-singlet pairing and instead points to an odd-parity spin-triplet state. Superconductivity observed in multiple single crystals appears to be present only within the antiferromagnetic phases and, although inhomogeneous, the zero-resistance state with persistent current is achieved. To resolve the superconducting properties at microkelvin temperatures, the team developed a novel ultra-sensitive four-terminal ohmmeter based on a superconducting quantum interference device (SQUID). This device enabled accurate, simultaneous measurement of sample resistance and temperature down to 400 microkelvin. The work was supported by the European Microkelvin Platform, a consortium supporting research in ultra–low-temperature environments. Why the magnetism underpins the superconductivity in YbRh₂Si₂ remains an open question, currently under active investigation at MPI-CPfS thanks to a Marie Skłodowska-Curie Fellowship.

This is not the first odd-parity superconductor associated with MPI-CPfS. In 2016, scientists of the institute and their international collaborators reported the discovery of the heavy-fermion multiphase superconductor CeRh₂As₂, which hosts an odd-parity superconducting order parameter while exhibiting spin‑singlet pairing. This possibility, however, is enabled by its locally non‑centrosymmetric crystal structure. In contrast, in YbRh₂Si₂ the evidence indicates intrinsic spin-triplet pairing, likely mediated by magnetic fluctuations.

Topology is a central research focus at MPI‑CPfS and odd-parity topological superconductors are of fundamental interest of the Physics of Quantum Materials department. They may offer routes toward fault‑tolerant quantum devices. In topological platforms, the superconducting state can be intrinsically protected against certain perturbations, potentially lowering error rates in qubits. While practical applications remain a long-term goal, establishing YbRh₂Si₂ as a candidate spin‑triplet topological superconductor provides a new benchmark for theory and experiment, and motivates searches for related behavior in similar materials.

MPI CPfS (MB, JK)