Time-Reversal Symmetry Breaking in the Centrosymmetric Nonsymmorphic Superconductor RuB2
Abstract
Centrosymmetric nonsymmorphic AB2 (A = Ru, Os) borides exhibit anomalous superconducting properties, but the microscopic nature of their superconducting ground state remains unresolved. Here, we investigate RuB2 using zero- and transverse-field muon spin relaxation (muSR), first-principles electronic-structure calculations, and Ginzburg-Landau symmetry analysis. Zero-field muSR reveals spontaneous internal magnetic fields below Tc = 1.6 K, providing direct evidence for time-reversal symmetry breaking (TRSB). Transverse-field measurements establish a fully gapped superconducting state, with a gap magnitude close to the BCS weak-coupling limit. Density-functional calculations show that RuB2 is a multiband Z2 topological metal with appreciable spin-orbit coupling. Guided by the experimental observations and the symmetry of the orthorhombic crystal structure, our Ginzburg-Landau analysis suggests that an internally antisymmetric, nonunitary spin-triplet state is a promising candidate for the superconducting order parameter. This candidate is consistent with both the observed TRSB and the fully gapped superconducting state. These results establish RuB2 as a promising platform for investigating unconventional superconductivity arising from the interplay of crystal symmetry, multiband electronic structure, and spin-orbit coupling, and motivate further studies of related nonsymmorphic borides. The two muSR measurements provide complementary constraints: zero-field data probe spontaneous magnetism associated with TRSB, while transverse-field data characterize superconducting field distribution and support a full gap. The topological electronic structure and spin-orbit coupling provide context for unconventional pairing. Although these results do not uniquely determine the order parameter, they narrow candidates and highlight that nonsymmorphic symmetry may shape superconductivity in RuB2.
Related
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BibTeX
@misc{gupta2026timereversal,
title = {{Time-Reversal Symmetry Breaking in the Centrosymmetric Nonsymmorphic Superconductor RuB2}},
author = {Ritu Gupta and Reena and Dibyendu Samanta and Jaskaran Singh and Thomas J. Hicken and Hubertus Luetkens and Jonas Krieger and Sudeep Kumar Ghosh and Yogesh Singh},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.26264},
eprint = {2610.26264},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.26264},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Time-Reversal Symmetry Breaking in the Centrosymmetric Nonsymmorphic Superconductor RuB2
AU - Ritu Gupta
AU - Reena
AU - Dibyendu Samanta
AU - Jaskaran Singh
AU - Thomas J. Hicken
AU - Hubertus Luetkens
AU - Jonas Krieger
AU - Sudeep Kumar Ghosh
AU - Yogesh Singh
PY - 2026
DA - 2026/10/26
PB - arXiv
DO - 10.48550/arxiv.2610.26264
UR - https://arxiv.org/abs/2610.26264
AB - Centrosymmetric nonsymmorphic AB2 (A = Ru, Os) borides exhibit anomalous superconducting properties, but the microscopic nature of their superconducting ground state remains unresolved. Here, we investigate RuB2 using zero- and transverse-field muon spin relaxation (muSR), first-principles electronic-structure calculations, and Ginzburg-Landau symmetry analysis. Zero-field muSR reveals spontaneous internal magnetic fields below Tc = 1.6 K, providing direct evidence for time-reversal symmetry breaking (TRSB). Transverse-field measurements establish a fully gapped superconducting state, with a gap magnitude close to the BCS weak-coupling limit. Density-functional calculations show that RuB2 is a multiband Z2 topological metal with appreciable spin-orbit coupling. Guided by the experimental observations and the symmetry of the orthorhombic crystal structure, our Ginzburg-Landau analysis suggests that an internally antisymmetric, nonunitary spin-triplet state is a promising candidate for the superconducting order parameter. This candidate is consistent with both the observed TRSB and the fully gapped superconducting state. These results establish RuB2 as a promising platform for investigating unconventional superconductivity arising from the interplay of crystal symmetry, multiband electronic structure, and spin-orbit coupling, and motivate further studies of related nonsymmorphic borides. The two muSR measurements provide complementary constraints: zero-field data probe spontaneous magnetism associated with TRSB, while transverse-field data characterize superconducting field distribution and support a full gap. The topological electronic structure and spin-orbit coupling provide context for unconventional pairing. Although these results do not uniquely determine the order parameter, they narrow candidates and highlight that nonsymmorphic symmetry may shape superconductivity in RuB2.
ER -