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First type-I superconductor found breaking time-reversal symmetry

First type-I superconductor found breaking time-reversal symmetry

New Capabilities

Muon measurements show YbSb2 generates a weak internal field as it turns superconducting, a property previously limited to type-II materials

September 23rd, 2026: PRL publishes TRS-breaking report

Overview

Updated Yesterday

YbSb2 is the first type-I superconductor found to break time-reversal symmetry. As single crystals cooled into their superconducting state at 0.95 kelvin, muon probes detected a spontaneous magnetic field of about 0.44 gauss.

That property was previously seen only in type-II superconductors. The finding, published in Physical Review Letters, points to an exotic pairing state and topological surface modes that researchers can now test directly.

Why it matters

If it holds up, the finding widens the hunt for triplet-paired topological superconductors from type-II to type-I materials, a step toward fault-tolerant quantum computing.

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Key Indicators

0.95 K
Superconducting transition temperature
YbSb2 becomes superconducting at 0.95 kelvin, just above absolute zero.
0.44 G
Spontaneous internal field
Internal magnetic field detected by zero-field muon spin relaxation, evidence of broken time-reversal symmetry.
1
Type-I superconductors with confirmed TRS breaking
YbSb2 is the first; no other type-I superconductor has shown this before.

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People Involved

Organizations Involved

Timeline

2012 September 2026

3 events Latest: September 23rd, 2026 · 1 week ago
  1. PRL publishes TRS-breaking report

    Latest Publication

    Physical Review Letters publishes evidence that YbSb2 is the first type-I superconductor with broken time-reversal symmetry.

  2. Preprint posted to arXiv

    Publication

    The IISER Bhopal-led team posts its muon spin relaxation study of YbSb2 to arXiv as 2601.07460.

  3. YbSb2 superconductivity first reported

    Discovery

    Zhao et al. report bulk superconductivity near 1.3 K with signs of a second transition around 0.41 K.

Scenarios

1

Independent labs confirm TRS breaking in YbSb2

Likely Resolves by Mar 23, 2028

Discussed by: The paper's authors and the muon spectroscopy community; replication is standard for first-of-kind claims.

A second group repeats zero-field muon spin relaxation on YbSb2 crystals and reports the same spontaneous internal field near the superconducting transition. Confirmation would cement YbSb2 as the first type-I TRS-breaking superconductor and push the INT state hypothesis forward.

2

Topological surface states detected in YbSb2

Possible Resolves by Sep 23, 2028

Discussed by: The authors predict zero-bias conductance peaks in tunneling or point-contact spectroscopy; surface-sensitive probes would test this.

Scanning tunneling microscopy or angle-resolved photoemission detects the predicted Majorana surface modes at YbSb2 surfaces. Detection would confirm the topological superconducting state and open the material to quantum computing studies.

3

More type-I superconductors found breaking time-reversal symmetry

Possible Resolves by End of 2028

Discussed by: The paper flags the RSb2 family (R = Ca, Yb) and related nonsymmorphic compounds as candidates.

Researchers scan CaSb2 and similar square-net compounds, finding at least one more type-I superconductor with spontaneous TRS breaking. This would show YbSb2 is part of a broader class rather than an isolated anomaly.

Historical Context

2 moments from history that rhyme with this story — and how they unfolded.

1998

Sr2RuO4 and the first TRS-breaking superconductors (1998)

Zero-field muon spin relaxation suggested that Sr2RuO4, a type-II superconductor, generates spontaneous internal fields on entering its superconducting state, evidence of broken time-reversal symmetry. The interpretation has been debated since.

Then

The finding made Sr2RuO4 the archetypal test case for TRS breaking in superconductors.

Now

For nearly three decades, spontaneous TRS breaking was observed only in type-II materials.

Why this matters now

YbSb2 is the first type-I superconductor to show the same signature, breaking the pattern Sr2RuO4 helped establish.

2012–2016

LaNiGa2 and the INT state (2012–2016)

Hillier et al. reported time-reversal symmetry breaking in the superconductor LaNiGa2 via muon spin relaxation, and Weng et al. proposed the internally antisymmetric nonunitary triplet state to explain it.

Then

The INT state became a working model for orbital-dependent triplet pairing in intermetallics.

Now

It provided the framework now applied to YbSb2, which shares the same point-group symmetry.

Why this matters now

YbSb2's proposed superconducting state is the same INT state first developed for LaNiGa2, making that earlier work the direct intellectual precursor.

Sources

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