[Paper Review] Multiband Superconductivity in the time reversal symmetry broken superconductor Re6Zr

Pradnya Parab, D. Singh+4 more|arXiv (Cornell University)|Apr 20, 2017

Rare-earth and actinide compounds3 citations

TL;DR

This study presents point contact Andreev reflection (PCAR) measurements on high-quality Re6Zr single crystals, revealing two distinct superconducting gaps (Δ₁ ≈ 0.79 meV and Δ₂ ≈ 0.22 meV), indicating multiband superconductivity. Combined with prior muon spin relaxation evidence of time reversal symmetry breaking, the results suggest an unconventional superconducting state, likely a multiband singlet pairing with time reversal symmetry breaking or a triplet state dominated by interband pairing.

ABSTRACT

We report point contact Andreev Reflection (PCAR) measurements on a high-quality single crystal of the non-centrosymmetric superconductor Re6Zr. We observe that the PCAR spectra can be fitted by taking two isotropic superconducting gaps with Delta_1 ~ 0.79 meV and Delta_2 ~ 0.22 meV respectively, suggesting that there are at least two bands which contribute to superconductivity. Combined with the observation of time reversal symmetry breaking at the superconducting transition from muon spin relaxation measurements (Phys. Rev. Lett. 112, 107002 (2014)), our results imply an unconventional superconducting order in this compound: A multiband singlet state that breaks time reversal symmetry or a triplet state dominated by interband pairing.

Motivation & Objective

• To investigate the superconducting pairing symmetry in Re6Zr, a non-centrosymmetric superconductor known to break time reversal symmetry (TRSB).
• To determine whether multiple electronic bands contribute to superconductivity in Re6Zr through direct spectroscopic measurements.
• To clarify the nature of the superconducting order parameter by identifying the number and magnitude of superconducting gaps.
• To reconcile the observed TRSB with the multiband superconducting state, distinguishing between singlet and triplet pairing scenarios.

Proposed method

• Point contact Andreev reflection (PCAR) spectroscopy was performed on a high-quality single crystal of Re6Zr to probe the local density of states near the superconducting gap.
• The measured PCAR spectra were fitted using a two-gap model with two isotropic superconducting gaps.
• The fitting procedure assumed s-wave pairing symmetry for both bands, consistent with isotropic gap behavior.
• The results were interpreted in light of prior muon spin relaxation (μSR) measurements that confirmed time reversal symmetry breaking at the superconducting transition.
• The analysis focused on distinguishing between multiband singlet pairing with broken TRSB and interband triplet pairing as possible origins of the unconventional state.

Experimental results

Research questions

• RQ1 Does Re6Zr exhibit multiband superconductivity, as indicated by multiple superconducting gaps?
• RQ2 What is the pairing symmetry and nature of the superconducting order parameter in Re6Zr, given its time reversal symmetry breaking?
• RQ3 Can the observed TRSB be consistently explained by a multiband singlet pairing state or a triplet pairing state?
• RQ4 How do the two distinct superconducting gaps (Δ₁ ≈ 0.79 meV and Δ₂ ≈ 0.22 meV) relate to the electronic bands in Re6Zr?

Key findings

• Two isotropic superconducting gaps were identified in Re6Zr: Δ₁ ≈ 0.79 meV and Δ₂ ≈ 0.22 meV, indicating at least two contributing electronic bands.
• The presence of two distinct gaps strongly supports a multiband superconducting state in Re6Zr.
• The observed multiband superconductivity is consistent with time reversal symmetry breaking, as previously established by muon spin relaxation (μSR) measurements.
• The data suggest an unconventional superconducting state, either a multiband singlet state that breaks time reversal symmetry or a triplet state dominated by interband pairing.
• The absence of gap anisotropy in the PCAR spectra supports isotropic s-wave pairing in both bands.