Evidence for non-unitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi

summary

Video file (mp4)

The gist

The gist: The study investigates the superconducting properties of noncentrosymmetric ternary superconductors TaXSi (X = Re, Ru) using muon spin rotation/relaxation and density functional theory

In short

The study investigated superconducting properties in noncentrosymmetric ternary superconductors TaXSi (TaReSi and TaRuSi) using muon spin rotation ($μ$SR) and density functional theory. While TaReSi shows isotropic s-wave superconductivity, TaRuSi exhibits multi-gap superconductivity and broken time-reversal symmetry (TRS). DFT suggests this TRS breaking in TaRuSi arises from a non-unitary triplet pairing state, which is consistent with the relatively weak spin-orbit coupling in that material.

Key concepts

Noncentrosymmetric Superconductors (NCS)
These are materials where the crystal structure lacks a center of inversion. This structural feature allows for electronic antisymmetric spin-orbit coupling (ASOC), which is crucial because it enables the formation of exotic superconducting states, such as mixtures of spin-singlet and spintriplet Cooper pairs, going beyond conventional BCS theory.
Broken Time-Reversal Symmetry (TRS)
TRS breaking means that the physical properties of a material are not the same when time is reversed. In TaRuSi, the observation of small spontaneous magnetic fields in the superconducting state indicates TRS breaking. This phenomenon is linked to a non-unitary triplet pairing state, suggesting an unconventional superconducting mechanism.
Spin-Orbit Coupling (SOC)
SOC describes the interaction between an electron's spin and its orbital motion within the atom. The paper found that SOC is strong in TaReSi but relatively weak in TaRuSi. This difference in SOC strength influences the allowed pairing symmetries, as strong ASOC can drive systems toward unconventional superconductivity.
Non-unitary Triplet Pairing State
This refers to a specific type of superconducting pairing where the Cooper pairs have both spin and orbital angular momentum, and the state is not described by a simple scalar function. Its presence in TaRuSi is attributed to the broken TRS and is consistent with symmetry analysis for systems with C2v point-group symmetry.

Terminology used across episodes

This episode discusses

The paper

Evidence for non-unitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi · Read on arXiv

S. Sharma, Sajilesh K. P., A. D. S. Richards, J. Gautreau, M. Pula, J. Beare, K. M Kojima, S Yoon, § Y Cai, § R K Kushwaha, § T Agarwal, § E S Sørensen, R P Singh, * G M Luke

Department of Physics and Astronomy, McMaster University · Department of Physics, Indian Institute of Science Education and Research Bhopal · TRIUMF · Department of Physics, Sungkyunkwan University · Quantum Matter Institute, The University of British Columbia

DOI: 10.1103/PhysRevB.108.144510

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: We're looking at the paper: "Evidence for non-unitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi".

Mira: Here's the gist of the paper. The gist: The study investigates the superconducting properties of noncentrosymmetric ternary superconductors TaXSi (X = Re, Ru) using muon spin rotation/relaxation and density functional theory calculations to determine if they exhibit conventional or unconventional pairing symmetries.

Kai: Superconducting States in TaReSi and TaRuSi The transverse-field µSR measurements indicate isotropic s-wave superconductivity in TaReSi and multi-gap superconductivity in TaRuSi Zero-field µSR measurements find no evidence for spontaneous fields in the superconducting state of TaReSi, whereas small spontaneous fields that onset with superconductivity indicate broken time-reversal symmetry (TRS) superconductivity in TaRuSi Density functional theory calculations attribute the broken time-reversal symmetry (TRS) in TaRuSi to a non-unitary triplet pairing state This finding is consistent with the expectation that such a state is not allowed in the presence of strong spin-orbit coupling, as spin-orbit coupling is relatively weak in TaRuSi and strong in TaReSi.

Mira: Role of Crystal Structure and Spin-Orbit Coupling The role of the underlying crystal structure has been recognized for its role in determining the allowed symmetries of the superconducting state In noncentrosymmetric superconductors (NCS), parity is not a good quantum number, and electronic antisymmetric spin-orbit coupling (ASOC) is allowed by symmetry.

Kai: This relaxed symmetry in NCS systems, in the presence of strong ASOC, can give rise to the formation of a mixture of spin-singlet and spintriplet Cooper pairs Such a superconducting state can go beyond conventional BCS-like superconductivity with various exotic features like time-reversal symmetry (TRS) breaking, zeros or multiple gaps in the energy spectrum, and topologically protected nontrivial surface states.

Paper summary: Mira: So, when we wrap up this discussion on "Evidence for non-unitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi", the authors conclude that the superconducting state for TaRuSi is a non-unitary triplet-pairing state.

Kai: They also suggest that this finding makes TaRuSi the first system other than LaNiC2 with the point group C2v to host such a nonunitary triplet pairing state.

Lev: That comparison to LaNiC2 is interesting because both systems break time reversal symmetry and share the same point group, C2v, so the symmetry analysis they used for TaRuSi should apply there too.

Mira: Essentially, this research points toward Hund’s coupling between electrons on Ni orbitals as a possible pairing mechanism for that non-unitary triplet pairing in TaRuSi.

Kai: That means we need to look at those Ni orbitals and how Hund’s coupling might be involved when we try to understand these kinds of unconventional superconductivity.

Conclusion: Kai: So we've been looking at this paper on TaRuSi and TaReSi, and they're calling it "Evidence for non-unitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi".

Mira: It seems like the authors are putting forward that the weird stuff—the broken time reversal symmetry—isn't just a fluke in one material, but something tied to how spin-orbit coupling behaves differently between Re and Ru.

Lev: From an error correction standpoint, if this non-unitary triplet pairing is real, it means our usual assumptions about the pairing symmetry might be wrong for these systems.

Kai: Right, so they're suggesting that the specific crystal structure of TaRuSi allows for a pairing state that isn't just a simple spin-singlet or spin-triplet.

Mira: Exactly, and they link it to density functional theory calculations showing how the electron-spin-orbit coupling strength is way different in these two compounds.

Lev: That difference in SOC strength would mean that any experimental realization of this state would have a very specific energy landscape to work with for our hardware.

Kai: The comparison between TaRuSi and TaReSi, where one has isotropic s-wave and the other shows signs of time-reversal symmetry breaking, is key here.

Mira: It implies that the noncentrosymmetric structure itself can tune the pairing mechanism to favor these different states depending on which element you put in.

Lev: If we're building qubits based on this physics, it means we have to account for this complexity when designing our superconducting circuits.

Kai: So, they're suggesting that understanding these materials could open up new ways to engineer exotic superconducting states.

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