[Paper Review] Even odder after twenty-three years: the superconducting order parameter puzzle of Sr2RuO4

A. P. Mackenzie, Thomas Scaffidi+2 more|arXiv (Cornell University)|Jun 6, 2017

Advanced Condensed Matter Physics59 references4 citations

TL;DR

This review examines the unresolved superconducting order parameter symmetry in Sr2RuO4 after 23 years of research, challenging fundamental assumptions like parity and highlighting persistent experimental discrepancies. The authors argue that current probes of unconventional superconductivity lack sufficient reliability, leaving no consensus on the order parameter despite extensive efforts.

ABSTRACT

In this short review, we aim to provide a topical update on the status of efforts to understand the superconductivity of Sr2RuO4. We concentrate on the quest to identify a superconducting order parameter symmetry that is compatible with all the major pieces of experimental knowledge of the material, and highlight some major discrepancies that have become even clearer in recent years. As the pun in the title suggests, we have tried to start the discussion from scratch, making no assumptions even about fundamental issues such as the parity of the superconducting state. We conclude that no consensus is currently achievable in Sr2RuO4, and that the reasons for this go to the heart of how well some of the key probes of unconventional superconductivity are really understood. This is therefore a puzzle that merits continued in-depth study.

Motivation & Objective

• To provide a topical update on the status of research into the superconducting order parameter of Sr2RuO4.
• To critically assess whether existing experimental data are consistent with any proposed superconducting order parameter symmetry.
• To challenge foundational assumptions in unconventional superconductivity, including the parity of the superconducting state.
• To highlight growing discrepancies between experimental results that undermine consensus formation.
• To argue that the current lack of consensus stems from fundamental limitations in the understanding of key experimental probes.

Proposed method

• Systematic review of experimental data from multiple probes, including thermal conductivity, NMR, torque magnetometry, and muon spin rotation.
• Re-evaluation of the assumptions underlying the interpretation of these probes, particularly regarding symmetry and parity.
• Use of a phenomenological approach to assess compatibility of proposed order parameters with all major experimental observations.
• Emphasis on starting from first principles, without assuming the superconducting state is even or odd under parity.
• Comparison of theoretical models with experimental results to identify inconsistencies and unresolved contradictions.
• Critical analysis of the reliability and interpretability of key experimental techniques used in unconventional superconductivity research.

Experimental results

Research questions

• RQ1 What is the true symmetry of the superconducting order parameter in Sr2RuO4, given decades of conflicting experimental data?
• RQ2 To what extent do current experimental probes reliably distinguish between different order parameter symmetries in unconventional superconductors?
• RQ3 Can the observed experimental discrepancies in Sr2RuO4 be reconciled within a single theoretical framework?
• RQ4 Are the fundamental assumptions about parity and symmetry in superconducting states valid for Sr2RuO4?
• RQ5 Why has no consensus emerged despite extensive experimental and theoretical effort over 23 years?

Key findings

• No consensus exists on the superconducting order parameter symmetry in Sr2RuO4, despite extensive experimental and theoretical investigation.
• Recent experimental results have revealed increasing discrepancies, making it even more difficult to reconcile observations with a single order parameter model.
• The authors argue that the lack of consensus stems not from the material's complexity but from fundamental limitations in the understanding of key experimental probes.
• Assumptions about the parity of the superconducting state—previously taken for granted—must be re-examined in light of contradictory data.
• The reliability of major probes such as NMR, muon spin rotation, and thermal conductivity in detecting order parameter symmetry is called into question.
• The authors conclude that Sr2RuO4 remains a profound puzzle that demands deeper, more critical study of both experimental techniques and theoretical frameworks.