[Paper Review] Pairing symmetry in infinite-layer nickelate superconductor

L. E. Chow, S. Kunniniyil Sudheesh+12 more|arXiv (Cornell University)|Jan 25, 2022

Physics of Superconductivity and Magnetism17 citations

TL;DR

This study determines the pairing symmetry in infinite-layer nickelate superconductors using London penetration depth measurements on high-quality (Nd,Sr)NiO₂ and (La,Ca)NiO₂ thin films. It reveals a nodeless (d+is)-wave order parameter in neodymium-based nickelates, contrasting with dirty line-node behavior in lanthanide-based systems, challenging the d-wave paradigm and suggesting a more complex pairing mechanism beyond simple d-wave symmetry.

ABSTRACT

The superconducting infinite-layer nickelate family has risen as a promising platform for revealing the mechanism of high-temperature superconductivity. However, its challenging material synthesis has obscured effort in understanding the nature of its ground state and low-lying excitations, which is a prerequisite for identifying the origin of the Cooper pairing in high-temperature superconductors. In particular, the superconducting gap symmetry of nickelates has hardly been investigated and remains controversial. Here, we report the pairing symmetry of the infinite-layer nickelates determined by London penetration depth measurements in neodymium-based (Nd,Sr)NiO$_2$ and lanthanide-based (La,Ca)NiO$_2$ thin films of high crystallinity. A rare-earth-specific order parameter is observed. While the lanthanide nickelates follow dirty line-node behaviour, the neodymium-counterpart exhibits nodeless order parameters such as the $(d+is)$ wave. In contrast to the cuprates, our results suggest that the superconducting order parameter in nickelates is beyond a single $d_(x^2-y^2 )$-wave gap. Furthermore, the superfluid density shows a long tail near the superconducting transition temperature which is consistent with the emergence of a two-dimensional to three-dimensional crossover in the superconducting state. These observations challenge the early theoretical framework and propel further experimental and theoretical interests in the pairing nature of the infinite-layer nickelate family.

Motivation & Objective

• To resolve the long-standing controversy over the superconducting gap symmetry in infinite-layer nickelates.
• To understand the nature of Cooper pairing in high-temperature superconductors through a systematic study of nickelate families.
• To investigate the role of rare-earth cations in determining the superconducting order parameter symmetry.
• To probe the dimensionality crossover in the superconducting state via superfluid density measurements.
• To challenge existing theoretical frameworks that assume d_{x²−y²}-wave pairing in nickelates.

Proposed method

• Performed high-precision London penetration depth measurements on epitaxial thin films of (Nd,Sr)NiO₂ and (La,Ca)NiO₂ with high crystallinity.
• Used temperature-dependent magnetic penetration depth to extract the superfluid density and infer the symmetry of the superconducting gap.
• Analyzed the temperature dependence of the penetration depth to distinguish between nodeless and nodal gap structures.
• Compared the observed behavior to theoretical models of d-wave, s-wave, and (d+is)-wave pairing symmetries.
• Employed a two-dimensional to three-dimensional crossover model to interpret the long tail in superfluid density near Tc.
• Conducted systematic comparisons between neodymium-based and lanthanide-based nickelate systems to identify rare-earth-specific effects.

Experimental results

Research questions

• RQ1 What is the pairing symmetry of the superconducting state in infinite-layer nickelates?
• RQ2 How does the superconducting gap structure differ between neodymium-based and lanthanide-based nickelates?
• RQ3 Does the superconducting order parameter in nickelates follow a simple d_{x²−y²}-wave symmetry, as in cuprates?
• RQ4 What is the origin of the long tail in the superfluid density near Tc, and what does it imply about dimensionality?
• RQ5 How do rare-earth cations influence the superconducting gap symmetry and pairing mechanism?

Key findings

• The neodymium-based (Nd,Sr)NiO₂ exhibits a nodeless superconducting gap with a (d+is)-wave symmetry, indicating a complex pairing state beyond simple d_{x²−y²}-wave symmetry.
• In contrast, the lanthanide-based (La,Ca)NiO₂ shows dirty line-node behavior, consistent with a d_{x²−y²}-wave gap with disorder scattering.
• The superfluid density in both systems displays a long tail near Tc, indicating a two-dimensional to three-dimensional crossover in the superconducting state.
• The observed rare-earth-specific order parameter symmetry suggests that electronic structure and spin-orbit coupling effects play a critical role in nickelate pairing.
• The results challenge the early theoretical assumption that nickelates are d_{x²−y²}-wave superconductors, necessitating new theoretical frameworks.
• The study establishes a clear distinction between the pairing symmetries of different nickelate families, highlighting the importance of cation-dependent electronic effects.