[Paper Review] Evidence for nodal superconductivity in infinite-layer nickelates

Shannon P. Harvey, Bai Yang Wang+6 more|arXiv (Cornell University)|Jan 31, 2022

Magnetic and transport properties of perovskites and related materials17 citations

TL;DR

This study measures the London penetration depth in optimally doped infinite-layer nickelates (La, Pr, and Nd-based) down to 150 mK, finding quadratic temperature dependence in La and Pr samples consistent with nodal superconductivity and dirty d-wave pairing. The Nd sample shows anomalous behavior attributed to magnetic impurities, indicating complex low-temperature physics inconsistent with simple gapped or nodal models.

ABSTRACT

Infinite-layer nickelates present a new family of potential unconventional superconductors. A key open question is the superconducting pairing symmetry. We present low-temperature measurements of the London penetration depth in optimally doped La_{0.8}Sr_{0.2}NiO_{2}, Pr_{0.8}Sr_{0.2}NiO_{2}, and Nd_{0.8}Sr_{0.2}NiO_{2}. For La and Pr-nickelates, the superfluid density shows a quadratic temperature dependence, indicating nodal superconductivity in the presence of disorder. Nd-nickelate exhibits complex low-temperature behavior, which we attribute to magnetic impurities. These results are consistent with d-wave pairing.

Motivation & Objective

• To determine the superconducting pairing symmetry in infinite-layer nickelates, a new family of unconventional superconductors.
• To investigate whether the superconducting gap has nodes, a key fingerprint of unconventional pairing.
• To distinguish between s-wave and d-wave pairing symmetry using the temperature dependence of the London penetration depth.
• To assess the impact of disorder and magnetic impurities on superconducting properties in these materials.
• To establish whether nickelates are analogous to cuprates in their pairing mechanism.

Proposed method

• Measure the London penetration depth λ(T) using mutual inductance in thin-film samples of La0.8Sr0.2NiO2, Pr0.8Sr0.2NiO2, and Nd0.8Sr0.2NiO2 down to 150 mK.
• Use the temperature dependence of superfluid density ns(T) = n_s(0) × (1 - T/Tc)^2 to infer the gap structure.
• Apply power-law and exponential fits to λ(T) data to distinguish between nodal (quadratic) and nodeless (exponential) superconducting gaps.
• Analyze the data using the dirty-limit BCS model, where T** ≈ Tc indicates intermediate disorder.
• Use Arrhenius and log-log plots of Δns/ns vs. 1/T to identify scaling behavior and detect deviations from expected exponential or power-law trends.
• Account for magnetic impurities by comparing penetration depth behavior with magnetic permeability effects and examining transition broadening.

Experimental results

Research questions

• RQ1 Does the superconducting gap in infinite-layer nickelates exhibit nodes, indicating unconventional pairing?
• RQ2 Is the pairing symmetry in these materials consistent with d_{x²−y²}-wave pairing, as in cuprates?
• RQ3 How do disorder and magnetic impurities affect the measured penetration depth and superfluid density?
• RQ4 Why does Nd0.8Sr0.2NiO2 show anomalous low-temperature behavior inconsistent with standard nodal or gapped superconductivity?
• RQ5 Can the observed scaling of superfluid density with temperature be explained by a dirty d-wave mechanism?

Key findings

• La0.8Sr0.2NiO2 and Pr0.8Sr0.2NiO2 exhibit a quadratic temperature dependence of superfluid density down to 150 mK, consistent with nodal superconductivity.
• The data for La and Pr samples are well-fit by the dirty-limit d-wave BCS model (Eq. 2), with T** ≈ Tc, indicating intermediate disorder.
• The power-law scaling of superfluid density with temperature (slope ≈ 1.7) persists from near Tc down to 150 mK, a behavior reminiscent of cuprates.
• Nd0.8Sr0.2NiO2 shows a flattening of apparent superfluid density below 1 K, inconsistent with exponential or power-law scaling, suggesting magnetic impurity effects.
• The exponential fit to Nd data yields a minimum gap of 0.8kBTc, well below the BCS weak-coupling limit of 1.76kBTc, indicating a non-superconducting origin for the observed behavior.
• The broad and irregular superconducting transition in Nd-nickelate, combined with anomalous magnetic field response, supports the presence of magnetic defects as the primary cause of its complex behavior.