[Paper Review] Superconductivity in the parent infinite-layer nickelate NdNiO$_2$

C. T. Parzyck, Y. Wu+15 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|Oct 2, 2024

Magnetic and transport properties of perovskites and related materialsMaterials Science3 citations

TL;DR

This study reports superconductivity with onset temperatures up to 11 K in undoped, parent-phase thin films of the infinite-layer nickelate NdNiO₂, achieved via oxide molecular-beam epitaxy and atomic hydrogen reduction. The superconductivity is proposed to be intrinsic but suppressed by disorder due to a nodal order parameter, or alternatively induced by random apical oxygen doping, challenging the conventional understanding of nickelate phase diagrams and suggesting new pathways for high-temperature superconductivity.

ABSTRACT

We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO$_2$. A combination of oxide molecular-beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to its nodal order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity.

Motivation & Objective

• To investigate the origin of superconductivity in the parent compound NdNiO₂, an infinite-layer nickelate previously thought to be metallic but non-superconducting.
• To determine whether superconductivity in NdNiO₂ is intrinsic or induced by defects or doping, given the material's complex electronic structure and synthesis challenges.
• To resolve the long-standing ambiguity in the nickelate phase diagram by identifying the true nature of the parent compound's electronic state.
• To explore the role of disorder and local apical oxygen atoms in enabling superconductivity in otherwise non-doped systems.
• To establish a reliable synthesis route for high-quality, low-resistivity NdNiO₂ films to enable systematic electronic and spectroscopic characterization.

Proposed method

• Growth of high-quality, epitaxial NdNiO₂ thin films on (001) SrTiO₃ and (LaAlO₃)₀.₃(Sr₂AlTaO₆)₀.₇ (LSAT) substrates using oxide molecular-beam epitaxy combined with in-situ atomic hydrogen reduction.
• Use of electrical transport measurements to identify superconducting transitions, including resistivity, magnetic field dependence, and pulsed IV measurements to assess current density and critical current density.
• Employment of scanning transmission electron microscopy (STEM) to assess crystallinity, thickness, and structural defects at atomic scale.
• Application of x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) at the Ni L₃-edge to probe local electronic structure, oxidation states, and spin excitations.
• Use of total electron yield (TEY) XAS to overcome substrate interference in measurements on LSAT substrates.
• Conducting density functional theory (DFT) calculations to model electronic structure and assess the role of rare-earth 5d and Ni 3d orbitals in the low-energy physics.

Experimental results

Research questions

• RQ1 Is superconductivity intrinsic to the undoped parent phase of NdNiO₂, or is it induced by defects or doping during synthesis?
• RQ2 What is the role of disorder, particularly in the form of apical oxygen atoms, in enabling superconductivity in the parent compound?
• RQ3 How does the nodal character of the superconducting order parameter influence the suppression of superconductivity in disordered samples?
• RQ4 To what extent do the electronic and magnetic properties of NdNiO₂ resemble those of cuprates, and what deviations suggest a distinct pairing mechanism?
• RQ5 Can the parent compound NdNiO₂ be considered a true 'parent' for superconductivity, or is it already doped by self-doping from rare-earth 5d states?

Key findings

• Superconducting transitions with onset temperatures up to 11 K were observed in NdNiO₂ thin films grown on LSAT substrates, with zero-resistance onset at 9.5 K in the best samples.
• The superconducting transition is hysteretic and suppressed by magnetic fields, with a critical field of ~1.5 T at 4 K, consistent with a BCS-like mechanism.
• A strong inverse correlation was found between the superconducting onset temperature (Tc,on) and the film's residual resistivity (ρres), indicating that disorder suppresses superconductivity.
• Resonant inelastic x-ray scattering (RIXS) measurements revealed spin excitations consistent with a d-wave-like order parameter, supporting a nodal superconducting gap.
• X-ray absorption spectroscopy (XAS) and STEM analysis revealed that apical oxygen atoms are randomly distributed in the superconducting films, suggesting a possible source of hole doping.
• Density functional theory (DFT) calculations confirmed that the Nd 5d and Ni 3d orbitals hybridize significantly, contributing to the metallic ground state and potentially enabling unconventional superconductivity.