Optical conductivity signatures of strong correlations and multiband superconductivity in infinite-layer nickelates

By analyzing the optical conductivity of $Nd_{1-x}Sr_xNiO_2$ thin films, this study demonstrates that infinite-layer nickelates exhibit a multiband electronic structure characterized by strongly correlated hole bands and electron bands, both of which contribute to the superconducting condensate.

The Gist

The Tale of Two Highways: Unlocking the Secret of Nickelate Superconductors

Imagine you are looking at a massive, high-speed highway system. In most materials, traffic (which we call electrons) moves in a single, predictable way. But in a newly discovered family of materials called "infinite-layer nickelates," the highway is much weirder and more complex.

Scientists have long been trying to understand why these materials can carry electricity with zero resistance (superconductivity). This paper uses a specialized "high-speed camera" (called spectroscopic ellipsometry) to watch how the traffic moves, and they discovered something groundbreaking.

Here is the breakdown of their discovery using simple analogies.

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1. The Two-Lane Highway (Multiband Nature)

In a standard material, you might have one single lane of traffic. If you add more cars (doping), the lane just gets more crowded.

However, the researchers found that nickelates actually have two distinct types of lanes running side-by-side:

• The "Wide, Slow Lane" (The Hole Band): This lane is filled with "holes"—think of these as empty spaces in a crowded parking lot. As you add more "cars" (Sr doping), this lane actually expands and becomes more organized.
• The "Narrow, Fast Lane" (The Electron Band): This is a separate lane of actual electrons.

The Big Discovery: For a long time, scientists argued about whether the "fast lane" even mattered, or if it was just a distraction. This paper proves that both lanes are essential. They aren't just bystanders; they are both active participants in the magic of superconductivity.

2. The "Traffic Jam" Effect (Strong Correlations)

In a normal material, electrons act like polite drivers who stay in their lanes. In nickelates, the electrons are "strongly correlated."

Think of this like a massive, synchronized dance troupe or a very intense traffic jam. When one electron moves, it forces everyone else to react instantly. The researchers saw this by watching "spectral weight transfer."

The Analogy: Imagine a crowded concert hall. If one person stands up to cheer, the energy doesn't just stay with them; it ripples through the entire crowd, changing the "vibe" of the whole room. In nickelates, when scientists changed the chemical makeup (doping), the energy didn't just move locally—it shifted from high-energy "vibes" to low-energy "movement" across the whole system. This is a classic sign of a "strongly correlated" material.

3. The Grand Parade (Multiband Superconductivity)

The most exciting part happens when the material becomes a superconductor (the state where electricity flows perfectly).

Usually, in similar materials (like the famous cuprates), superconductivity is like a single group of people marching in perfect unison. But in these nickelates, the researchers found that both the "hole" lane and the "electron" lane join the parade.

When the temperature drops to the superconducting level, the "traffic" in both lanes suddenly stops being chaotic and starts moving in a perfectly synchronized, frictionless flow. Because both lanes are contributing, we call this "multiband superconductivity."

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Why does this matter?

For decades, we have been studying "cuprates" (copper-based materials) to find the perfect superconductor. They are like a single-engine plane: powerful, but limited.

The nickelates are like a dual-engine jet. By proving that these materials use multiple "lanes" of electrons to achieve superconductivity, scientists now have a new blueprint. If we can learn to control both "engines" at once, we might be able to design new materials that work at much higher temperatures, potentially revolutionizing how we power our world—from ultra-fast maglev trains to lossless power grids.

Technical Summary

Technical Summary: Optical Conductivity Signatures of Strong Correlations and Multiband Superconductivity in Infinite-Layer Nickelates

1. Problem Statement

Since the discovery of superconductivity in infinite-layer nickelates ($Nd_{1-x}Sr_xNiO_2$), a central debate in condensed matter physics has been whether these materials are truly "cuprate analogs" or if their electronic structure is fundamentally different. While cuprates are generally described by a single-band Hubbard model (the $Cu\ 3d_{x^2-y^2}$ band), nickelates possess a complex multiband structure due to the coexistence of a Ni $3d_{x^2-y^2}$ hole band and rare-earth $5d$ electron bands.

The specific challenges addressed in this study are:

• Band Filling: Determining how the occupancy of these electron and hole bands evolves with Sr doping ($x$).
• Correlation Nature: Clarifying whether nickelates reside in a charge-transfer regime (like cuprates) or a Mott-Hubbard regime.
• Superconductivity Mechanism: Investigating whether the superconductivity is single-band or multiband by observing which carriers participate in the superconducting condensate.

2. Methodology

The researchers employed spectroscopic ellipsometry to measure the in-plane ($ab$-plane) optical conductivity ($\sigma_1(\omega)$) of $Nd_{1-x}Sr_xNiO_2$ (NSNO) thin films.

• Sample Preparation: Films were grown on LSAT substrates (chosen for optical transparency and epitaxial stability) using pulsed laser deposition (PLD) and subsequently reduced via topotactic reduction.
• Doping Range: The study spanned the full phase diagram from $x = 0.025$ to $x = 0.30$.
• Modeling: To interpret the low-energy response, the team utilized a two-band Drude-Lorentz model. This model decomposes the intraband response into two distinct Drude components: a "narrow" component (associated with electron bands) and a "broad" component (associated with the correlated hole band).
• Analysis: They calculated the effective electron number ($N_{eff}^*$) to quantify spectral weight transfer and analyzed temperature-dependent conductivity to observe the transition into the superconducting state.

3. Key Results

• Evidence of Strong Correlations: The optical spectra exhibit a significant spectral weight transfer from high energies ($>2.5$ eV) to low energies upon doping. This is marked by an isosbestic point at $\sim 2.5$ eV, a classic hallmark of strongly correlated electron systems.
• Mott-Hubbard Regime: The energy positions of interband transitions suggest that the on-site Coulomb repulsion ($U$) is smaller than the charge-transfer energy ($\Delta$), placing nickelates in the Mott-Hubbard regime rather than the charge-transfer regime typical of cuprates.
• Multiband Electronic Evolution:
  • The two-band Drude analysis confirms that as Sr doping increases, the spectral weight of the hole band increases (the hole pocket expands) while the spectral weight of the electron band decreases (the electron pockets shrink).
  • Crucially, the electron band does not completely deplete even in the overdoped regime, contradicting some theoretical models that predict total electron depletion before superconductivity.
• Multiband Superconductivity: In the optimally doped sample ($x = 0.15$), temperature-dependent measurements show that both the hole and electron bands contribute to the superconducting condensate. Both Drude components show a reduction in spectral weight and a sharp drop in scattering rate ($1/\tau$) below $T_c$, signifying that both carrier types condense into the superfluid.

4. Significance and Contributions

This work provides definitive experimental evidence that infinite-layer nickelates are multiband superconductors.

Major contributions include:

1. Distinguishing Nickelates from Cuprates: While they share structural similarities with cuprates, this study proves that nickelates are governed by different physics—specifically, a Mott-Hubbard multiband framework rather than a single-band charge-transfer framework.
2. Refining Theoretical Models: By showing that electron pockets persist and participate in superconductivity, the study suggests that the $5d$ electron bands are not "spectators" but are active participants in the superconducting state.
3. Mapping the Phase Diagram: The study provides a quantitative link between optical spectral weight redistribution and the changes in Fermi surface topology (evidenced by the Hall coefficient sign change).

In conclusion, the paper establishes that the interplay between Ni $3d$ and rare-earth $5d$ orbitals is fundamental to the superconducting mechanism in nickelates, supporting recent theoretical proposals of multiband pairing scenarios.