Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films

summary

Video file (mp4)

The gist

The gist The study probes the terahertz response of superconducting NbN thin films using atomic layer deposition to find signatures of Dynes superconductivity in their optical conductivity, which

In short

The study investigated THz optical conductivity of ALD-grown NbN films (4.5-20nm) to find signatures of Dynes superconductivity beyond standard BCS theory. For 20nm films, a distinct absorption step at half the zero-temperature spectral gap was found, which is fully explained by Dynes electrodynamics with a small, temperature-independent pair-breaking rate.

Key concepts

Optical Conductivity
This measures how much light (electromagnetic radiation) can be absorbed by a material at different frequencies. In superconductors, it reveals properties like the energy gap and superfluid density, which are crucial for understanding their behavior in the THz range.
BCS Theory
Bardeen-Cooper-Schrieffer theory is the standard model used to describe conventional superconductivity. The paper shows that this model fails to fully explain the observed absorption features in these disordered NbN films, necessitating a more complex approach.
Dynes Electrodynamics
This is a phenomenological model used to describe superconductors with disorder, introducing an extra pair-breaking rate ($\Gamma$). This modification allows the theory to accurately capture sub-gap absorption features, such as the characteristic step observed at half the energy gap.

Terminology used across episodes

This episode discusses

The paper

Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films · Read on arXiv

Frederik Bolle, Yayi Lin, Ozan Saritas, Martin Dressel, Ciprian Padurariu, Sahitya Varma Vegesna, Nitesh Yerra, Heidemarie Krüniger, Marc Scheffler

Physikalisches Institut, Universität Stuttgart · Center for Integrated Quantum Science and Technology (IQST), Universität Stuttgart · Institute for Complex Quantum Systems and IQST, Universität Ulm · Leibniz Institute of Photonic Technology, Jena · Institute for Solid State Physics, Friedrich-Schiller Universität Jena

The frequency-dependent complex optical conductivity reflects key properties of superconductors, such as the energy gap in the density of states (DOS) and the superfluid density. For disordered superconductors, the optical conductivity often can be described within Bardeen-Cooper-Schrieffer (BCS) theory, while in corresponding tunneling experiments, deviations in the observed DOS typically require modelling by the phenomenological Dynes formula. The implications of such Dynes DOS for optics were rarely discussed so far. Here we probe the terahertz conductivity of superconducting NbN thin films with thicknesses ranging from 4.5 to 20nm, which were grown by atomic layer deposition (ALD). Our frequency range from 0.3 to 2.1 THz covers energies below and above the spectral gap. For 20nm thick NbN, we find in the optical conductivity distinct deviations from the BCS model, including a step-like characteristic in the absorption at half the zero-temperature spectral gap. These observations can be fully captured by Dynes electrodynamics with a small and temperature-independent pair-breaking rate. For the other films, we also observe signs of Dynes electrodynamics, and we discuss the evolution of the energy gap, the superfluid density, and the pair-breaking rate as function of film thickness.

DOI: 10.1103/q3yw-r68s

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films".

Mira: The gist The study probes the terahertz response of superconducting NbN thin films using atomic layer deposition to find signatures of Dynes superconductivity in their optical conductivity,

Kai: First, who's behind it and why it matters.

Title and authors: Kai: Now let's talk about how this study suggests improvements for other related studies, specifically regarding the limitations of previous models <ref:2602.15003#pg5>.

Mira: They highlight that conventional BCS electrodynamics just can't describe the THz data, which is why they argue that including a finite Dynes pair-breaking parameter Γ helps get excellent agreement <ref:2602.15003#pg5>.

Lev: So for someone trying to model this, it means you need to go beyond just the standard BCS framework and explicitly include a term for that pair-breaking rate <ref:2602.15003#pg5>.

Kai: And they show that incorporating a small and temperature-independent pair-breaking rate Γ ∼ zero point zero three six ∆zero leads to the correct description of the optical conductivity, which is what they need to capture that step <ref:2602.15003#pg5>.

Mira: So the improvement is explicitly modeling this in the dynamical conductivity using a model based on Herman and Hlubina five and Equation one <ref:2602.15003#pg5>.

Lev: From an error correction side, if you were trying to run this on real hardware, you'd have to deal with that smearing of the energy gap <ref:2602.15003#pg5>.

Kai: And they show that this modification leads to a "smearing of the energy gap together with the emergence of a distinct absorption step at ∆" <ref:2602.15003#pg5>. That's the key feature they are trying to model correctly.

Mira: So, for our theoretical approach, it means we have to account for this specific feature in the optical conductivity when making predictions <ref:2602.15003#pg5>.

Lev: It means that whatever you're doing, you can't just ignore the pair-breaking term because it shows up in your results <ref:2602.15003#pg5>.

Kai: And they also showed that frequency-domain spectroscopy confirms this in the same way, reproducing the temperature and frequency dependence of the modes <ref:2602.15003#pg5>.

Mira: That means we have a solid experimental benchmark to check our theoretical predictions against when we include these effects <ref:2602.15003#pg5>.

Lev: And if you're aiming for actual hardware, you need to know that the pair-breaking rate is basically constant, which is helpful for designing stable systems <ref:2602.15003#pg5>.

Kai: So the whole point is that this paper gives us a concrete way to model those tricky sub-gap features using the Dynes formalism <ref:2602.15003#pg5>.

Mira: It means we have a better tool to quantify these in-gap states in disordered superconductors like transition-metal nitrides or carbides <ref:2602.15003#pg5>.

Lev: And that's a useful piece of information for designing systems where you need to understand the constraints on those pair-breaking terms <ref:2602.15003#pg5>.

The paper's summary: Kai: So wrapping up the discussion on "Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films," it seems like the main point is that we need to include the Dynes formalism when dealing with these types of superconductors <ref:2602.15003#pg5>.

Mira: They show that this formalism successfully reproduces the temperature and frequency dependence of the Fabry-P´erot transmission pattern observed in FDS <ref:2602.15003#pg5>.

Lev: And for us, it means we have a clear path forward for how to interpret these sub-gap features when they appear <ref:2602.15003#pg5>.

Kai: So essentially, the results indicate that Dynes scattering is relevant in the THz regime at zero magnetic field and reveals a constant Γ(T), which contrasts with previous tunneling and point-contact measurements <ref:2602.15003#pg5>.

Mira: This suggests that Dynes formalism might be the appropriate strategy to quantify sub-gap absorption that doesn't comply with Mattis-Bardeen predictions <ref:2602.15003#pg5>.

Lev: And for real hardware, it means we can build systems where we understand the constraints on those pair-breaking terms better <ref:2602.15003#pg5>.

Kai: We've seen some interesting physics here with this paper, and that's what we have for today <ref:2602.15003#pg1>.

The paper's improvements: Kai: So we just looked at how they found that the Dynes formalism helps us describe these sub-gap features in NbN films, and now we're going to talk about what this work suggests for improving other research <ref:2602.15003#pg5>.

Mira: They suggest that because conventional BCS electrodynamics just can't handle the THz data, you absolutely have to include that finite pair-breaking parameter Γ in your models <ref:2602.15003#pg5>.

Lev: From an engineering side, incorporating that requires you to model how the energy gap gets smeared out and how those absorption steps actually emerge when you try to run it on real hardware <ref:2602.15003#pg5>.

Kai: They show that this specific parameter, Γ ∼ zero point zero three six ∆zero is temperature-independent across both time-domain and frequency-domain measurements <ref:2602.15003#pg5>.

Mira: That constancy of the pair-breaking rate is pretty important because it gives us a solid experimental benchmark to check our theoretical predictions against when we include these effects <ref:2602.15003#pg5>.

Lev: If you're aiming for actual hardware, knowing that this rate doesn't change with temperature is helpful for designing stable systems <ref:2602.15003#pg5>.

Kai: So the whole point of their suggestion is that we have a better tool now to quantify those tricky sub-gap states in disordered superconductors like transition-metal nitrides or carbides <ref:2602.15003#pg5>.

Mira: That means for us, it’s a better way to model the dynamical conductivity when you're dealing with things that aren't perfectly clean systems <ref:2602.15003#pg5>.

Lev: And that moves the problem from just fitting data to actually understanding what microscopic processes are driving that constant rate of pair-breaking <ref:2602.15003#pg5>.

Kai: It’s definitely a useful piece of information for designing systems where you need to understand the constraints on those pair-breaking terms <ref:2602.15003#pg5>.

Mira: And we can use this approach to see if there are other similar signatures in different materials or regimes <ref:2602.15003#pg5>.

Lev: The next thing we need to figure out is where the limitations of this Dynes model lie when you try to push it into more complex superconducting scenarios <ref:2602.15003#pg5>.

Conclusion: Kai: So we're wrapping up on "Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films," and the main thing is that we’ve shown how including that finite pair-breaking rate Γ helps us model those sub-gap features <ref:2602.15003#pg5>.

Mira: It means for condensed matter theory, we have a better tool now to quantify those in-gap states in disordered superconductors like transition-metal nitrides or carbides <ref:2602.15003#pg5>.

Lev: I think the fact that this rate is temperature-independent across both TDS and FDS measurements is pretty robust, which makes it a good piece of data to work with on real hardware <ref:2602.15003#pg5>.

Kai: Yeah, because if you're building error correction for something like NbN, knowing that the pair-breaking term doesn't drift with temperature simplifies things <ref:2602.15003#pg5>.

Mira: Exactly. It removes one layer of complexity from the fitting process when we try to determine the actual superconducting gap or ns zero <ref:2602.15003#pg5>.

Lev: And that constant rate means the overhead for modeling it in a real system stays predictable, which is crucial for error correction architecture design <ref:2602.15003#pg5>.

Kai: It’s definitely a useful piece of information for designing systems where you need to understand the constraints on those pair-breaking terms <ref:2602.15003#pg5>.

Mira: We can use this approach to see if there are other similar signatures in different materials or regimes where standard BCS theory falls short <ref:2602.15003#pg5>.

Lev: The next thing we need to figure out is where the limitations of this Dynes model lie when you try to push it into more complex superconducting scenarios <ref:2602.15003#pg5>.

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