Transparency-controlled multiple charge transfer in superconducting junctions with local shot-noise scanning tunneling spectroscopy

summary

Video file (mp4)

The gist

The gist Shot noise scanning tunneling microscopy measurements enable access to different transparency regimes, allowing observation that shot noise evolves from a single electron tunneling regime to

In short

Researchers used shot noise scanning tunneling microscopy (noise-STM) to probe charge transport in superconducting junctions by tuning junction transparency at 2.2 K on Pb(111). They observed that as transparency increases, the shot noise signature evolves from single-electron tunneling to multiple charge transfer regimes, quantitatively matching theoretical predictions for Andreev reflections and multiple Andreev reflections.

Key concepts

Andreev Reflection (AR)
AR is a process in superconducting junctions where an electron incident on the junction is reflected as a hole. This reflection effectively transfers two elementary charges across the barrier, which is a key mechanism for charge transfer in these systems.
Multiple Andreev Reflections (MAR)
MAR occurs when multiple AR processes happen sequentially or simultaneously at low tunneling transparency. This allows for the transfer of multiple electron charges ($ne$), leading to an effective charge transfer significantly larger than one elementary charge.
Shot Noise
Shot noise measures fluctuations in the electrical current, providing direct information about the effective charge ($q$) carried by the tunneling carriers. In correlated systems like superconductors, this noise reveals deviations from the standard single-electron transport value of $1e$.

Terminology used across episodes

This episode discusses

The paper

Transparency-controlled multiple charge transfer in superconducting junctions with local shot-noise scanning tunneling spectroscopy · Read on arXiv

Yudai Sato, Maialen Ortego Larrazabal, Jian-Feng Ge, Ingmar Swart, Doohee Cho, Wolfgang Belzig, Juan Carlos Cuevas, Milan P. Allan,  and Jiasen Niu

Leiden Institute of Physics, Leiden University · FakultΤt f཮ r Physik, Ludwig-Maximilians-UniversitΤt Munich · Munich Center for Quantum Science and Technology · Center for Nano Science, Ludwig-Maximilian University Munich · Debye Institute of Nanomaterials Science, Utrecht University · Max Planck Institute for Chemical Physics of Solids · Department of Physics, Yonsei University · Fachbereich Physik, Universität Konstanz

Charge transport in superconducting junctions at finite voltages is governed by Andreev reflections, including multiple Andreev reflections, which are processes that enable multiple charge transfer, a hallmark that shot noise can directly quantify. Since the effective charge extracted from shot noise measurements varies with the transparency of the junction, systematic control of transparency is essential but experimentally challenging. Here, we present shot noise scanning tunneling microscopy measurements enabled by a newly developed amplifier, allowing access to different transparency regimes. We perform shot noise measurements on Pb(111) with tunable transparency at 2.2 K and observe that the shot noise evolves from a single electron tunneling regime to multiple charge transfer regime as transparency increases. Our results are quantitatively consistent with theoretical simulations of Andreev reflections and multiple Andreev reflections for a single-channel system. These results establish junction transparency as the key parameter governing the evolution of charge transport and demonstrate that noise-STM is a powerful platform for investigating microscopic charge transport mechanisms with controlled junction transparency at the atomic scale.

DOI: 10.1103/81fz-49c6

Transcript

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

Kai: We're looking at the paper: "Transparency-controlled multiple charge transfer in superconducting junctions with local shot-noise scanning tunneling spectroscopy".

Mira: Here's the gist of the paper. The gist Shot noise scanning tunneling microscopy measurements enable access to different transparency regimes, allowing observation that shot noise evolves from a single electron tunneling regime to multiple charge transfer regime as transparency increases In superconducting junctions, charge can be transferred in units larger than a single electron via multiple-charge transport processes.

Kai: For example, in normal metal (N) - insulator (I) - superconductor (S) junctions Andreev reflection (AR) occurs when a bias voltage V lower than the superconducting gap Δis applied across the junction, resulting in an effective transfer of charge 2e.

Mira: In superconductor–insulator–superconductor (SIS) junctions, the tunneling current can be mediated by multiple Andreev reflections (MAR), a process effectively transferring multiple charges across the junction.

Kai: A MAR process of order n corresponds to an effective charge transfer of ne and occurs at low tunneling junction transparency (τ) in the voltage range 2Δ/n < eV < 2Δ/(n-1) as shown in Fig.

Paper summary: Kai: So we're talking about this paper by Sato et al., "Transparency-controlled multiple charge transfer in superconducting junctions with local shot-noise scanning tunneling spectroscopy." The main claim is that they developed a new amplifier setup to access different transparency regimes in their noise measurements.

Mira: They are investigating how the effective charge extracted from these shot noise measurements changes depending on the transparency of the junction.

Kai: Specifically, they perform these measurements on Pb(one hundred eleven) at two point two Kelvin and observe this evolution as transparency is increased.

Lev: The process they are tracking is basically moving from a regime where you just see single electron tunneling to a regime where multiple charge transfer processes become significant.

Mira: The paper connects the measured noise behavior to theoretical simulations of Andreev reflections and multiple Andreev reflections for a single-channel system.

Kai: This gives us concrete evidence that this mechanism is happening in the real system, not just in a simplified model.

Lev: If they can control that transparency locally, it opens up new avenues for how we think about correlated charge transport in nanoscale devices and junctions.

Conclusion: Kai: Thinking about this paper, "Transparency-controlled multiple charge transfer in superconducting junctions with local shot-noise scanning tunneling spectroscopy," the authors are focusing heavily on how controlling the transparency affects the noise signature.

Mira: The implication is that we can use these measurements to engineer transport properties by adjusting how open or closed that junction is at specific spots.

Kai: So, instead of just measuring a fixed property, you can systematically change it and see exactly how the charge transfer mechanism shifts between single and multiple events.

Lev: For us working on real hardware, this suggests that understanding the transparency dependence isn't just academic; it tells us exactly what kind of noise signature we should expect when we build a device with a certain coupling strength.

Mira: It confirms that the transparency parameter is fundamentally important for describing charge transport in these superconducting interfaces, whether you are looking at SIS or SIN junctions.

Kai: So, to sum up this work is that shot noise scanning tunneling spectroscopy gives us a tool to map out the landscape of charge transfer as we change the junction parameters.

More episodes

← Home