Resistance measurement approach successfully observes topological signatures in multiterminal Josephson junctions by StuffsEarth

Multiterminal Josephson junctions, nanoscale devices with unique electronic properties, comprise non-superconducting metallic material coupled to three or more superconducting leads. These devices have proved to be promising platforms for the exploration of topological phenomena in condensed matter physics.

Researchers at Northwestern University and Aalto University recently proposed a new approach to studying the topological signatures of multiterminal Josephson junctions, which relies on the collection of resistance measurements.

Using their approach, outlined in a paper published in Physical Review Letters, they were able to observe these signatures, while also unveiling resistance patterns that are far richer than those predicted by physics theories.

“The subject of this paper was inspired by a talk given at the APS March Meeting, which explored the idea that the superconducting phase difference in a Josephson junction could play the role of electron momentum in a real crystal,” said Maxwell Wisne, first author of the paper.

“This is a tantalizing proposition to the experimentalist. Compared to the electron momentum, the Josephson phase is a degree of freedom amenable to direct manipulation, opening the possibility for us to explore the rich physics of the superconducting proximity effect and help answer the question: What is the connection between superconducting device physics and topology?”

The recent paper by Wisne and his colleagues draws inspiration from recent theoretical studies, which examined the similarity between electronic states in a crystal and those in multiterminal Josephson junctions. Building on these theories, the researchers first predicted the topological behavior of these devices.

“Our goal was to see whether we could observe signatures of such topological behavior by measuring the resistance of suitably designed devices rather than the tunneling density of states as had been done previously, as fabrication of devices for resistance measurements is simpler,” Venkat Chandrasekhar, senior author of the paper, told Phys.org.

To map the proximity-induced gap (i.e., an energy gap in the electronic density of states that form in non-superconducting material that is in contact with a superconductor), the team collected four-terminal conductance measurements. While this method of measuring conductance is fairly simple, to detect milliohm changes in resistance, the researchers first had to remove noise from the system.

“The on- chip field coils, on the other hand, were more fun to use,” explained Wisne. “They allowed us to induce separate, localized magnetic fields that pushed the phase anywhere we wanted in 2D space. For example, we could increment step-by-step the field from one coil while slowly sweeping the other coil back and forth for six days. This gave us the heat maps shown in the right panel.”

In one of his previous studies, Chandrasekhar had theoretically predicted the existence of a correspondence between the resistance and density of quasiparticles states. Building on this prediction, this new study measured the resistance of diffusive normal metals connected to three superconducting contacts.

“A key feature of our experiment was that the superconducting contacts were connected in loops, so that, by threading a magnetic field through the loops using on-chip field coils, we could independently and statically vary the phase differences between the superconducting contacts,” said Chandrasekhar.

“We observed a rich periodic structure of the resistance with these phase differences, reflecting the topological nature of the density of states in the normal metal.”

The data that the researchers collected in their experiments was more structured than what they had previously predicted. To compare their observations to theoretical predictions, the team first had to ensure that all complementing aspects of their experiments were adequately designed.

“Much effort was poured into refining the measurement technique and separately fine-tuning the simulations,” said Wisne. “While we’re unsure why our measurements show features absent in the theory, we can only trust the data and indulge our speculation: What new physics are we seeing here?”

The recent study by Wisne, Chandrasekhar and their colleagues highlights the promise of resistance measurements for studying the structure underlying the density of states. Using these measurements, they were able to pin-point the topological signatures of multiterminal Josephson junctions.

“We also observed that the experimentally measured resistance shows far richer structure than predicted by theory, indicating that our understanding of the underlying physics is not complete and will require further theoretical and experimental investigation,” said Chandrasekhar.

The methods employed by Wisne, Chandrasekhar and their collaborators as part of their recent study could soon be used by other research groups studying multiterminal Josephson junctions. Their findings could collectively improve the understanding of these devices, shedding new light on their unique topological behavior.

“Our paper shows that, despite the intense research the superconducting proximity effect has undergone in the last few decades, there’s still more to learn,” said Wisne.

“We think the energy structure from multiterminal Josephson junctions especially holds promise for the topological physics and quantum information communities. We’ll just need to continue to follow the data.”

As part of their recent study, the researchers used Josephson junctions with a normal metal in which electron transport is diffusive. In their future research, they will also try to examine different devices in which this electron transport is ballistic (i.e., scattering through the normal metal without scattering or colliding with impurities or phonons).

“For our next series of experiments, we plan to measure devices in which electron transport in the normal metal is ballistic, focusing on both resistance measurements as well as tunneling measurements to directly probe the quasiparticle density of states,” added Chandrasekhar.

“These devices will use graphene as the normal metal and will also employ on-chip field coils in order to enable independent, static control of the phase differences between the superconductors.”

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