Ingrid Fadelli is a writer for the website Phys.org.
Superconductors have many interesting and beneficial properties. Physicists and computer scientists are looking at their potential for different applications.
The Yu-Shiba-Rusinov (YSR) states can be produced by the combination of magnetic and superconductors. The YSR state undergoes a quantum phase transition when the impurities are added to the superconductor. YSR states and their quantum phase transition have been investigated by physicists over the past few years, but their effect on the ground state of superconductors is not well understood.
The study was carried out by researchers at the Max Planck Institute for Solid State Research, Ulm University, the GermanAerospace Center, Uppsala University, and the University of Madrid. The detailed observation of a change in the Josephson current as a signature of the YSR state's phase transition was the result of their study.
According to Christian Ast, one of the researchers who carried out the study, there are indications that the ground state changes have not been accounted for. It was never clear which side of the quantum phase transition was which. The YSR state is empty in the ground state and the impurity spin is free if the exchange is strong.
The p-Josephson junction is a reversal in the material's supercurrent that can be observed by measuring the Josephson current. Ast and his colleagues set out to measure the Josephson current across the state.
According to Ast, the Josephson current can tell us which side of the YSR state is in.
The researchers used a scanning tunneling microscope to measure a single impurity with a YSR state in their sample. The mK-STM is located at the Max Planck Institute for Solid State Research.
We had to use a very tricky detail to observe the reversal. There is a phase shift at the tunnel junction. The junctions are referred to as p-junctions because of the sign change which is equivalent to a phase shift.
The phase changes probed by Ast and his colleagues are hard to detect. A second tunnel junction is usually needed to detect these changes. Most researchers have been able to detect phase changes using a device called a SQUID.
SQUIDs are very sensitive and can detect subtle magnetic fields. The Josephson effect is what makes these devices work.
Ast said that they had mimicked the device by exploiting a second transport channel in the tunnel junction. "Constructive interference on one side of the quantum phase transition and destructive interference between the two channels on the other side manifest itself in a change in the magnitude of the Josephson current."
The smallest SQUID device has been introduced by Ast and his colleagues. They were able to detect the 0-p transition using this device.
The main difference between a conventional SQUID and our device is that we don't have a loop through which we can change the phase. The only thing we can detect is a sign change. We were able to detect the phase change in the 0-p transition of the YSR state.
A change in the Josephson current is indicative of a change in the ground state produced by the YSR state. Ast and his colleagues were able to detect this change by exploiting the interference between two tunneling channels in the Josephson effect for the first time.
The researchers hope to get new insight into phase changes in superconductors using the sensor introduced in their paper and other new devices. Their goal is to unveil new quantum limits by reducing systems to their bare minimum, suppressing interactions and decreasing them to the atomic level.
The physics of these systems can be modeled by simple theories. This is the first step in the quest for new quantum limits. We are looking to use phase sensitivity in the functionalized YSR tips to detect other exotic phenomena.
More information: Sujoy Karan et al, Superconducting quantum interference at the atomic scale, Nature Physics (2022). DOI: 10.1038/s41567-022-01644-6 Journal information: Nature PhysicsThere is a science network.
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