Japanese researchers find contradictions to existing theory on superconductors

RIKEN researcher Ching-Kai Chiu and his collaborator have revealed the origin of a mysterious experimental observation in superconductors that contradicts an existing theory, according to a RIKEN article published Jan. 30.

Superconductors conduct electricity, with no resistance, due to their electrons forming pairs with an energy gap. Placing a magnetic atom on top of a superconductor, however, creates a new state within the gap due to the atom's magnetism interacting with the superconductor's paired electrons. 

This state is known as the Yu-Shiba-Rusinov state or Shiba state. It has attracted much interest in the scientific community, because it could help them learn more about the emergence of a unique state in a topological superconductor, called the Majorana zero mode, which may be promising for the realization of fault-tolerant quantum computing.

The Japan-based researchers conducted a theoretical study that exposed the truth behind an observation in a superconductor with a magnetic impurity on top of it, finding that the interaction of the magnetic atom with the superconductor's bulk states creates a double crossing in the energy gap. 

The discovery could help in the creation of a robust quantum state, residing in a superconductor, that could be useful in quantum computers.

Experimentalists use scanning tunneling microscopes to study superconductors with magnetic atoms resting on them. Controlling the distance between the microscope tip and the atom allows them to control the strength, at which the atom interacts with the superconductor. One finding that has puzzled researchers is the observation that the Shiba states have two zero-energy crossings due to the varied interaction, which differs from a theory predicting only one such crossing.

"There's actually an inconsistency between theory and experiment," Chiu said. "That's the motivation behind our study."

Chiu and his collaborator conducted a theoretical analysis of such a system and discovered the origin of the double-crossing. They found that it's caused by the interaction of the magnetic atom with the superconductor's bulk states. Prior analyses missed this because they had only considered the superconductor's surface states.

"Our analysis has shown that you can't ignore the bulk theory," Chiu said. "You need to consider the entire system."

“Initially, we didn’t have any intention of explaining the two crossings,” Chiu continued. “It came as a real surprise to us. When we ran our simulations and we saw them, we thought it might have been an error. But further analysis showed that we had stumbled on the origins of the double-crossing.”