Breakthrough in superconducting technology: Physicists directly observe zero-magnetic field paired density waves for the first time

In the field of superconductivity - a phenomenon in which electrons can flow through materials with essentially zero resistance - the most important goal is a superconductor that can operate at everyday temperatures and pressures. This material could revolutionize modern life. But currently, even the "high-temperature" (high-temperature) superconductors that have been discovered must be kept very cold to function because such conditions are too cold for most applications.

Scientists still have a lot to learn before room-temperature superconductivity can be achieved, largely because superconductors are highly complex materials whose magnetic and electronic states are intertwined and sometimes competing with each other. These different states or stages can be difficult to sort out and explain.

One of these states is an alternating superconducting state of matter called a paired density wave (PDW), which is characterized by coupled electron pairs in constant motion. Until now, PDWs have been thought to arise only when superconductors are exposed to large magnetic fields.

Brookhaven Laboratory research team members (left to right) Raymond Blackwell, He Chao and Kazuhiro Fujita. Image source: Brookhaven National Laboratory

Recently, researchers at the U.S. Department of Energy's Brookhaven National Laboratory, Columbia University, and Japan's Advanced Institute of Industrial Technology directly observed PDWs in iron-based superconducting materials without a magnetic field. They describe their findings in the June 28, 2023, online edition of the journal Nature.

"Researchers in our field have speculated that PDWs may exist independently, but the evidence is murky at best," said Brookhaven physicist Kazuhiro Fujita, who was involved in the study. "This iron-based superconductor is the first with clear evidence of zero-magnetic field PDWs." materials. This is an exciting result that opens up new potential avenues for superconducting research and discovery.”

This material, the iron phosphide EuRbFe4As4 (Eu-1144), has a layered crystal structure and is also remarkable because it naturally exhibits superconductivity and ferromagnetism. This unusual dual identity initially attracted the group to the material and guided their research.

"We wanted to understand if this magnetism is related to superconductivity? In general, superconductors are destabilized by magnetic order, so when superconductivity and magnetism are both present in a single compound, it was interesting to see how the two coexist. Very interesting," said physicist Abhay Pasupathy, one of the paper's co-authors. Affiliated with Brookhaven and Columbia Universities. "It's conceivable that these two phenomena existed in different parts of the compound and were unrelated to each other. But instead, we found a wonderful connection between the two."

Pasupathy and colleagues studied Eu-1144 using a state-of-the-art spectral imaging scanning tunneling microscope (SI-STM) at Brookhaven's Ultralow Vibration Laboratory.

"As the voltage between the tip and the surface changes, the microscope measures the number of electrons traveling back and forth between the sample surface and the SI-STM tip at specific locations in the material's 'channel,'" Fujita said. "These measurements allow us to create sample crystals." A graph of the lattice and the number of electrons of different energies at each atomic position.”

They measured the sample as the temperature increased, passing two critical points: the magnetic temperature, below which the material behaves ferromagnetically, and the superconducting temperature, below which the material is able to carry electrical current with zero resistance. ).

Below the sample's critical superconducting temperature, the measurements showed gaps in the electron spectrum. This gap is an important signature because its size corresponds to the energy required to break apart the electron pairs that carry the superconducting current. The modulation of the gap reveals changes in the electron binding energy, which oscillates between a minimum and a maximum. These energy gap modulations are direct characteristics of PDWs.

The discovery points researchers in some new directions, such as trying to reproduce the phenomenon in other materials. Other aspects of PDWs can also be studied, such as attempts to indirectly detect the motion of electron pairs through features revealed in other properties of the material.