The first direct observation of a zero-field pair density wave is a major advancement in superconductivity.

The most convincing evidence to date that this unusual superconducting state of matter exists in an iron-based superconductor without a magnetic field is shown via tunnelling spectroscopy.

 In a non-magnetic environment, researchers found an alternative superconducting state known as a pair density wave (PDW), which cast doubt on earlier theories of superconductivity. This discovery of an iron-based superconductor with ferromagnetism opens up new possibilities for superconductivity research and may completely change the discipline.

The "holy grail" of discovery in superconductivity is a superconductor that can function at normal temperatures and pressures. Superconductivity is the phenomenon in which electrons can flow through a material with nearly negligible resistance. Modern existence may be completely transformed by such a substance. But at the moment, even the "high-temperature" (high-Tc) superconductors that have been identified need to be maintained extremely cold to work—too cold for the majority of uses.

Due to the fact that superconductors are extremely complicated materials with intertwined and occasionally conflicting magnetic and electronic states, there is still a great deal that scientists need to learn before room-temperature superconductivity can be achieved. It may be quite challenging to sort through and comprehend all of these varied moods or phases.

One such state is the so-called pair density wave (PDW), an alternative superconducting state of matter in which linked pairs of electrons are in continual motion. Up until recently, PDWs were supposed to occur only when a superconductor was surrounded by a strong magnetic field.

Recently, scientists from Columbia University, the National Institute of Advanced Industrial Science and Technology in Japan, and the Brookhaven National Laboratory of the U.S. Department of Energy directly saw a PDW in an iron-based superconducting material in the absence of a magnetic field. In the journal Nature's online issue from June 28, 2023, they discuss their findings.

The evidence has been at best vague, according to Kazuhiro Fujita, a physicist at Brookhaven who took part in the study. "Researchers in our field have theorised that a PDW could exist on its own, but the evidence has been ambiguous at best," he said. The first material in which the data unmistakably supports a zero-magnetic-field PDW is this iron-based superconductor. This is an interesting breakthrough that offers up new possible directions for superconductivity research and discovery.

Due to its inherent superconductivity and ferromagnetism, the iron pnictide EuRbFe4As4 (Eu-1144), which has a layered crystalline structure, is also highly remarkable. The group had been drawn to the material and had begun to explore it because of its peculiar dual characteristics.

"We wanted to determine whether the superconductivity and this magnetism were related. Since magnetic order typically destabilises superconductors, it is intriguing to see how the two phenomena coexist when they are present in the same compound, according to physicist Abhay Pasupathy, one of the paper's co-authors and a member of both Brookhaven and Columbia. "It's possible that the two occurrences exist in separate locations inside the complex and are unrelated. But instead, we discovered an excellent connection between the two.

Upon the emergence of the magnetic, the spatially modulated superconductivity was discovered.

In Brookhaven's ultra-low vibration lab, Pasupathy and his colleagues used a cutting-edge spectroscopic-imaging scanning tunnelling microscope (SI-STM) to study Eu-1144.

According to Fujita, "This microscope counts the number of electrons that 'tunnel' back and forth between the surface of the sample and the tip of the SI-STM when the voltage between the tip and the surface is changed. These observations enable us to map the sample's crystal structure as well as the number of electrons present at various energies at each atomic position.

They took readings of their sample when the temperature was raised, passing through two critical points: the superconducting temperature and the magnetism temperature, below which the material, is capable of carrying current without resistance and exhibiting ferromagnetic behaviour accordingly. The observations identified a gap in the spectrum of electron energies below the sample's essential superconducting temperature. Because of its magnitude being equal to the energy required to separate the electron pairs that convey the superconducting current, this gap is a significant landmark. The electrons' binding energies change with time, oscillating between a minimum and a maximum, as seen by changes in the gap. A PDW may be identified directly by these energy gap modulations.

Researchers may try to replicate this phenomenon in different materials as a result of this study, among other potential possibilities. There are more PDW-related issues that might be researched, such as attempting to infer the mobility of the electron pairs from signals that appear in other features of the material.

Many of our partners have expressed a keen interest in our study and are preparing various experiments on this material, including those utilising x-rays and muons, according to Pasupathy.

Along with He Zhao and Raymond Blackwell from Brookhaven Lab, Xiaoyang Zhu and Morgan Thinel from Columbia University, Shigeyuki Ishida from the National Institute of Advanced Industrial Science and Technology of Japan, Taketo Handa from Columbia University, Akira Iyo from the National Institute of Advanced Industrial Science and Technology of Japan, and Hiroshi Eisaki from the National Institute of Advanced Industrial Science and Technology of Japan are also members of this research team. The DOE Office of Science (BES), the National Science Foundation, the Office of Scientific Research of the Air Force, and the Japan Society for the Promotion of Science all provided funding for the project.