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Tunneling Spectroscopy Shows First Direct Visualization of Pair Density Wave

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Jul 4 2023Reviewed by Skyla Baily

Superconductivity occurs when electrons can flow through a material with fundamentally zero resistance. Researchers have chased the “holy grail” of breakthroughs, a superconductor that can function under daily pressures and temperatures. Such a material has the potential to transform modern life. However, at present, even the “high-temperature” (high-T_c) superconductors that have been found must be kept very cold to work with—extremely cold for a majority of applications.

In this illustration of the superconducting material Eu-1144, the blue and magenta wave shown above the crystal lattice represents how the energy level of the electron pairs (yellow spheres) spatially modulates as these electrons move through the crystal.

Still, researchers have a lot to learn before room-temperature superconductivity can be realized, mostly because superconductors are highly complicated materials with interwoven and, at times, competing electronic and magnetic states. Such varied states, or phases, could be very challenging to interpret and untangle.

One such state is a substitutive superconducting state of matter called a pair density wave (PDW), defined by coupled pairs of electrons that are in movement continuously. PDWs are believed to only emerge when a superconductor is kept within a huge magnetic field—until now, that is.

Scientists from the US Department of Energy’s Brookhaven National Laboratory, Columbia University, and Japan’s National Institute of Advanced Industrial Science and Technology directly noticed a PDW in an iron-based superconducting material without the existence of a magnetic field. They explain their outcomes in the June 28^th, 2023, online edition of the journal Nature.

Researchers in our field have theorized that a PDW could exist on its own, but the evidence has been ambiguous at best. This iron-based superconductor is the first material in which the evidence clearly points to a zero-magnetic-field PDW. This is an exciting result that opens new potential avenues of research and discovery for superconductivity.

Kazuhiro Fujita, Physicist, Brookhaven National Laboratory

The material, the iron pnictide EuRbFe₄As₄ (Eu-1144), has a layered crystalline structure and is also fairly remarkable as it naturally reveals both ferromagnetism and superconductivity. This rare dual identity is what directed the team to the material in the first place.

We wanted to see, is this magnetism linked to the superconductivity? In general, superconductors are destabilized by magnetic order, so when both superconductivity and magnetism exist together in a single compound, it is interesting to see how the two of them coexist. It’s conceivable that the two phenomena exist in different parts of the compound and have nothing to do with each other. But, instead, we found that there is a beautiful connection between the two.

Abhay Pasupathy, Study Co-Author, Brookhaven National Laboratory

Abhay Pasupathy is affiliated with both Brookhaven and Columbia.

At Brookhaven’s ultra-low vibration laboratory, Pasupathy and his co-workers studied Eu-1144 with the use of an advanced spectroscopic-imaging scanning tunneling microscope (SI-STM).

This microscope measures how many electrons at a specific location in the material ‘tunnel’ back and forth between the sample’s surface and the tip of the SI-STM as the voltage between the tip and the surface is varied. These measurements allow us to create a map of both the sample’s crystal lattice and the number of electrons at different energies at each atomic location.

Kazuhiro Fujita, Physicist, Brookhaven National Laboratory

As its temperature increased, they conducted measurements on their sample, which passed between two critical points: the magnetism temperature, below which the material shows ferromagnetism, and the superconducting temperature, below which the material could carry current with zero resistance.

“This is an exciting result that opens new potential avenues of research and discovery for superconductivity,” adds Fujita.

Below the critical superconducting temperature of the sample, the measurements have shown a gap in the spectrum of electron energies. This gap is an essential marker as its size is equivalent to the energy it consumes to split apart the electron pairs that carry the superconducting current. Modulations in the gap show differences in the binding energies of electrons, oscillating between a maximum and minimum. Such energy gap modulations are a direct signature of a PDW.

This breakthrough points scientists in some new directions, like attempting to repeat this occurrence in other materials. Also, there are other factors of a PDW that could be examined, like attempting to indirectly sense the mobility of the electron pairs through signatures that reveal other characteristics of the material.

“Many of our collaborators have shown great interest in our work and are already planning different types of experiments on this material, such as using x-rays and muons,” stated Pasupathy.

This study team also included He Zhao (Brookhaven Lab), Raymond Blackwell (Brookhaven Lab), Morgan Thinel (Columbia University), Taketo Handa (Columbia University), Shigeyuki Ishida (National Institute of Advanced Industrial Science and Technology, Japan), Xiaoyang Zhu (Columbia University), Akira Iyo (National Institute of Advanced Industrial Science and Technology, Japan), and Hiroshi Eisaki (National Institute of Advanced Industrial Science and Technology, Japan). The study was financially aided by the DOE Office of Science (BES), the National Science Foundation, the Air Force Office of Scientific Research, and the Japan Society for the Promotion of Science.