Breakthrough in Superconductivity: Direct Visualization of Zero-Field Pair Density Wave

In the crystal lattice of Eu-1144, a superconducting material, the movement of electron pairs (represented by yellow spheres) is visually demonstrated as a blue and magenta wave, illustrating the spatial modulation of their energy levels within the crystal. Credit: Brookhaven National Laboratory

Through tunneling spectroscopy, scientists have obtained compelling evidence for the existence of an exotic superconducting state called a pair density wave (PDW) in an iron-based superconductor, even without the presence of a magnetic field. This groundbreaking discovery challenges previous understandings of superconductivity and paves the way for transformative research in the field.

The pursuit of superconductivity, where electrons can flow through a material without encountering any resistance, has long been focused on discovering a superconductor that can operate under everyday temperatures and pressures. Such a material could revolutionize numerous applications. However, the currently known “high-temperature” (high-Tc) superconductors still require extremely low temperatures to function, limiting their practicality.

Achieving room-temperature superconductivity remains a significant scientific challenge due to the complex nature of superconductors, which involve intricate interactions between magnetic and electronic states. Untangling and comprehending these different phases pose substantial difficulties.

Among these states, the pair density wave (PDW) represents an alternate form of superconductivity characterized by moving electron pairs. Until now, PDWs were believed to emerge exclusively under the influence of strong magnetic fields.

In a recent study published in the journal Nature on June 28, 2023, researchers from the U.S. Department of Energy’s Brookhaven National Laboratory, Columbia University, and Japan’s National Institute of Advanced Industrial Science and Technology have directly observed a PDW in an iron-based superconducting material, EuRbFe4As4 (Eu-1144), without the presence of a magnetic field. This material is particularly noteworthy as it naturally exhibits both superconductivity and ferromagnetism, offering a unique avenue for investigation.

The research team aimed to investigate the relationship between magnetism and superconductivity in Eu-1144, as the coexistence of these two phenomena within a single compound presents intriguing possibilities. Historically, superconductivity tends to be destabilized by magnetic order. However, the team discovered an unexpected connection between the two states.

To examine Eu-1144, the scientists employed a cutting-edge spectroscopic-imaging scanning tunneling microscope (SI-STM) in Brookhaven’s ultra-low vibration laboratory. This advanced microscope measures electron tunneling between the sample’s surface and the microscope’s tip while varying the voltage. These measurements allow the creation of maps depicting the crystal lattice and the distribution of electrons at different energy levels in the material.

As the temperature of the sample increased, the researchers conducted measurements crossing two critical points: the magnetism temperature, where ferromagnetism emerges, and the superconducting temperature, where the material exhibits zero electrical resistance.

Below the critical superconducting temperature, the measurements revealed a gap in the spectrum of electron energies. This gap serves as a crucial indicator, representing the energy required to disrupt the electron pairs carrying the superconducting current. Modulations in the gap disclose variations in the electrons’ binding energies, oscillating between minimum and maximum values. These energy gap modulations directly signify the presence of a PDW.

This remarkable finding opens up new research directions, including attempts to reproduce this phenomenon in other materials. Additionally, investigating various aspects of PDWs, such as indirectly detecting the movement of electron pairs through signatures in other material properties, offers further avenues for exploration.

The significance of this discovery has garnered immense interest from numerous collaborators, who are already planning diverse experiments utilizing techniques such as x-rays and muons, as mentioned by physicist Abhay Pasupathy, one of the co-authors of the study.

Reference: “Smectic pair-density-wave order in EuRbFe4As4” by He Zhao, Raymond Blackwell, Morgan Thinel, Taketo Handa, Shigeyuki Ishida, Xiaoyang Zhu, Akira Iyo, Hiroshi Eisaki, Abhay N. Pasupathy, and Kazuhiro Fujita, 28 June 2023, Nature.
DOI: 10.1038/s41586-023-06103-7

The research team comprises 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). Funding for this work was provided 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.

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• Nature: Smectic pair-density-wave order in EuRbFe4As4
• Brookhaven National Laboratory
• Columbia University
• National Institute of Advanced Industrial Science and Technology, Japan