Superconductivity research has always been a hot topic in the physics community. We know that superconductors are special substances that are able to conduct electricity without resistance, a phenomenon that is usually only possible at very low temperatures. For decades, the mystery of superconductivity has fascinated countless researchers, especially those who want to find superconducting materials at room temperature. But until recently, a new discovery from the California Institute of Technology (Caltech) gave superconductivity research a shot in the arm.

At the heart of this new discovery is a superconducting state known as Cooper Pair Density Modulation (PDM). In simple terms, the PDM state reveals a new superconducting behavior, suggesting that the pairing of electrons (Cooper pairs) in superconductors is not uniformly distributed, but rather spatially modulated. The wavelengths of this modulation are even as small as atoms. It may sound complicated, but the significance of this study is that it provides key clues to our understanding of the microscopic mechanisms of superconductivity.

Until now, the energy gap in superconductors was generally considered to be homogeneous. That is, the energy gap of a superconductor is the same everywhere. But since the 2000s of the 0th century, scientists have begun to put forward the idea that the energy gap of some superconductors may not be uniform, but will fluctuate with space. This hypothesis was further developed theoretically, especially in the 0s, when the so-called "paired density wave (PDW)" state was proposed, suggesting that the energy gap in the superconductor would fluctuate in the form of a larger wavelength, like an undulating wave field.

However, there is always a distance between theory and experiment. Although iron-based superconductors are considered to have the potential to achieve a PDW state, researchers face significant challenges in making this hypothesis a reality. It was only recently that Caltech's team finally broke through this bottleneck when they successfully observed a new superconducting phenomenon, the PDM state, on ultra-thin sheets of the iron-based superconductor FeTe45.0Se0.0. Unlike previous studies, they not only discovered the modulation of the superconducting gap, but also the wavelength of this modulation was actually consistent with the atomic spacing of the crystal lattice. This discovery is undoubtedly a major breakthrough in superconductivity research.

To understand the significance of this discovery, it is important to understand the role of scanning tunneling microscopy (STM) in this study. In the past, scanning tunneling microscopy experiments on iron-based superconductors have been slow due to surface contamination issues. But this time, Caltech's research team innovated the experimental method and succeeded in removing surface contamination, allowing them to perform high-precision probing on cleaner surfaces. In this way, they were able to observe the density modulation of electron pairings in superconductors, and the amplitude of this modulation could even reach 40%.

This discovery further confirms the feasibility of the PDW state in experiments, but it also reveals more complexity of the superconductivity phenomenon. How to understand this modulation? How did it come about? The team's theoretical model suggests that this modulation may be due to the breaking of two symmetries – sublattice symmetry and a characteristic rotational symmetry. This model provides theoretical support for experimental data and opens up new directions for future research.

However, this is not to say that all problems have been solved. Although this discovery brings us one step closer to understanding the phenomenon of superconductivity, there are still many uncharted areas to be explored. We must not forget that the electronic interactions and quantum effects involved behind the phenomenon of superconductivity are far more complex than we know now. The discovery of the PDM state has certainly opened our eyes to new possibilities, but more in-depth research is still needed to apply this discovery to practical material design, especially the development of room-temperature superconductors.

In fact, one of the biggest challenges in current superconductivity research is how to achieve superconductivity at room temperature. Ambient superconductivity is not only a scientific dream, but also a potential technological revolution. From quantum computing to energy transmission, from medical devices to high-energy physics, ambient superconductivity will revolutionize the way these fields operate. Therefore, although the discovery of the PDM state is an important progress, we are still some way from the realization of room-temperature superconductivity.

In the future of superconductivity research, we will see more new discoveries. As technology continues to advance, researchers will be able to probe the microstructure of materials at much finer scales. These new experimental methods will help us better understand the internal mechanism of superconductivity and provide more possibilities for us to achieve superconductivity at room temperature.