New superconducting states discovered: Cooper pair density modulation

Superconductivity is a quantum physical state in which a metal can conduct electricity completely without resistance. In the most well-known application, a powerful magnet within an MRI machine allows doctors to create a magnetic field that allows them to see inside our bodies. So far, the material has achieved superconductivity at very low temperatures, close to absolute zero (dozens or cold, below tens of Kelvin).

However, physicists dream of superconducting materials that may one day operate at room temperature. Such materials could open up entirely new possibilities in areas such as quantum computing, the energy sector, and medical technology.

“Understanding the mechanisms that lead to the formation of superconductivity and discovery of exotic new superconducting phases is not only one of the most exciting pursuits in the fundamental research of quantum materials, but is also driven by this ultimate dream of achieving room temperature superconductivity.”

The team led by Nadj-Perge, which now includes Lingyuan Kong, the AWS Quantum Postdoctoral Scholar Associate and other colleagues at Caltech, has provided discoveries that offer a new puzzle behind this mysterious but powerful phenomenon. The paper on the work was published in Nature on March 19th.

In normal metals, individual electrons collide with ions as they move across a metal lattice structure made up of oppositely charged ions. Each collision causes electrons to lose energy and increases electrical resistance. On the other hand, in superconductors, electrons are weakly attracted and bonded to each other, forming a duo called a Cooper pair.

As long as the electrons remain within a certain relatively small range of energy levels known as the energy gap, the electrons remain in pairs and do not lose energy in collisions. Therefore, it is within a relatively small energy gap that superconductivity occurs.

Typically, the energy gap of a superconductor is the same everywhere in the material. For example, in a superconducting crystal without impurities, all parts of the crystal have the same energy gap.

However, from the 1960s, scientists began theorizing that the energy gaps in some superconducting materials could be adjusted in space. This means that gaps can be stronger in some regions and weaker in others. Later, in the 2000s, the idea was further developed, with the proposal of what is called paired density wave (PDW) states. This suggests that superconducting states can occur where the energy gap is modulated at long wavelengths.

Over the past decade, this concept has attracted great experimental interest in numerous materials, including iron-based superconductors being investigated as potential hosts in the PDW state.

In collaboration with the very thin flakes of Fete0.55Se0.45, now an iron-based superconductor, Nadj-Perge and his colleagues have discovered modulation of the superconducting gap with the smallest wavelength that matches the spacing of the crystal atoms. They named it the Cooper Pair Density Modulation (PDM) state.

“The observed gap modulation reaches up to 40%, representing the most powerful ever reported, leading to the clearest experimental evidence to date, and gap modulation may also be present at the atomic scale,” says Kong, the lead author of the new paper.

This unexpected discovery was made possible by the first successful realization of scan tunneling microscopy experiments of iron-based superconductors on specialized devices to study such thin flakes. Such experiments have been hampered by the presence of severe surface contamination for nearly 20 years, but the Caltech team working at the Kavli Nanoscience Institute (KNI) has developed a new experimental approach that allows for a sufficiently clean surface for microscope probes.

Maikawa Papaji, assistant professor of physics at the University of Houston, and Patrick A. Lee Lee, California, professor of physics in California, who is also the author of a new paper, have developed a theoretical model to explain the possible origins of observed gap adjustments. “Their model suggests that modulation of the PDM state arises from both the disruption of sublattic symmetry and the unique rotational symmetry characteristic of thin flakes,” says Nadj-Perge.