Pan Jianwei, Yao Xingcan, Chen Yuao and others from the University of Science and Technology of China observed the pseudogap generated by many-body pairing for the first time based on the strongly interacting uniform Fermi gas. This study establishes the existence of a pairing pseudogap for the first time, provides support for the electron pre-pairing hypothesis in the high-temperature superconducting mechanism, takes an important step towards understanding the high-temperature superconducting mechanism, and is an example of using quantum simulation to solve important physical problems. On February 8, this result was published in the authoritative international academic journal Nature under the title "Observation and quantification of pseudo-gap in unitary Fermi gas".
Figure 1: The two carps with jade beads on their heads symbolize a pair of fermions with opposite spins; the dragon gate represents superfluid phase transition and pseudo-energy gap. The carp jumped over the gantry, indicating that pairing occurs above the superfluid phase transition temperature. This pairing phenomenon in turn leads to the emergence of pseudoenergy gaps. /Drawing: Chen Lei
The generation of energy gap is the iconic phenomenon of superconductivity. In conventional superconductors, the energy gap exists below the superconducting phase transition temperature. With the discovery of cuprate high-temperature superconductors, energy gaps can still be observed even above the superconducting phase transition temperature. This phenomenon is called a pseudogap. The origin and properties of the pseudogap can provide key clues to answer the mechanism of high-temperature superconductivity. Academic circles generally believe that there are two main possible pseudogap mechanisms: one is derived from electron multi-body pre-pairing above the superconducting phase transition temperature; the other is derived from various quantum ordered phases found in high-temperature superconductors, such as antiferromagnetic order, stripe order and pairing density waves. However, because the real high-temperature superconducting material system is very complex and various possible sources of mechanisms compete with each other, it has not been clear which mechanism is at work.
The ultracold Fermi gas in the strong interaction (unitary) limit provides an ideal quantum simulation platform for the study of the mechanism of pseudogaps due to its purity and controllability. On the one hand, the strong attractive interaction between Fermi atoms creates favorable conditions for many-body pairing; on the other hand, the system can avoid competition among multiple quantum-ordered phases. Therefore, whether a pseudogap can be observed in this system will become a decisive verification of the many-body pairing mechanism. However, the realization of this scientific goal faces two major technical challenges, which are also the reasons why previous work has not achieved breakthroughs: first, it is necessary to prepare high-quality, uniform-density unitary Fermi gas; second, it is necessary to develop measurement technology similar to angle-resolved photoelectron spectroscopy in ultracold atomic systems.
After years of hard work, the research team established an ultracold lithium-dysprosium atomic quantum simulation platform and achieved the world's leading preparation of uniform Fermi gas. The research team has also developed stabilization technology for large magnetic fields. Under a magnetic field of about 700G, its short-term fluctuation is better than 25μG, and the relative magnetic field stability is close to 10-8, which is more than an order of magnitude higher than the previous international best results. Under this ultra-stable magnetic field, the research team was able to successfully implement microwave spectroscopy technology that can resolve the momentum of ultracold atoms. On this basis, the research team systematically measured the single-particle spectral function of unitary Fermi gas at different temperatures and successfully observed the existence of a pseudogap, which provided support for the electron pre-pairing hypothesis (as shown in Figure 2).
Figure 2. Schematic of single particle spectrum. The connected and independent balls represent Cooper pairs and single particles respectively, and the surface gap is the pseudo energy gap. /Drawing: Chen Lei
This research work not only advances the study of strongly correlated multi-body systems, but also provides important experimental basis for improving the multi-body theory. In addition, the ultracold atom quantum control technology developed in this work laid a technical foundation for the next step of studying other important condensed matter physics phenomena, such as single-band superfluidity, stripe phase, FFLO superfluidity, etc. The reviewers of Nature magazine unanimously agreed that "this work solves an important long-standing physical problem and is a milestone in quantum simulation research."
Relevant research teams at the University of Science and Technology of China have carried out fruitful work in quantum simulations based on ultracold atoms in recent years, and have published 10 high-quality papers in Nature and Science. Based on the accumulation of previous technologies, ultracold atom quantum simulation has begun to show significant effectiveness in revealing the laws of complex physical systems including high-temperature superconducting mechanisms, paving the way for the construction of a dedicated quantum simulator capable of solving practical problems in the near future.
Hu Hui from Swinburne University of Technology and Chen Qijin from University of Science and Technology of China are theoretical collaborators on this work. This research was supported by the Ministry of Science and Technology, National Natural Science Foundation of China, Chinese Academy of Sciences, Anhui Province, Shanghai Municipality and New Cornerstone Science Foundation.
Paper link: https://doi.org/10.1038/s41586-023-06964-y
(School of Physics, Hefei National Research Center for Microscale Physical Sciences, Institute of Quantum Information and Quantum Technology Innovation, Chinese Academy of Sciences, Research Department)