1 Ultracold research confirms pseudogap pairing due to strong attraction by Hui Hu

Chinese physicists have made a new experimental observation, confirming the many-particle pairing of fermions, before they reach a critical temperature and exhibit remarkable quantum superfluidity.

In a paper published in Nature [1], researchers at the University of Science and Technology of China (USTC) observed and quantified the pseudogap pairing in a strongly attractively interacting cloud of fermionic lithium atoms, which may help unlock the mysterious microscopic mechanism of high-temperature superconductivity.

Quantum superfluidity/superconductivity is the most intriguing phenomenon of quantum physics. In the superfluid state, quantum particles collectively flow in a frictionless way without losing any kinetic energy. High-temperature superconducting materials therefore hold the prospect of solving the current world’s energy crisis. However, despite substantial efforts spanning the past 38 years, the origin of high-temperature superconductivity, especially the emergence of an energy gap in the normal state preceding superconductivity, continues to elude researchers.

This energy gap without superconducting is the well-known pseudogap, whose origin is controversial and is the subject of a long-lasting debate in the condensed matter community. In one of the two mainstream interpretations, the scenario of preformed pairs, the pseudogap is produced by incoherent fluctuations of the pairing field that describes the entangled fermions due to attraction; however, superfluidity does not arise because large phase fluctuations of the pairing field cannot allow quantum particles to march in the exact same manner above the critical temperature.

The central aim of the USTC researchers’ work was to quantum emulate a simple text-book model to examine the scenario of preformed pairs, by using a table-top system of ultracold atoms.

Quantum simulation using ultracold atoms is a rapidly developing field that was established following the Nobel Prize-winning research on laser cooling of atoms in the late 1980s and on Bose-Einstein condensation in 1995. A cloud of atoms can be cooled down to incredibly low temperatures of a billionth of a degree Kelvin, just above absolute 0. At these temperatures, fermionic atoms in different hyperfine states, as the atomic analog of electrons and protons which are the building blocks of all matter, can bind together due to attraction to form Cooper pairs and consequently condense into a macroscopic quantum state, precisely following the rule of quantum mechanics.

The unique advantage of ultracold atoms is their unprecedented controllability. By utilizing Feshbach resonances and applying a magnetic field at the right strength, interactions between atoms can be controlled with great precision—from arbitrarily weak to arbitrarily strong. This allows scientists to monitor, step by step, the emergence and the evolution of the pairing field in a strongly interacting Fermi cloud of about a million atoms, with decreasing temperature.

At USTC, Professor Xing-Can Yao, Professor Jian-Wei Pan, and their colleagues confined strongly attracting fermionic lithium-6 atoms into a uniform box potential and cooled them down to near absolute 0. By developing a novel technology of momentum-resolved microwave spectroscopy with high energy resolution, the pseudogap pairing above the critical superfluid transition temperature was clearly revealed for the first time.

The investigation of pseudogap pairing with ultracold atoms was attempted in 2010. However, at that time the cloud of fermionic atoms was not homogeneously distributed, leading to an inhomogeneity-related spectral broadening in the spectroscopy. Moreover, atoms that were experimentally probed suffered from some additional, unwanted interatomic collisions during the spectroscopic imaging. The two obstacles made the previous observation of pseudogap inconclusive.

In the USTC experiment, state-of-the-art methods were utilized for preparing the homogeneous Fermi cloud and for removing unwanted interatomic collisions, with ultra-stable magnetic field control at unprecedented levels of a quarter of ppm (i.e., less than one millionth). As a result of these technical advances, the suppression of spectral weight near the Fermi surface in the normal state was unequivocally revealed in microwave spectroscopy. This is the smoking gun of a pseudogap, without the need to invoke any specific microscopic theories to fit the experimental data.

This discovery will undoubtedly have far-reaching implications for the future study of strongly interacting Fermi systems and could lead to potential applications in future quantum technologies.

An immediate application is to probe the long-sought inhomogeneous Fermi superfluidity in a spin-population imbalanced Fermi gas, where the standard pairing field will be modified and become spatially inhomogeneous. This exotic pairing scenario was proposed by Fulde, Ferrell, Larkin, and Ovchinnikov 60 years ago. It may underlie the imbalanced nucleon superfluidity of neutron-proton pairs at the core of neutron stars and may have some relevance in explaining the sudden variations (glitches) in the rotation periods of pulsars.



  1. A White Paper from the Office of National Higher Education Science Research and Innovation Policy Council, Thailand S&T Strategy. https://www.nxpo.or.th/th/frontier-research-3/

  2. N. Ratsuntia, A. Songmunnak, N. Ritjoho, T. Sanghangthum and C. Kobdej, In Study of imaging system in proton computed tomography with data acquisition from a monolithic active pixel sensor, Siam Physics Congress 2022: Carbon Neutrality (SPC 2022). J. Phys. Conf. Ser., IOP Publishing. 2431(2023), 012093 (2022). https://doi.org/10.1088/1742-6596/2431/1/012093

  3. A. Wisitsorasak, A. Shimizu, S. Sangaroon, K. Ogawa, B. Chatthong, S. Suksaengpanomrung, A. Tamman, N. Poolyarat, M. Isobe, Feasibility study of a Heavy Ion Beam Probe for the Thailand Tokamak-1. Fusion Eng. Des. 198, 114068 (2024)

  4. P. Poramapijitwat, P. Thana, P. Sukum, Y. Liangdeng, C. Kuensaen, ...D. Boonyawan, Selective Cytotoxicity of Lung Cancer Cells—A549 and H1299—Induced by Ringer’s Lactate Solution Activated by a Non‑thermal Air Plasma Jet Device, Nightingale®. Plasma Chem. Plasma Process. 43, 805–830 (2023)

  5. The Brainpower Congress, Program Management Unit for Human Resources & Institutional Development, Research and Innovation (PMU-B), Dec 18-20, 2023 (2023)

  6. M. S. Bahramy, O. J. Clark, B.-J. Yang, J. Feng, L. Bawden, J. M. Riley, I. Marković, F. Mazzola, V. Sunko, D. Biswas, S. P. Cooil, M. Jorge, J. W. Wells, M. Leandersson, T. Balasubramanian, J. Fujii, I. Vobornik, J. E. Rault, T. K. Kim, M. Hoesch, K. Okawa, M. Asakawa, T. Sasagawa, T. Eknapakul, W. Meevasana & P. D. C. King, Ubiquitous formation of bulk Dirac cones and topological surface states from a single orbital manifold in transition-metal dichalcogenides. Nat. Mater. 17(1), 21–27 (2018)

  7. G. Gitgeatpong, Y. Zhao, P. Piyawongwatthana, Y. Qiu, L. W. Harriger, N. P. Butch, T. J. Sato, and K. Matan, Nonreciprocal Magnons and Symmetry-Breaking in the Noncentrosymmetric Antiferromagnet. Phys. Rev. Lett. 119, 047201 (2017)

  8. C. Thongbai, N. Khangrang, W. Jaikla, P. Kitisri, K. Kongmali, M.W. Rhodes, S. Rimjaem, J. Saisut, S. Supasin, S. Suphakul, Development of an Undulator Magnetic Field Measurement System for the Free-Electron Laser Facility at Chiang-Mai University. Physica Status Solidi (A) Applications and Materials Science. 220(10), 2200398 (2023)

  9. W. Passatorntaschakorn, W. Khampa, W. Musikpan, C. Bhoomanee, A. Ngamjarurojana, S. Rimjaem, Atcharawon Gardchareona,c, C. Rodwihok, H. S. Kim, N. Khambunkoed, R. Supruangnet, H. Nakajima, L. Srathongsian, P. Kanjanaboos, A. Intaniwet, A. Kaewprajak, P. Kumnorkaew, F. Goubard, P. Ruankham, D. Wongratanaphisan, A novel carbon electrode for up-scaling flexible perovskite solar cells. Appl. Mater. Today. 34, 101895 (2023)

  10. For additional details, please visit: https://www.thep-center.org