March 1, 2024

Unraveling mysteries of quantum superconductivity with ultracold fermions

Researchers have made a landmark discovery in quantum physics by observing and quantitatively characterizing the many-body pairing pseudogap in unitary Fermi gases, a topic of debate for nearly two decades. This discovery not only resolves long-standing questions about the nature of the pseudogap in these gases, but also suggests a potential link to the pseudogap observed in high-temperature superconductors. Credit: SciTechDaily.com

Researchers have conclusively observed the many-body pairing pseudogap in unitary Fermi gases, advancing our understanding of the mechanisms of superconductivity.

A research team led by professors Jianwei Pan, Xingcan Yao and Yu’ao Chen from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences has observed and quantitatively characterized many-body pairing for the first time. pseudogap in unitary Fermi gases.

This achievement, pursued by the ultraglued atomic community for nearly two decades, resolves long-standing debates about the existence of a pairing pseudogap in these gases. It also supports pairing as a possible origin of the pseudogap in high-temperature superconductors, within the framework of the preformed pair theory of superconductivity.

Published in Nature on February 7, this study coincides with the upcoming Year of the Dragon. Interestingly, the physics behind this achievement can be vividly illustrated by the iconic Chinese myth of the “Carp Leaping Over the Dragon Gate,” symbolizing great success in Chinese culture.

Carp jumping over Dragon Gate fermions

In this artistic representation, two carps, each holding a jade bead in their mouth, symbolize fermions with opposite spins. The Dragon Gate represents both the superfluid transition and the pseudogap. The depiction of the carp leaping over the Dragon’s Gate suggests pairing above the superfluid phase transition temperature. This pairing phenomenon, in turn, leads to the appearance of the pseudogap. Credit: Lei Chen

Unraveling the mysteries of superconductivity

The existence of an energy gap is a characteristic phenomenon of superconductivity. In conventional superconductors, the energy gap exists below the superconducting transition temperature (Tw). Notably, in high-temperature cuprate superconductors, the energy gap can still be observed even above Twa phenomenon known as pseudogap.

Understanding the origin and nature of the pseudogap is crucial to understanding the mechanism of high-temperature superconductivity, particularly with regard to how Cooper pairs form and establish long-range phase coherence.

There are two main hypotheses for the origin of the pseudogap: It results from strong pair fluctuations, manifesting as preformed electron pairs above Tw and serving as a precursor to the condensation of coherent pairs; and arises from various quantum orders in high-temperature superconductors, such as antiferromagnetic order, band phase, and pair density wave. However, the complexity of high-temperature superconducting materials leaves these questions largely unanswered.

Single particle spectral function measured above critical temperature

The red and blue spheres symbolize fermionic atoms with spins up and down, respectively. The curved surfaces with grids represent the energy-momentum landscapes for quasi-particles. Paired fermions inhabit the lower surface, while unpaired fermions occupy the upper surface. The gap between the surfaces means pseudogap, indicating that a minimum amount of energy is required to break the fermion pairs. Blurred fermion pairs in the gap suggest partial filling of the pseudogap. Credit: Lei Chen

Quantum simulation sheds new light

Unitary Fermi gases provide an ideal quantum simulation platform to investigate the existence and characteristics of a pairing pseudogap. This can be attributed to its controllability, unprecedented purity and, most importantly, the presence of known short-range attractive interactions.

Furthermore, the absence of a lattice structure in Fermi gases eliminates the influence of competing quantum orders. In this context, previous experiments have measured the trap-averaged single-particle spectral function of strongly interacting Fermi gases.

However, these experiments did not provide convincing evidence of a pseudogap, mainly due to the inhomogeneity of the trap and the serious problems arising from final-state interactions in commonly used RF spectroscopy.

After years of dedicated work, the USTC research team established a quantum simulation platform using ultracold lithium and dysprosium atoms, and achieved a state-of-the-art preparation of homogeneous Fermi gases (Science).

Furthermore, this team developed new techniques to stabilize the necessary magnetic fields. At a magnetic field of approximately 700 G, the short-term fluctuations achieved are less than 25 μG, resulting in record-breaking relative magnetic field stability. This ultrastable magnetic field allowed the research team to use microwave pulses to excite atoms to high energy states that do not interact with the initial states, thus realizing moment-resolved photoemission spectroscopy.

With these two crucial technical advances, the research team systematically measured the single-particle spectral function of unitary Fermi gases at different temperatures and observed the existence of the pairing pseudogap, supporting the role of preformed pairing as a precursor to superfluidity.

Furthermore, the research team determined the pairing gap, pair lifetime, and single-particle scattering rate from the measured spectral function, which are essential quantities for characterizing the behavior of strongly interacting quantum systems. These findings not only advance the study of strongly correlated systems, but also provide valuable insights and information for establishing a suitable many-body theory.

The techniques developed in this work lay the foundation for future exploration and study of other important low-temperature quantum phases, such as single-band superfluidity, stripe phases, and Fulde–Ferrel–Larkin–Ovchinnikov superfluidity.

Reference: February 7, 2024, Nature.
DOI: 10.1038/s41586-023-06964-y

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