[Paper Review] Observation and quantification of pseudogap in unitary Fermi gases

Xi Li, Shuai Wang+10 more|arXiv (Cornell University)|Oct 21, 2023

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This study observes and quantifies a pair-fluctuation-driven pseudogap in a homogeneous unitary Fermi gas of 6Li atoms using momentum-resolved microwave spectroscopy, which eliminates final-state effects present in rf spectroscopy. The experiment reveals a large, T-independent pseudogap above Tc, with inverse pair lifetime showing thermally activated behavior consistent with virtual pair breaking and recombination, and a single-particle scattering rate approaching the Planckian limit, confirming preformed pairs as a precursor to superfluidity.

ABSTRACT

The nature of pseudogap lies at the heart of strongly-interacting superconductivity and superfluidity. With known pairing interactions, unitary Fermi gases provide an ideal testbed to verify whether a pseudogap can arise from many-body pairing. Here we report the observation of the long-sought pair-fluctuation-driven pseudogap in homogeneous unitary Fermi gases of lithium-6 atoms, by precisely measuring the spectral function through momentum-resolved microwave spectroscopy without the serious effects of final-state effect. We find a large pseudogap above the superfluid transition. The inverse pair lifetime exhibits a thermally-activated exponential behavior, uncovering the microscopic virtual pair breaking and recombination mechanism. The obtained large, T-independent single-particle scattering rate is comparable with that set by the Planckian limit. Our findings quantitatively characterize the pseudogap in strongly-interacting Fermi gases, highlighting the role of preformed pairing as a precursor to superfluidity.

Motivation & Objective

• To resolve the long-standing debate on the existence of a pseudogap in strongly correlated Fermi systems.
• To experimentally verify that the pseudogap in unitary Fermi gases arises from preformed Cooper pairs due to strong pair fluctuations.
• To provide quantitative measurements of the pairing gap Δ(T), inverse pair lifetime Γ₀, and single-particle scattering rate Γ₁ in a homogeneous system.
• To eliminate experimental artifacts such as final-state effects and trap inhomogeneity that hindered prior spectroscopic measurements.
• To establish a benchmark for many-body theories of strongly interacting Fermi gases and high-Tc superconductors.

Proposed method

• Employed a cylindrical box trap to achieve a homogeneous unitary Fermi gas of 6Li atoms, eliminating spatial inhomogeneity from harmonic traps.
• Developed momentum-resolved microwave spectroscopy to probe the spectral function A(k,ω), avoiding final-state interactions that plague rf spectroscopy.
• Used a high-stability magnetic field to enable ultrahigh energy resolution in the microwave transition between hyperfine states.
• Fitted the measured spectral function using a minimal fermion self-energy model: Σ(k,ω) = Δ² / (iħΓ₀ - iħΓ₁ + [ξ(k) - εk + μ]), where ξ(k) = ħ²k²/2m* + U - μ.
• Extracted Δ(T), Γ₀(T), and Γ₁ from both quasiparticle dispersion fitting and energy distribution curve (EDC) analysis for cross-validation.
• Performed systematic temperature sweeps from T ≈ 0.77Tc to 1.51Tc to map the T-dependent behavior of the pseudogap and relaxation rates.

Experimental results

Research questions

• RQ1 Does a pair-fluctuation-driven pseudogap exist in a homogeneous unitary Fermi gas, and can it be experimentally observed?
• RQ2 What is the microscopic origin of the inverse pair lifetime Γ₀, and does it follow a thermally activated behavior?
• RQ3 How does the single-particle scattering rate Γ₁ behave near Tc, and does it approach the Planckian limit?
• RQ4 Can the spectral function reveal two distinct BCS-like quasiparticle branches above Tc, indicating preformed pairing?
• RQ5 To what extent do the extracted parameters Δ(T), Γ₀(T), and Γ₁ provide a quantitative benchmark for many-body theories?

Key findings

• The pairing gap Δ(1.11Tc) = 0.195(3)EF is observed above Tc, confirming a large pseudogap persisting up to 1.51Tc, indicating a substantial pseudogap window.
• The inverse pair lifetime Γ₀ exhibits a thermally activated exponential dependence with activation energy Ea = 2Δ₀, where Δ₀ = 0.39(8)EF, consistent with virtual pair breaking and recombination.
• The single-particle scattering rate Γ₁ ≈ 0.3EF/ħ near Tc, approximately 1.7 times the Planckian limit (kB T/ħ), indicating strong scattering due to unitary interactions.
• Γ₁ shows weak temperature dependence across Tc, suggesting it is largely insensitive to pairing, and is consistent with viscous relaxation rates in unitary Fermi gases.
• The pseudogap extracted from dispersion fitting and EDC fitting agree within error, confirming quantitative accuracy and robustness of the measurements.
• The observation of two BCS-like quasiparticle branches in A(k,ω) above Tc provides direct evidence for preformed Cooper pairs as a precursor to superfluidity.