Biss Hauke, Sobirey Lennart, Luick Niclas, Bohlen Markus, Kinnunen Jami J, Bruun Georg M, Lompe Thomas, Moritz Henning
Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
Phys Rev Lett. 2022 Mar 11;128(10):100401. doi: 10.1103/PhysRevLett.128.100401.
Ultracold atomic gases are a powerful tool to experimentally study strongly correlated quantum many-body systems. In particular, ultracold Fermi gases with tunable interactions have allowed to realize the famous BEC-BCS crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of weakly bound Cooper pairs. However, large parts of the excitation spectrum of fermionic superfluids in the BEC-BCS crossover are still unexplored. In this work, we use Bragg spectroscopy to measure the full momentum-resolved low-energy excitation spectrum of strongly interacting ultracold Fermi gases. This enables us to directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. We also use our spectra to determine the evolution of the superfluid gap and find excellent agreement with previous experiments and self-consistent T-matrix calculations both in the BEC and crossover regime. However, toward the BCS regime a calculation that includes the effects of particle-hole correlations shows better agreement with our data.
超冷原子气体是实验研究强关联量子多体系统的有力工具。特别是，具有可调相互作用的超冷费米气体使得人们能够实现从分子的玻色 - 爱因斯坦凝聚（BEC）到弱束缚库珀对的巴丁 - 库珀 - 施里弗（BCS）超流体的著名BEC - BCS交叉。然而，在BEC - BCS交叉中费米子超流体激发谱的大部分仍然未被探索。在这项工作中，我们使用布拉格光谱来测量强相互作用超冷费米气体的全动量分辨低能激发谱。这使我们能够直接观察在BEC - BCS交叉中发生的从玻色子超流体到费米子超流体的平滑转变。我们还利用我们的光谱来确定超流能隙的演化，并发现与之前在BEC和交叉区域的实验以及自洽T矩阵计算有很好的一致性。然而，在接近BCS区域时，包含粒子 - 空穴关联效应的计算与我们的数据显示出更好的一致性。
