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1. Vortex structure and spectrum of an atomic Fermi superfluid in a spherical bubble trap

   https://doi.org/10.1103/PhysRevA.108.053303  

   He, Yan; Chien, Chih-Chun (November 2023, Physical Review A)

   The structures of multiply quantized vortices (MQVs) of an equal-population atomic Fermi superfluid in a rotating spherical bubble trap approximated as a thin shell are analyzed by solving the Bogoliubov-de Gennes (BdG) equation throughout the BCS-Bose Einstein condensation (BEC) crossover. Consistent with the Poincare-Hopf theorem, a pair of vortices emerge at the poles of the rotation axis in the presence of azimuthal symmetry, and the compact geometry provides confinement for the MQVs. While the single-vorticity vortex structure is similar to that in a planar geometry, higher-vorticity vortices exhibit interesting phenomena at the vortex center, such as a density peak due to accumulation of a normal Fermi gas and reversed circulation of current due to in-gap states carrying angular momentum, in the BCS regime but not the BEC regime because of the subtle relations between the order parameter and density. The energy spectrum shows the number of the in-gap state branches corresponds to the vorticity of a vortex, and an explanation based on a topological correspondence is provided. 

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2. Absence of a BCS-BEC crossover in the cuprate superconductors

   https://doi.org/10.1038/s41535-023-00550-1  

   Sous, John; He, Yu; Kivelson, Steven A. (May 2023, npj Quantum Materials)

   Abstract We examine key aspects of the theory of the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensation (BEC) crossover, focusing on the temperature dependence of the chemical potential,μ. We identify an accurate method of determining the change ofμin the cuprate high temperature superconductors from angle-resolved-photoemission data (along the ‘nodal’ direction), and show thatμvaries by less than a few percent of the Fermi energy over a range of temperatures from far below to several times above the superconducting transition temperature,T_c. This shows, unambiguously, that not only are these materials always on the BCS side of the crossover (which is a phase transition in thed-wave case), but are nowhere near the point of the crossover (where the chemical potential approaches the band bottom). 

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3. Neural-network quantum states for ultra-cold Fermi gases

   https://doi.org/10.1038/s42005-024-01613-w  

   Kim, Jane; Pescia, Gabriel; Fore, Bryce; Nys, Jannes; Carleo, Giuseppe; Gandolfi, Stefano; Hjorth-Jensen, Morten; Lovato, Alessandro (May 2024, Communications Physics)

   Abstract Ultra-cold Fermi gases exhibit a rich array of quantum mechanical properties, including the transition from a fermionic superfluid Bardeen-Cooper-Schrieffer (BCS) state to a bosonic superfluid Bose-Einstein condensate (BEC). While these properties can be precisely probed experimentally, accurately describing them poses significant theoretical challenges due to strong pairing correlations and the non-perturbative nature of particle interactions. In this work, we introduce a Pfaffian-Jastrow neural-network quantum state featuring a message-passing architecture to efficiently capture pairing and backflow correlations. We benchmark our approach on existing Slater-Jastrow frameworks and state-of-the-art diffusion Monte Carlo methods, demonstrating a performance advantage and the scalability of our scheme. We show that transfer learning stabilizes the training process in the presence of strong, short-ranged interactions, and allows for an effective exploration of the BCS-BEC crossover region. Our findings highlight the potential of neural-network quantum states as a promising strategy for investigating ultra-cold Fermi gases. 

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4. Fragility of surface states in topological superfluid 3He

   https://doi.org/10.1038/s41467-021-21831-y  

   Heikkinen, P. J.; Casey, A.; Levitin, L. V.; Rojas, X.; Vorontsov, A.; Sharma, P.; Zhelev, N.; Parpia, J. M.; Saunders, J. (December 2021, Nature Communications)

   null (Ed.)

   Abstract Superfluid 3 He, with unconventional spin-triplet p-wave pairing, provides a model system for topological superconductors, which have attracted significant interest through potential applications in topologically protected quantum computing. In topological insulators and quantum Hall systems, the surface/edge states, arising from bulk-surface correspondence and the momentum space topology of the band structure, are robust. Here we demonstrate that in topological superfluids and superconductors the surface Andreev bound states, which depend on the momentum space topology of the emergent order parameter, are fragile with respect to the details of surface scattering. We confine superfluid 3 He within a cavity of height D comparable to the Cooper pair diameter ξ 0 . We precisely determine the superfluid transition temperature T c and the suppression of the superfluid energy gap, for different scattering conditions tuned in situ, and compare to the predictions of quasiclassical theory. We discover that surface magnetic scattering leads to unexpectedly large suppression of T c , corresponding to an increased density of low energy bound states. 

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5. Bose polarons near quantum criticality

   https://doi.org/10.1126/science.aax5850  

   Yan, Zoe Z.; Ni, Yiqi; Robens, Carsten; Zwierlein, Martin W. (April 2020, Science)

   The emergence of quasiparticles in interacting matter represents one of the cornerstones of modern physics. However, in the vicinity of a quantum critical point, the existence of quasiparticles comes under question. Here, we created Bose polarons near quantum criticality by immersing atomic impurities in a Bose-Einstein condensate (BEC) with near-resonant interactions. Using radiofrequency spectroscopy, we probed the energy, spectral width, and short-range correlations of the impurities as a function of temperature. Far below the superfluid critical temperature, the impurities formed well-defined quasiparticles. Their inverse lifetime, given by their spectral width, increased linearly with temperature at the so-called Planckian scale, consistent with quantum critical behavior. Close to the BEC critical temperature, the spectral width exceeded the impurity’s binding energy, signaling a breakdown of the quasiparticle picture. 

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