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1. Universal sound diffusion in a strongly interacting Fermi gas

   https://doi.org/10.1126/science.aaz5756  

   Patel, Parth B.; Yan, Zhenjie; Mukherjee, Biswaroop; Fletcher, Richard J.; Struck, Julian; Zwierlein, Martin W. (December 2020, Science)

   Transport of strongly interacting fermions is crucial for the properties of modern materials, nuclear fission, the merging of neutron stars, and the expansion of the early Universe. Here, we observe a universal quantum limit of diffusivity in a homogeneous, strongly interacting atomic Fermi gas by studying sound propagation and its attenuation through the coupled transport of momentum and heat. In the normal state, the sound diffusivity D monotonically decreases upon lowering the temperature, in contrast to the diverging behavior of weakly interacting Fermi liquids. Below the superfluid transition temperature, D attains a universal value set by the ratio of Planck’s constant and the particle mass. Our findings inform theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons, and quarks. 

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2. Emergence of Sound in a Tunable Fermi Fluid

   https://doi.org/10.1103/PhysRevX.15.011074  

   Huang, Songtao; Ji, Yunpeng; Repplinger, Thomas; Assumpção, Gabriel_G T; Chen, Jianyi; Schumacher, Grant L; Vivanco, Franklin J; Kurkjian, Hadrien; Navon, Nir (March 2025, Physical Review X)

   Landau’s Fermi-liquid (FL) theory has been successful at the phenomenological description of the normal phase of many different Fermi systems. Using a dilute atomic Fermi fluid with tunable interactions, we investigate the microscopic basis of Landau’s theory with a system describable from first principles. We study transport properties of an interacting Fermi gas by measuring its density response to a periodic external perturbation. In an ideal Fermi gas, we measure for the first time the celebrated Lindhard function. As the system is brought from the collisionless to the hydrodynamic regime, we observe the emergence of sound and find that the experimental observations are quantitatively understood with a first-principle transport equation for the FL. When the system is more strongly interacting, we find deviations from such predictions. Finally, we measure the momentum-space shape of the quasiparticle excitations and see how it evolves from the collisionless to the collisional regime. Our study establishes this system as a clean platform for studying Landau’s theory of the FL and paves the way for extending the theory to more exotic conditions, such as nonlinear dynamics and FLs with strong correlations in versatile settings. Published by the American Physical Society2025 

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3. Generalized Bose–Fermi mapping and strong coupling ansatz wavefunction for one dimensional strongly interacting spinor quantum gases

   https://doi.org/10.1088/1751-8121/aca302  

   Yang, Li; Alam, Shah Saad; Pu, Han (November 2022, Journal of Physics A: Mathematical and Theoretical)

   Abstract Quantum many-body systems in one dimension (1D) exhibit some peculiar properties. In this article, we review some of our work on strongly interacting 1D spinor quantum gas. First, we discuss a generalized Bose–Fermi mapping that maps the charge degrees of freedom to a spinless Fermi gas and the spin degrees of freedom to a spin chain model. This also maps the strongly interacting system into a weakly interacting one, which is amenable for perturbative calculations. Next, based on this mapping, we construct an ansatz wavefunction for the strongly interacting system, using which many physical quantities can be conveniently calculated. We showcase the usage of this ansatz wavefunction by considering the collective excitations and quench dynamics of a harmonically trapped system. 

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4. Experimental realization of one dimensional helium

   https://doi.org/10.1038/s41467-022-30752-3  

   Del Maestro, Adrian; Nichols, Nathan S.; Prisk, Timothy R.; Warren, Garfield; Sokol, Paul E. (December 2022, Nature Communications)

   Abstract As the spatial dimension is lowered, locally stabilizing interactions are reduced, leading to the emergence of strongly fluctuating phases of matter without classical analogues. Here we report on the experimental observation of a one dimensional quantum liquid of 4 He using nanoengineering by confining it within a porous material preplated with a noble gas to enhance dimensional reduction. The resulting excitations of the confined 4 He are qualitatively different than bulk superfluid helium, and can be analyzed in terms of a mobile impurity allowing for the characterization of the emergent quantum liquid beyond the Luttinger liquid paradigm. The low dimensional helium system offers the possibility of tuning via pressure—from weakly interacting, all the way to the super Tonks-Girardeau gas of strongly interacting hard-core particles. 

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5. Thermal transport of helium-3 in a strongly confining channel

   https://doi.org/10.1038/s41467-020-18662-8  

   Lotnyk, D.; Eyal, A.; Zhelev, N.; Abhilash, T. S.; Smith, E. N.; Terilli, M.; Wilson, J.; Mueller, E.; Einzel, D.; Saunders, J.; et al (December 2020, Nature Communications)

   null (Ed.)

   Abstract The investigation of transport properties in normal liquid helium-3 and its topological superfluid phases provides insights into related phenomena in electron fluids, topological materials, and putative topological superconductors. It relies on the measurement of mass, heat, and spin currents, due to system neutrality. Of particular interest is transport in strongly confining channels of height approaching the superfluid coherence length, to enhance the relative contribution of surface excitations, and suppress hydrodynamic counterflow. Here we report on the thermal conduction of helium-3 in a 1.1  μ m high channel. In the normal state we observe a diffusive thermal conductivity that is approximately temperature independent, consistent with interference of bulk and boundary scattering. In the superfluid, the thermal conductivity is only weakly temperature dependent, requiring detailed theoretical analysis. An anomalous thermal response is detected in the superfluid which we propose arises from the emission of a flux of surface excitations from the channel. 

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