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1. 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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2. Transport in helical Luttinger liquids in the fractional quantum Hall regime

   https://doi.org/10.1038/s41467-021-25631-2  

   Wang, Ying; Ponomarenko, Vadim; Wan, Zhong; West, Kenneth W.; Baldwin, Kirk W.; Pfeiffer, Loren N.; Lyanda-Geller, Yuli; Rokhinson, Leonid P. (September 2021, Nature Communications)

   Abstract Domain walls in fractional quantum Hall ferromagnets are gapless helical one-dimensional channels formed at the boundaries of topologically distinct quantum Hall (QH) liquids. Naïvely, these helical domain walls (hDWs) constitute two counter-propagating chiral states with opposite spins. Coupled to an s-wave superconductor, helical channels are expected to lead to topological superconductivity with high order non-Abelian excitations^1–3. Here we investigate transport properties of hDWs in theν = 2/3 fractional QH regime. Experimentally we found that current carried by hDWs is substantially smaller than the prediction of the naïve model. Luttinger liquid theory of the system reveals redistribution of currents between quasiparticle charge, spin and neutral modes, and predicts the reduction of the hDW current. Inclusion of spin-non-conserving tunneling processes reconciles theory with experiment. The theory confirms emergence of spin modes required for the formation of fractional topological superconductivity. 

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3. Artificial intelligence “sees” split electrons

   https://doi.org/10.1126/science.abm2445  

   Perdew, John P. (December 2021, Science)

   Chemical bonds between atoms are stabilized by the exchange-correlation (xc) energy, a quantum-mechanical effect in which “social distancing” by electrons lowers their electrostatic repulsion energy. Kohn-Sham density functional theory (DFT) ( 1 ) states that the electron density determines this xc energy, but the density functional must be approximated. This is usually done by satisfying exact constraints of the exact functional (making the approximation predictive), by fitting to data (making it interpolative), or both. Two exact constraints—the ensemble-based piecewise linear variation of the total energy with respect to fractional electron number ( 2 ) and fractional electron z -component of spin ( 3 )—require hard-to-control nonlocality. On page 1385 of this issue, Kirkpatrick et al. ( 4 ) have taken a big step toward more accurate predictions for chemistry through the machine learning of molecular data plus the fractional charge and spin constraints, expressed as data that a machine can learn. 

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4. Weyl spin-momentum locking in a chiral topological semimetal

   https://doi.org/10.1038/s41467-024-47976-0  

   Krieger, Jonas A; Stolz, Samuel; Robredo, Iñigo; Manna, Kaustuv; McFarlane, Emily C; Date, Mihir; Pal, Banabir; Yang, Jiabao; B_Guedes, Eduardo; Dil, J Hugo; et al (December 2024, Nature Communications)

   Abstract Spin-orbit coupling in noncentrosymmetric crystals leads to spin-momentum locking – a directional relationship between an electron’s spin angular momentum and its linear momentum. Isotropic orthogonal Rashba spin-momentum locking has been studied for decades, while its counterpart, isotropic parallel Weyl spin-momentum locking has remained elusive in experiments. Theory predicts that Weyl spin-momentum locking can only be realized in structurally chiral cubic crystals in the vicinity of Kramers-Weyl or multifold fermions. Here, we use spin- and angle-resolved photoemission spectroscopy to evidence Weyl spin-momentum locking of multifold fermions in the chiral topological semimetal PtGa. We find that the electron spin of the Fermi arc surface states is orthogonal to their Fermi surface contour for momenta close to the projection of the bulk multifold fermion at the Γ point, which is consistent with Weyl spin-momentum locking of the latter. The direct measurement of the bulk spin texture of the multifold fermion at the R point also displays Weyl spin-momentum locking. The discovery of Weyl spin-momentum locking may lead to energy-efficient memory devices and Josephson diodes based on chiral topological semimetals. 

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5. Bogoliubov quasiparticle on the gossamer Fermi surface in electron-doped cuprates

   https://doi.org/10.1038/s41567-023-02209-x  

   Xu, Ke-Jun; Guo, Qinda; Hashimoto, Makoto; Li, Zi-Xiang; Chen, Su-Di; He, Junfeng; He, Yu; Li, Cong; Berntsen, Magnus H.; Rotundu, Costel R.; et al (September 2023, Nature Physics)

   Electron-doped cuprates consistently exhibit strong antiferromagnetic correlations, leading to the prevalent belief that antiferromagnetic spin fluctuations mediate Cooper pairing in these unconventional superconductors. However, early investigations showed that although antiferromagnetic spin fluctuations create the largest pseudogap at hot spots in momentum space, the superconducting gap is also maximized at these locations. This presented a paradox for spin-fluctuation-mediated pairing: Cooper pairing is strongest at momenta where the normal-state low-energy spectral weight is most suppressed. Here we investigate this paradox and find evidence that a gossamer—meaning very faint—Fermi surface can provide an explanation for these observations. We study Nd2–xCexCuO4 using angle-resolved photoemission spectroscopy and directly observe the Bogoliubov quasiparticles. First, we resolve the previously observed reconstructed main band and the states gapped by the antiferromagnetic pseudogap around the hot spots. Within the antiferromagnetic pseudogap, we also observe gossamer states with distinct dispersion, from which coherence peaks of Bogoliubov quasiparticles emerge below the superconducting critical temperature. Moreover, the direct observation of a Bogoliubov quasiparticle permits an accurate determination of the superconducting gap, yielding a maximum value an order of magnitude smaller than the pseudogap, establishing the distinct nature of these two gaps. We propose that orientation fluctuations in the antiferromagnetic order parameter are responsible for the gossamer states. 

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