 Award ID(s):
 1822258
 Publication Date:
 NSFPAR ID:
 10296017
 Journal Name:
 Nature Communications
 Volume:
 12
 Issue:
 1
 ISSN:
 20411723
 Sponsoring Org:
 National Science Foundation
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BACKGROUND Landau’s Fermi liquid theory provides the bedrock on which our understanding of metals has developed over the past 65 years. Its basic premise is that the electrons transporting a current can be treated as “quasiparticles”—electronlike particles whose effective mass has been modified, typically through interactions with the atomic lattice and/or other electrons. For a long time, it seemed as though Landau’s theory could account for all the manybody interactions that exist inside a metal, even in the socalled heavy fermion systems whose quasiparticle mass can be up to three orders of magnitude heavier than the electron’s mass. Fermi liquid theory also lay the foundation for the first successful microscopic theory of superconductivity. In the past few decades, a number of new metallic systems have been discovered that violate this paradigm. The violation is most evident in the way that the electrical resistivity changes with temperature or magnetic field. In normal metals in which electrons are the charge carriers, the resistivity increases with increasing temperature but saturates, both at low temperatures (because the quantized lattice vibrations are frozen out) and at high temperatures (because the electron mean free path dips below the smallest scattering pathway defined by the lattice spacing).more »

Abstract The lowtemperature properties of a wide range of manyfermion systems spanning metals, quantum gases and liquids to nuclear matter are well understood within the framework of Landau’s theory of Fermi liquids. The lowenergy physics of these systems is governed by interacting fermionic quasiparticles with momenta and energies near a Fermi surface in momentum space. Nonequilibrium properties are described by a kinetic equation for the distribution function for quasiparticles proposed by Landau. Quasiparticle interactions with other quasiparticles, phonons, or impurities lead to internal forces acting on a distribution of nonequilibrium quasiparticles, as well as collision processes that ultimately limit the transport of mass, heat, charge, and magnetization, as well as limiting the coherence times of quasiparticles. For Fermi liquids that are close to a secondorder phase transition, e.g., Fermi liquids that undergo a superfluid transition, incipient Cooper pairs—longlived fluctuations of the ordered phase—provide a new channel for scattering quasiparticles, as well as corrections to internal forces acting on the distribution of nonequilibrium quasiparticles. We develop the theory of quasiparticle transport for Fermi liquids in the vicinity of a BCStype superfluid transition starting from Keldysh’s field theory for nonequilibrium, strongly interacting fermions. The leading corrections to Fermiliquid theory for nonequilibrium quasiparticle transportmore »

We report the optical conductivity in highquality crystals of the chiral topological semimetal CoSi, which hosts exotic quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different types of quasiparticles. The lowfrequency intraband response is captured by a narrow Drude peak from a highmobility electron pocket of double Weyl quasiparticles, and the temperature dependence of the spectral weight is consistent with its Fermi velocity. By subtracting the lowfrequency sharp Drude and phonon peaks at low temperatures, we reveal two intermediate quasilinear interband contributions separated by a kink at 0.2 eV. Using Wannier tightbinding models based on firstprinciple calculations, we link the optical conductivity above and below 0.2 eV to interband transitions near the double Weyl fermion and a threefold fermion, respectively. We analyze and determine the chemical potential relative to the energy of the threefold fermion, revealing the importance of transitions between a linearly dispersing band and a flat band. More strikingly, below 0.1 eV our data are best explained if spinorbit coupling is included, suggesting that at these energies, the optical response is governed by transitions between a previously unobserved fourfold spin3/2 nodemore »

Because Fermi liquids are inherently noninteracting states of matter, all electronic levels below the chemical potential are doubly occupied. Consequently, the simplest way of breaking the Fermiliquid theory is to engineer a model in which some of those states are singly occupied, keeping timereversal invariance intact. We show that breaking an overlooked1 localinmomentum space ℤ2 symmetry of a Fermi liquid does precisely this. As a result, although the Mott transition from a Fermi liquid is correctly believed to arise without breaking any continuous symmetry, a discrete symmetry is broken. This symmetry breaking serves as an organizing principle for Mott physics whether it arises from the tractable Hatsugai–Kohmoto model or the intractable Hubbard model. Through a renormalizationgroup analysis, we establish that both are controlled by the same fixed point. An experimental manifestation of this fixed point is the onset of particle–hole asymmetry, a widely observed2,3,4,5,6,7,8,9,10 phenomenon in strongly correlated systems. Theoretically, the singly occupied region of the spectrum gives rise to a surface of zeros of the singleparticle Green function, denoted as the Luttinger surface. Using Khomology, we show that the Bott topological invariant guarantees the stability of this surface to local perturbations. Our proof demonstrates that the strongly coupled fixedmore »

We have measured the superconducting penetration depth (T ) in the heavyfermion/intermediatevalent superconducting alloy series Ce1−xYbxCoIn5 using a transversefield muon spin relaxation to study the effect of intermediatevalent Yb doping on Fermiliquid renormalization. From (T ) we determine the superfluid density ρs (T ) and find that it decreases continuously with increasing nominal Yb concentration x, i.e., with increasing intermediate valence. The temperaturedependent renormalization of the “normal” fluid density ρN (T ) = ρs (0) − ρs (T ) in both the heavyfermion and intermediate valence limits is proportional to the temperaturedependent renormalization of the specific heat. This indicates that the temperaturedependent Fermiliquid Landau parameters of the superconducting quasiparticles entering the two different physical quantities are the same. These results represent an important advance in understanding of both intermediate valence and heavyfermion phenomena in superconductors.