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    Temperature dependences of resistivity and magnetoresistivity for half-metallic ferromagnets
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    We provide a comprehensive theoretical investigation of the Fermi liquid quasiparticle description in two-dimensional electron gas interacting via the long-range Coulomb interaction by calculating the electron self-energy within the leading-order approximation, which is exact in the high-density limit. We find that the quasiparticle energy is larger than the imaginary part of the self-energy up to very high energies, implying that the basic Landau quasiparticle picture is robust up to far above the Fermi energy. We find, however, that the quasiparticle picture becomes fragile in a small discrete region around a critical wave vector where the quasiparticle spectral function strongly deviates from the expected quasiparticle Lorentzian line shape with a vanishing renormalization factor. We show that such a non-Fermi liquid behavior arises due to the coupling of quasiparticles with the collective plasmon mode. This situation is somewhat intermediate between the one-dimensional interacting electron gas (i.e., Luttinger liquid), where the Landau Fermi liquid theory completely breaks down since only bosonic collective excitations exist, and three-dimensional electron gas, where quasiparticles are well-defined and more stable against interactions than in one and two dimensions. We use a number of complementary definitions for a quasiparticle to examine the interacting spectral function, contrasting two-dimensional and three-dimensional situations critically.
    The propagation characteristics of quasiparticles in bulk superconducting Pb single crystals is studied. A transition from quasiparticle diffusion to diffusion of the combined gas of quasiparticles and phonons is observed as the temperature is increased. The intrinsic quasiparticle recombination time, as well as the decay time of the quasiparticle density, is determined. The latter is found to be at least an order of magnitude longer than this quasiparticle recombination time.
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    The carriers of electric current in a metal are quasiparticles dressed by electron-electron interactions, which have a larger effective mass $m^*$ and a smaller quasiparticle weight $z$ than non-interacting carriers. If the momentum dependence of the self-energy can be neglected, the effective mass enhancement and quasiparticle weight of quasiparticles at the Fermi energy are simply related by $z=m/m^*$ ($m$=bare mass). We propose that both superconductivity and ferromagnetism in metals are driven by quasiparticle 'undressing', i.e., that the correlations between quasiparticles that give rise to the collective state are associated with an increase in $z$ and a corresponding decrease in $m^*$ of the carriers. Undressing gives rise to lowering of kinetic energy, which provides the condensation energy for the collective state. In contrast, in conventional descriptions of superconductivity and ferromagnetism the transitions to these collective states result in $increase$ in kinetic energy of the carriers and are driven by lowering of potential energy and exchange energy respectively.
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    The quasiparticle band structures of the ordered L12 and L10 phases of NixPt1-x (x = 0.25, 0.5, and 0.75) have been calculated and compared to the band structures obtained using density functional theory. For each alloy, an increase in the curvature of the band structure is generally seen when the quasiparticle correction is made. Non-dispersive regions are shown to include a dispersive component under the quasiparticle correction. The quasiparticle weights are k-dependent and greater towards the Γ point. Non-linear analytical modelling of the quasiparticle correction is shown to be complex.
    Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices subjected to local quasiparticle injection. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the timescale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order of magnitude reduction in quasiparticle loss for a given injected quasiparticle power.
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