L. H. GreeneM. ApriliM. CovingtonE. BadicaD. E. PugelH. AubinYuanyang XiaM. B. SalamonSha JainD. G. Hinks
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T-symmetry
Recent experiments in the spectroscopy of free or impurity-bound electrons and excitons in semiconductor alloys show that these quasiparticles are differently affected by the same disorder state. In this paper, we present a formal theory to account for disorder effects on the properties of quasiparticles. To this end, a new alloy model is described. Based on a representation of the composition fluctuations within the quasiparticle extent, this theory treats the effective alloy medium "felt" by the quasiparticle of a given spatial extent. Its relations with the single-site and virtual-crystal approximation are obtained in the two limiting cases (site-localized limit and highly extended quasiparticle cases). Trends in disorder effects on quasiparticles as functions of their extent are briefly discussed from a simple model of propagating quasiparticles.
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Dynamics
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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.
Momentum (technical analysis)
Fermi energy
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This paper reports on the the discovery of a novel magnetoresistance called tunnelling anisotropic magnetoresistance (TAMR), which may be harnessed for device applications as both volatile and non-volatile memory. TAMR arises when tunnelling into a material with large spin orbit coupling and magnetic anisotropy such as the ferromagnetic semiconductor (Ga,Mn)As. It results from the strong coupling between the holes and the Mn system, which translates the magnetic anisotropy into an anisotropy in the transport density of states (DOS). This effect was first observed in a Au/AlO x /(Ga,Mn)As tunnel structure.
Tunnel magnetoresistance
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A previously unrecognised non-equilibrium phenomenon in superconducting tunnel junctions is identified. The situation may arise in low-loss, low-gap, multi-tunnelling devices in which quasiparticles retain accumulated energy through several cycles of sequential forward and back tunnelling. As a consequence, a stable non-equilibrium distribution is established, in which many quasiparticles have energies exceeding the 3Δ threshold for breaking Cooper pairs, resulting in a large excess current. By solving the system of coupled kinetic equations for quasiparticles and phonons, we have modelled the quasiparticle distribution and excess current, and have confirmed the predictions through experiments on an appropriate superconducting tunnel junction.
Cooper pair
Tunnel junction
Tunnel effect
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Particle (ecology)
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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.
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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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