We report temperature and density dependences of the spin susceptibility of strongly interacting electrons in Si inversion layers. We measured (i) the itinerant electron susceptibility χ∗ from the Shubnikov-de Haas oscillations in crossed magnetic fields and (ii) thermodynamic susceptibility χTsensitive to all the electrons in the layer. Both χ∗ and χT are strongly enhanced with lowering the electron density in the metallic phase. However, there is no sign of divergency of either quantity at the density of the metal-insulator transition nc. Moreover, the value of χT, which can be measured across the transition down to very low densities deep in the insulating phase, increases with density at n<nc, as expected. In the absence of magnetic field, we found the temperature dependence of χ∗ to be consistent with Fermi-liquid-based predictions, and to be much weaker than the power law predicted by non-Fermi-liquid models. We attribute a much stronger temperature dependence of χT to localized spin droplets. In strong enough in-plane magnetic field, we found the temperature dependence of χ∗to be stronger than that expected for the Fermi liquid interaction corrections.
We argue that quantum fluctuations of the phase of the order parameter may strongly affect the electron density of states (DOS) in ultrathin superconducting wires. We demonstrate that the effect of such fluctuations is equivalent to that of a quantum dissipative environment formed by soundlike plasma modes propagating along the wire. We derive a nonperturbative expression for the local electron DOS in superconducting nanowires which fully accounts for quantum phase fluctuations. At any nonzero temperature these fluctuations smear out the square-root singularity in DOS near the superconducting gap and generate quasiparticle states at subgap energies. Furthermore, at sufficiently large values of the wire impedance this singularity is suppressed down to T=0 in which case DOS tends to zero at subgap energies and exhibits the power-law behavior above the gap. Our predictions can be directly tested in tunneling experiments with superconducting nanowires.
Quantum phase slips (QPSs) generate voltage fluctuations in superconducting nanowires. Employing the Keldysh technique and making use of the phase-charge duality arguments, we develop a theory of QPS-induced voltage noise in such nanowires. We demonstrate that quantum tunneling of the magnetic flux quanta across the wire yields quantum shot noise which obeys Poisson statistics and is characterized by a power-law dependence of its spectrum SΩ on the external bias. In long wires, SΩdecreases with increasing frequency Ω and vanishes beyond a threshold value of Ω at T→0. The quantum coherent nature of QPS noise yields nonmonotonous dependence of SΩ on T at small Ω.
The development of active and passive plasmonic devices is challenging due to the high level of dissipation in normal metals. One possible solution to this problem is using alternative materials. Graphene is a good candidate for plasmonics in the near-infrared region. In this paper, we develop a quantum theory of a graphene plasmon generator. We account for quantum correlations and dissipation effects, thus we are able to describe such regimes of a quantum plasmonic amplifier as a surface plasmon emitting diode and a surface plasmon amplifier using stimulated radiation emission. Switching between these generation types is possible in situ with a variance of the graphene Fermi level. We provide explicit expressions for dissipation and interaction constants through material parameters, and we identify the generation spectrum and the second-order correlation function, which predicts the laser statistics.