The Transition Between Quantum Coherence and Incoherence

We show that a transformed Caldeira-Leggett Hamltonian has two distinct families of fixed points, rather than a single unique fixed point as often conjectured based on its connection to the anisotropic Kondo model. The two families are distinguished by a sharp qualitative difference in their quantum coherence properties and we argue that this distinction is best understood as the result of a transition in the model between degeneracy and non-degeneracy in the spectral function of the ``spin-flip'' operator.Comment: some typos corrected and a reference adde

Evidence for a common physical description of non-Fermi-liquid behavior in f-electron systems

The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat $C(T)$ and magnetic susceptibility $\chi(T)$ of f-electron systems is analyzed within the context of a recently developed theory based on Griffiths singularities. Measurements of $C(T)$ and $\chi(T)$ in the systems $Th_{1-x}U_{x}Pd_{2}Al_{3}$, $Y_{1-x}U_{x}Pd_3$, and $UCu_{5-x}M_{x}$ (M = Pd, Pt) are found to be consistent with $C(T)/T \propto \chi(T) \propto T^{-1+\lambda}$ predicted by this model with $\lambda <1$ in the NFL regime. These results suggest that the NFL properties observed in a wide variety of f-electron systems can be described within the context of a common physical picture.Comment: 4 pages, 4 figure

Theory of magnetoresistance in films of dilute magnetic alloys

Earlier a magnetic anisotropy for magnetic impurities nearby the surface of non-magnetic host was proposed in order to explain the size dependence of the Kondo effect in dilute magnetic alloys. Recently Giordano has measured the magnetoresistance of dilute Au(Fe) films for different thicknesses well above the Kondo temperature $T_K$. In this way he verified the existence of that anisotropy even for such a case where the Kondo effect is not dominating. For detailed comparison of that suggestion with experiments, the magnetic field dependence of the magnetoresistance is calculated in the lowest approximation, thus in the second order of the exchange coupling. The strength of the anisotropy is very close to earlier estimates deduced from the size dependence of the Kondo resistivity amplitude.Comment: (11 pages, 8 figures, essential changes compared to the old version

Quantum interference in asymmetric superconducting nanowire loops

Macroscopic phase coherence in superconductors enables quantum interference and phase manipulation at realistic device length scales. Numerous superconducting electronic devices are based on the modulation of the supercurrent in superconducting loops. While the overall behavior of symmetric superconducting loops have been studied, the effects of asymmetries in such devices remain under-explored and poorly understood. Here we report on an experimental and theoretical study of the flux modulation of the persistent current in a doubly-connected asymmetric aluminum nanowire loop. A model considering the length and electronic cross-section asymmetries in the loop provides a quantitative account of the observations. Comparison with experiments give essential parameters such as persistent and critical currents as well as the amount of asymmetry which can provide feedback into the design of superconducting quantum devices.Comment: to appear in EP

Phase diagram and magnetic correlations in one-dimensional repulsive Hubbard model with magnetic field

[[abstract]]The generalized self-consistent field (GSCF) theory for the one-dimensional repulsive Hubbard model at half-filling is examined in the presence of magnetic field h in wide range of interaction strength U / t . The evolution of the energy gap in the presence of magnetc field describes a magnetic crossover from itinerant magnetism of weakly bound electron-hole pairs with k F =π/2 to the localized magnetic regime, with the Bose condensation of local electron-hole pairs ( k F =0).[[notice]]補正完畢[[journaltype]]國

Superconductivity, magnetism and mapping in entire parameter space between the attractive and repulsive Hubbard models

[[abstract]]The exact Bethe-ansatz and generalized self-consistent field (GSCF) ground-state properties of the one-dimensional (1d) Hubbard model for U >0 and U 0 and U <0 models.[[notice]]補正完畢[[journaltype]]國