The 2-Channel Kondo Model II: CFT Calculation of Non-Equilibrium Conductance through a Nanoconstriction containing 2-Channel Kondo Impurities

Recent experiments by Ralph and Buhrman on zero-bias anomalies in quenched Cu nanoconstrictions (reviewed in the preceding paper, I), are in accord with the assumption that the interaction between electrons and nearly degenerate two-level systems in the constriction can be described, for sufficiently small voltages and temperatures (V,T < \Tk), by the 2-channel Kondo (2CK) model. Motivated by these experiments, we introduce a generalization of the 2CK model, which we call the nanoconstriction 2-channel Kondo model (NTKM), that takes into account the complications arising from the non-equilibrium electron distribution in the nanoconstriction. We calculate the conductance $G(V,T)$ of the constriction in the weakly non-equilibrium regime of V,T \ll \Tk by combining concepts from Hershfield's $Y$-operator formulation of non-equilibrium problems and Affleck and Ludwig's exact conformal field theory (CFT) solution of the 2CK problem (CFT technicalities are discussed in a subsequent paper, III). Finally, we extract from the conductance a universal scaling curve $\Gamma(v)$ and compare it with experiment. Combining our results with those of Hettler, Kroha and Hershfield, we conclude that the NTKM achieves quantitative agreement with the experimental scaling data.Comment: Final published version (minor revisions only), 41 pages RevTeX, 9 encapsulated postscript figure

Floquet topological phases coupled to environments and the induced photocurrent

We consider the fate of a helical edge state of a spin Hall insulator and its topological transition in presence of a circularly polarized light when coupled to various forms of environments. A Lindblad type equation is developed to determine the fermion occupation of the Floquet bands. We find by using analytical and numerical methods that non-secular terms, corresponding to 2-photon transitions, lead to a mixing of the band occupations, hence the light induced photocurrent is in general not perfectly quantized in the presence of finite coupling to the environment, although deviations are small in the adiabatic limit. Sharp crossovers are identified at frequencies $\Omega$ and $\frac{1}{2}\Omega$ ($\Omega$ is the strength of light-matter coupling) with the former resembling to a phase transition.Comment: 7+4 pages, 6+2 figure