Results on the superconductor to insulator transition in two-dimensional films are analyzed in terms of coupling of the system to a dissipative bath. Upon lowering the temperature the parameter that controls this coupling becomes relevant and a wide range of metallic phase is recovered.PACS numbers: 73.40.Hm Quantum phase transitions (QPT) continue to attract intense theoretical and experimental interest. These transitions -where changing an external parameter in the Hamiltonian of the system induces a transition from one quantum ground state to another, fundamentally different one -have been invoked to explain data from various experiments, including quantum-Hall liquid to insulator, metal to insulator and superconductor to insulator experiments. In this paper we use experiments done on thin superconducting films which undergo a socalled Superconductor-Insulator (SI) transition to propose a new phase diagram for a QPT that includes a dissipation axis. The implications of our analysis go much beyond this problem and in fact bear on all QPT in two dimensions.The most common treatment of the SI transition is to map it onto the so-called "dirty-bosons" model. In this model Cooper pairs are the Bose particles with well formed (non-fluctuating) pair amplitudes. At zero temperature an SI transition is expected as a function of disorder, with vortex-antivortex pairs activated by both quantum fluctuations and disorder. Vortices of one vorticity are also induced by a magnetic field; with no disorder an Abrikosov lattice of these vortices realizes the superconducting phase. However, an arbitrary amount of disorder disrupts the lattice and a true superconducting phase is assumed to be recovered only at T=0. In the superconducting phase vortices are localized into a so-called vortex-glass phase [3] and the Cooper pairs are delocalized. Upon increasing the magnetic field above some critical field H c , vortices will delocalize with Cooper pairs localizing into a Bose-Glass phase. Further increase of the magnetic field dissociates the Cooper pairs, and fermion degrees of freedom then determine the properties of the system. At the SI transition, both vortices and Cooper pairs are delocalized in a "Bose-Metal" phase; thus, a finite conductivity is expected at the transition. The above scenario was the basis for a scaling theory proposed by Fisher [3] in which a field-tuned transition was considered as a continuous transition with an associated diverging length ξ ∼ (H c − H) −ν , as well as a finite, universal conductivity at the transition [4].The above "dirty-bosons" model was used to describe numerous experimental data in both disorder and field tuned transitions [1,5,6]. However, a difficulty in applying this model arose when the vortex glass phase displayed a temperature independent resistance that could be interpreted as quantum tunneling of vortices [2,7,8]. In particular, in previous experiments on amorphous MoGe [1] we showed scaling for a range of disorder; yet the superconducting phase at lower fields showed finit...

We report measurements of the nonequilibrium electron energy distribution in carbon nanotubes. Using tunneling spectroscopy via a superconducting probe, we study the shape of the local electron distribution functions, and hence energy relaxation rates, in nanotubes that have bias voltages applied between their ends. At low temperatures, electrons interact weakly in nanotubes of a few microns channel length, independent of end-to-end-conductance values. Surprisingly, the energy relaxation rate can increase substantially when the temperature is raised to only 1.5 K.

1The Kondo-effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin 1 . Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons and at low temperature this leads to a zero-bias conductance anomaly 2,3 . This has become a common signature in bias-spectroscopy of single-electron transistors, observed in GaAs quantum dots 4,5,6,7,8,9 as well as in various single-molecule transistors 10,11,12,13,14,15 . While the zero-bias Kondo effect is well established it remains uncertain to what extent Kondo correlations persist in non-equilibrium situations where inelastic processes induce decoherence. Here we report on a pronounced conductance peak observed at finite bias-voltage in a carbon nanotube quantum dot in the spin singlet ground state. We explain this finite-bias conductance anomaly by a nonequilibrium Kondo-effect involving excitations into a spin triplet state. Excellent agreement between calculated and measured nonlinear conductance is obtained, thus strongly supporting the correlated nature of this nonequilibrium resonance.For quantum dots accommodating an odd number of electrons, a suppression of chargefluctuations in the Coulomb blockade regime leads to a local spin-1/2 degree of freedom and, when temperature is lowered through a characteristic Kondo-temperature T K , the Kondoeffect shows up as a zero-bias peak in the differential conductance. In a dot with an even number of electrons, the two electrons residing in the highest occupied level may either form a singlet or promote one electron to the next level to form a triplet, depending on the relative magnitude of the level splitting δ and the ferromagnetic intradot exchange energy J. For J > δ, the triplet state prevails and gives rise to a zero-bias Kondo peak 7,8 , but when δ > J the singlet state is lower in energy and no Kondo effect is expected in the linear conductance. Nevertheless, spin-flip tunneling becomes viable when the applied bias is large enough to induce transitions from singlet to triplet state. Such inter-lead exchange-tunneling may give rise to Kondo correlations and concomitant conductance peaks near V ∼ ±δ/e. However, since the tunneling involves excited states with a rather limited life-time, the question remains to what extend the coherence of such inelastic spin-flips and hence the Kondo-effect is maintained?In the context of a double-dot system, a qualitative description of a finite-bias Kondoeffect, leaving out decoherence and nonequilibrium effects, was given already in Ref.16 and 2 finite bias conductance peaks have already been observed in carbon nanotubes 11,14,17 as well as in GaAs quantum dots 8,9,18 . However, for lack of a quantitative theory for this nonequilibrium resonance no characterization of the phenomenon has yet been possible. As pointed out in Refs.19,20,21, a bias induced population of the excited state (here the triplet), may change a simple finite-bias cotunneling step into a cusp in the nonlinear conductance. Therefore, in order...

Systems of superconducting islands placed on normal metal films offer tunable realizations of twodimensional (2D) superconductivity 1, 2 ; they can thus elucidate open questions regarding the nature of 2D superconductors and competing states. In particular, island systems have been predicted to exhibit zero-temperature metallic states 3-5 . Although evidence exists for such metallic states in some 2D systems 6, 7 , their character is not well understood: the conventional theory of metals cannot explain them 8 , and their properties are difficult to tune 7,9 . Here, we characterize the superconducting transitions in mesoscopic island-array systems as a function of island thickness and spacing. We observe two transitions in the progression to superconductivity; both transition temperatures exhibit unexpectedly strong depression for widely spaced islands. These depressions are consistent with the system approaching zero-temperature metallic states. The nature of the transitions and the state between them is explained using a phenomenological model involving the stabilization of superconductivity on each island via a weak coupling to and feedback from its neighbors.Conventional zero-temperature ( 0  T ) metallic states do not exist in 2D systems possessing any disorder, because of Anderson localization 8,9 . To reconcile this fact with experimental evidence for 0  T metals in 2D, it has been proposed that the experimental observations do not pertain to conventional metals, but rather to spatially inhomogeneous superconducting (or, more generally, correlated) states 3, 4, 10 . Inhomogeneity is thought to arise in some of these systems because of phase