Low-Frequency Optical Conductivity in Inhomogeneous d-wave Superconductors

Motivated by the recent optical conductivity experiments on Bi_2Sr_2CaCu_2O_{8+delta} films, we examine the possible origin of low-frequency dissipation in the superconducting state. In the presence of spatial inhomogeneity of the local phase stiffness rho_s, it is shown that some spectral weight is removed from omega=0 to finite frequencies and contribute to dissipation. A case where both rho_s and the local normal fluid density are inhomogeneous is also considered. We find an enhanced dissipation at low frequency if the two variations are anti-correlated.Comment: To appear in Phys. Rev.

Double Fe-impurity charge state in the topological insulator Bi2_2Se3_3

The influence of individual impurities of Fe on the electronic properties of topological insulator Bi$_2$Se$_3$ is studied by Scanning Tunneling Microscopy. The microscope tip is used in order to remotely charge/discharge Fe impurities. The charging process is shown to depend on the impurity location in the crystallographic unit cell, on the presence of other Fe impurities in the close vicinity, as well as on the overall doping level of the crystal. We present a qualitative explanation of the observed phenomena in terms of tip-induced local band bending. Our observations evidence that the specific impurity neighborhood and the position of the Fermi energy with respect to the Dirac point and bulk bands have both to be taken into account when considering the electron scattering on the disorder in topological insulators.Comment: 10 pages, accepted for publication in Applied Physics Letters, minor bugs were correcte

Fluctuation Dominated Josephson Tunneling with a Scanning Tunneling Microscope

We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 k$\Omega$. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltages. In the presence of microwave fields (f=15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory.Comment: 4 pages, REVTeX, submitted to PR

Long range coherent magnetic bound states in superconductors

The quantum coherent coupling of completely different degrees of freedom is a challenging path towards creating new functionalities for quantum electronics. Usually the antagonistic coupling between spins of magnetic impurities and superconductivity leads to the destruction of the superconducting order. Here we show that a localized classical spin of an iron atom immersed in a superconducting condensate can give rise to new kind of long range coherent magnetic quantum state. In addition to the well-known Shiba bound state present on top of an impurity we reveal the existence of a star shaped pattern which extends as far as 12 nm from the impurity location. This large spatial dispersion turns out to be related, in a non-trivial way, to the superconducting coherence length. Inside star branches we observed short scale interference fringes with a particle-hole asymmetry. Our theoretical approach captures these features and relates them to the electronic band structure and the Fermi wave length of the superconductor. The discovery of a directional long range effect implies that distant magnetic atoms could coherently interact leading to new topological superconducting phases with fascinating properties

Confinement of superconducting fluctuations due to emergent electronic inhomogeneities

The microscopic nature of an insulating state in the vicinity of a superconducting state, in the presence of disorder, is a hotly debated question. While the simplest scenario proposes that Coulomb interactions destroy the Cooper pairs at the transition, leading to localization of single electrons, an alternate possibility supported by experimental observations suggests that Cooper pairs instead directly localize. The question of the homogeneity, granularity, or possibly glassiness of the material on the verge of this transition is intimately related to this fundamental issue. Here, by combining macroscopic and nano-scale studies of superconducting ultrathin NbN films, we reveal nanoscopic electronic inhomogeneities that emerge when the film thickness is reduced. In addition, while thicker films display a purely two-dimensional behaviour in the superconducting fluctuations, we demonstrate a zero-dimensional regime for the thinner samples precisely on the scale of the inhomogeneities. Such behavior is somehow intermediate between the Fermi and Bose insulator paradigms and calls for further investigation to understand the way Cooper pairs continuously evolve from a bound state of fermionic objects into localized bosonic entities.Comment: 29 pages 9 figure

Spectroscopic evidence for strong correlations between local superconducting gap and local Altshuler-Aronov density-of-states suppression in ultrathin NbN films

Disorder has different profound effects on superconducting thin films. For a large variety of materials, increasing disorder reduces electronic screening which enhances electron-electron repulsion. These fermionic effects lead to a mechanism described by Finkelstein: when disorder combined to electron-electron interactions increases, there is a global decrease of the superconducting energy gap $\Delta$ and of the critical temperature $T_c$, the ratio $\Delta$/$k_BT_c$ remaining roughly constant. In addition, in most films an emergent granularity develops with increasing disorder and results in the formation of inhomogeneous superconducting puddles. These gap inhomogeneities are usually accompanied by the development of bosonic features: a pseudogap develops above the critical temperature $T_c$ and the energy gap $\Delta$ starts decoupling from $T_c$. Thus the mechanism(s) driving the appearance of these gap inhomogeneities could result from a complicated interplay between fermionic and bosonic effects. By studying the local electronic properties of a NbN film with scanning tunneling spectroscopy (STS) we show that the inhomogeneous spatial distribution of $\Delta$ is locally strongly correlated to a large depletion in the local density of states (LDOS) around the Fermi level, associated to the Altshuler-Aronov effect induced by strong electronic interactions. By modelling quantitatively the measured LDOS suppression, we show that the latter can be interpreted as local variations of the film resistivity. This local change in resistivity leads to a local variation of $\Delta$ through a local Finkelstein mechanism. Our analysis furnishes a purely fermionic scenario explaining quantitatively the emergent superconducting inhomogeneities, while the precise origin of the latter remained unclear up to now.Comment: 11 pages, 4 figure

High tunnel magnetoresistance in spin-polarized scanning tunneling microscopy of Co nanoparticles on Pt(111)

We employ variable-temperature spin-polarized scanning tunneling microscopy in constant current mode to read the magnetic state of monodomain cobalt nanoparticles on Pt(111). In order to avoid stray fields we use in situ prepared antiferromagnetically (Cr) coated W tips. The contrast in apparent height between nanoparticles with opposite magnetization is typically Delta z=0.20 +/- 0.05 A, but can reach up to 1.1 A, indicating 80% spin-polarization of the nanoparticles and 850% magnetoresistance of the tip-sample tunnel junction with tip and sample at 300 K and 160 K, respectively. There is no zero-bias anomaly. These results suggest state-selective tunneling which is expected to lead to very high magnetoresistance values. (C) 2005 American Institute of Physics

Scanning tunneling spectroscopy study of the proximity effect in a disordered two-dimensional metal

The proximity effect between a superconductor and a highly diffusive two-dimensional metal is revealed in a scanning tunneling spectroscopy experiment. The in situ elaborated samples consist of superconducting single crystalline Pb islands interconnected by a nonsuperconducting atomically thin disordered Pb wetting layer. In the vicinity of each superconducting island the wetting layer acquires specific tunneling characteristics which reflect the interplay between the proximity-induced superconductivity and the inherent electron correlations of this ultimate diffusive two-dimensional metal. The observed spatial evolution of the tunneling spectra is accounted for theoretically by combining the Usadel equations with the theory of dynamical Coulomb blockade; the relevant length and energy scales are extracted and found in agreement with available experimental dataWe thank Hermann Grabert for useful discussions. This work was supported by grants from the University Pierre et Marie Curie (UPMC) ‘‘Emergence’’ and by CNRS Ph.D. Grant (L. S.-G.). J. C. C. and F. S. B. acknowledge financial support from the Spanish MICINN (Contracts No. FIS2011-28851-C02-01 and No. FIS2011-28851- C02-02

Oxidation induced enhanced magnetic susceptibility of Co islands on Pt(111)

We have studied the evolution of the magnetic properties of monolayer-high cobalt islands on a Pt(111) surface as function of their exposure to oxygen. We observe a sequential quenching of magnetic anisotropy and magnetic moment. For minute exposures to oxygen this leads to an enhancement of the maximum susceptibility of up to 66% for chi". We show that the reason for the reduction of the anisotropy before the one of the moment is due to preferential oxygen adsorption at the island edges, which are the main source of magnetic anisotropy. Our example shows how the enhanced chemical reactivity, combined with the enhanced anisotropy of low coordinated atoms, leads to surprising changes of magnetic properties upon exposure to oxygen