Phase diagram and upper critical field of homogenously disordered epitaxial 3-dimensional NbN films

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal-insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder we observe that as kFl-->1 the superconducting transition temperature (Tc) and minimum conductivity (sigma_0) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal-insulator transition and the superconductor-insulator transition occur at a single quantum critical point at kFl~1.Comment: To appear in Journal of Superconductivity and Novel Magnetism (ICSM2010 proceedings

An atomically thin oxide layer on the elemental superconductor Ta(001) surface

Recently the oxygen-reconstructed tantalum surface Ta(001)-p(3$\times$3)-O has experienced considerable attention due its use as a potential platform for Majorana physics in adatom chains. Experimental studies using scanning tunneling microscopy and spectroscopy found rich atomic and electronic structures already for the clean Ta(001)-O surface, which we combine here with $ab~initio$ methods. We discover two metastable superstructures at the root of the different topographic patterns, discuss its emergence during annealing, and identify the electronic properties. The latter is determined as the sole origin for the contrast reversal seen at positive bias. The observed effects are essentially connected to the two distinct oxygen states appearing on the surface in different geometries. The second superstructure was found in simulations by introducing oxygen vacancies, what was also observed in tantalum pentoxide systems. Additionally we study the charge distribution on the oxidized surface and underline its importance for the adsorption process of polarizable atoms and molecules.Comment: 5 pages, 4 figures, supplementary informatio

Universal scaling of the order-parameter distribution in strongly disordered superconductors

We investigate theoretically and experimentally the statistical properties of the inhomogeneous order-parameter distribution (OPD) at the verge of the superconductor-insulator transition (SIT). We find within two prototype fermionic and bosonic models for disordered superconductors that one can identify a universal rescaling of the OPD. By performing scanning-tunneling microscopy experiments in three samples of NbN with increasing disorder we show that such a rescaling describes also with an excellent accuracy the experimental data. These results can provide a breakthrough in our understanding of the SIT.Comment: 11 pages, 8 figures, revised version submitted to PR

Tunneling studies in a homogeneously disordered s-wave superconductor: NbN

We report the evolution of superconducting properties as a function of disorder in homogeneously disordered epitaxial NbN thin films grown on (100) MgO substrates, studied through a combination of electrical transport, Hall Effect and tunneling measurements. The thickness of all our films are >50nm much larger than the coherence length ~5nm. The effective disorder in different films encompasses a large range, with the Ioffe-Regel parameter varying in the range kFl~1.38-8.77. Tunneling measurements on films with different disorder reveals that for films with large disorder the bulk superconducting transition temperature (Tc) is not associated with a vanishing of the superconducting energy gap, but rather a large broadening of the superconducting density of states. Our results provide strong evidence of the loss of superconductivity via phase-fluctuations in a disordered s-wave superconductor.Comment: pdf file including figure

Correlation of Yu-Shiba-Rusinov States and Kondo Resonances in Artificial Spin Arrays on an s-Wave Superconductor

Mutually interacting magnetic atoms coupled to a superconductor have gained enormous interest due to their potential for the realization of topological superconductivity. Individual magnetic impurities produce states within the superconducting energy gap known as Yu-Shiba-Rusinov (YSR) states. Here, using the tip of a scanning tunneling microscope, we artificially craft spin arrays consisting of an Fe adatom interacting with an assembly of interstitial Fe atoms (IFA) on a superconducting oxygen-reconstructed Ta(100) surface and show that the magnetic interaction between the adatom and the IFA assembly can be tuned by adjusting the number of IFAs in the assembly. The YSR state experiences a characteristic crossover in its energetic position and particle-hole spectral weight asymmetry when the Kondo resonance shows spectral depletion around the Fermi energy. By the help of slave-boson mean-field theory (SBMFT) and numerical renormalization group (NRG) calculations we associate the crossover with the transition from decoupled Kondo singlets to an antiferromagnetic ground state of the Fe adatom spin and the IFA assembly effective spin. © 2021 The Authors. Published by American Chemical Society.The authors thank Markus Ternes for fruitful discussions. This work was supported by the European Research Council Advanced Grant ADMIRE (Project 786020). R.W. and J.W. acknowledge funding by the Cluster of Excellence “Advanced Imaging of Matter” (EXC 2056, Project 390715994) as well as by the SFB 925-B9 “Light induced dynamics and control of correlated quantum systems” of the Deutsche Forschungsgemeinschaft (DFG). R.Ž. acknowledges support by the Slovenian Research Agency (ARRS) under Program P1-0044. M.V. and R.M. acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) through Projects SFB668-A3 and -A1 and from the European Union’s Horizon 2020 research and innovation program under Grant Agreement 696656—GrapheneCore1. The work of M.V. was additionally funded through 2019 Equal Opportunity Fund of University of Hamburg. The DFT computations were performed with resources provided by the North-German Supercomputing Alliance (HLRN). S.P. and J.F thank Stiftelsen Olle Engqvist Byggmästare and Vetenskapsrådet for financial support

Moire-induced electronic structure modifications in monolayer V2S3 on Au(111)

Contains fulltext : 232260.pdf (Publisher’s version ) (Open Access

Measurement of magnetic penetration depth and superconducting energy gap in very thin epitaxial NbN films

We report the evolution of the magnetic penetration depth (λ) and superconducting energy gap (Δ) in epitaxial NbN films with thickness (d) varying between 51-3 nm. With decrease in film thickness Tc and Δ (0) monotonically decreases, whereas λ (0) monotonically increases. Our results show that while the values of Δ (0) and λ (0) are well described by Bardeen-Cooper-Schrieffer theory, at elevated temperatures, films with d≤6.5 nm show sudden drop in superfluid density associated with the Kosterlitz-Thouless-Berezinski transition. We discuss the implication of these results on the time response of superconducting bolometers made out of ultrathin NbN films. © 2010 American Institute of Physics