Tunneling studies in a homogeneously disordered<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>s</mml:mi></mml:math>-wave superconductor: NbN

Chockalingam, S. P.; Chand, Madhavi; Kamlapure, Anand; Jesudasan, John; Mishra, Archana; Tripathi, Vikram; Raychaudhuri, Pratap

doi:10.1103/physrevb.79.094509

Cited by 49 publications

(60 citation statements)

References 32 publications

(20 reference statements)

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“…At higher disorder, one observes that the conductivity becomes a strongly increasing function of ω, which is a signature of incipient localization in a disordered metal [17][18][19]. Consistent with this, dc transport has shown that as T c is suppressed, the resistivity of all lower T c samples show a negative temperature coefficient at low T [16]. Localization modified Drude and Drude-Simith models have been proposed to include localization effects in such disordered systems and can reproduce the spectra with positive slope [18].…”

supporting

confidence: 58%

“…Details of fabrication and measurement can be found in Ref [16]. The spectra were fitted by using the theory of Larkin and Ovchinnikov and Feigelman and Skvortsov [27,29].…”

Section: Extraction Of Superconducting Gap ∆ and De-pairing Factormentioning

confidence: 99%

“…Tunneling spectra in moderately disordered conventional superconductors reveals a conventional BCS density of states with its square root singularities and a clean gap. With increasing disorder -reminiscent of the situation in optics -the density of states broadens [30,31,33] and although the energy separating the coherence peaks (2∆) maintains a ratio 2∆/k B T c ≈ 4 up to high disorder levels [16], the peaks become smeared and a tunneling conductance develops at lower energies (See SI).…”

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confidence: 99%

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Anomalous gap-edge dissipation in disordered superconductors on the brink of localization

Cheng

Laurita

et al. 2016

Phys. Rev. B

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Superconductivity in disordered systems close to an incipient localization transition has been an area of investigation for many years. It has been noted that in such highly disordered superconductors, anomalous spectral weight develops in their conductivity near and below the superconducting gap energy. In this work we investigate the low frequency conductivity in disordered superconducting NbN thin films close to the localization transition with time-domain terahertz spectroscopy. In the normal state, strong deviations from the Drude form due to incipient localization are found. In the superconducting state we find substantial spectral weight at frequencies well below the superconducting gap scale derived from tunneling. We analyze this spectral weight in the context of a model of disorder induced broadening of the quasiparticle density of states and effective pair-breaking. We find that although aspects of the optical and tunneling data can be consistently modeled in terms of this effect of mesoscopic disorder, the optical conductivity returns to the normal state value much faster than any model predicts. This points to the non-trivial interplay of superconductivity and disorder close to localization.The manifestation of superconductivity in systems close to a disorder-driven localization transition has been an area of investigation for many years, yet many even central topics are not understood. The electrodynamic response of such systems is a fundamental probe of their low-energy physics but wide-open issues exist here as well. The optical conductivity corresponding to the mean-field Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity was worked out in the context of the celebrated Matthis-Bardeen (MB) theory [1]. A central prediction of the MB theory is the presence of a zero-frequency delta function and a gap 2∆ of a form that depends non-trivially on the BCS coherence factors in the real part of optical conductivity (σ 1 ). This theory works exceptionally well for many superconductors in the "dirty" limit, where the normal state scattering rate (1/τ ) is much larger than the gap, but which are still far from a localization transition [2][3][4]. The MB theory predicts that in the limit of zero temperature, e.g. the gap is clean, there is no spectral weight in σ 1 for frequencies below 2∆. However, it has been noticed for many years that in highly disordered superconductors, for instance in thin-film systems near the superconductor-insulator transition, anomalous spectral weight develops near and below the expected gap edge. This has been observed in many different systems including granular superconductors [5][6][7][8][9], amorphous thin films [10][11][12], and high-temperature superconductors with intrinsic disorder [13,14].In this work we studied the low-frequency conductivity of strongly disordered superconducting NbN thin films close to the localization transition. In the normal state, strong deviations from the Drude form are found, which are indicative of incipient localization in these film...

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supporting

confidence: 58%

“…Details of fabrication and measurement can be found in Ref [16]. The spectra were fitted by using the theory of Larkin and Ovchinnikov and Feigelman and Skvortsov [27,29].…”

Section: Extraction Of Superconducting Gap ∆ and De-pairing Factormentioning

confidence: 99%

mentioning

confidence: 99%

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Anomalous gap-edge dissipation in disordered superconductors on the brink of localization

Cheng

Laurita

et al. 2016

Phys. Rev. B

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“…24 for the critical superconductive state (section 5.4). This is due to the averaging of the peak position eV = ∆ 0 + ∆ (j) P over a broad distribution P (∆ P ) of the local pairing gaps ∆ We emphasize again that the results obtained in this section give the averaged tunneling conductance which can be measured with a large-area junctions, such as the ones used in [41]. They may differ significantly from the local tunneling conductance measured by a small tip and discussed in the following subsection.…”

Section: Tunneling In a Superconductormentioning

confidence: 77%

Fractal superconductivity near localization threshold

Фейгельман¹,

Ioffe²,

Kravtsov³

et al. 2010

Annals of Physics

233

360

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We develop a semi-quantitative theory of electron pairing and resulting superconductivity in bulk "poor conductors" in which Fermi energy E F is located in the region of localized states not so far from the Anderson mobility edge E c . We assume attractive interaction between electrons near the Fermi surface. We review the existing theories and experimental data and argue that a large class of disordered films is described by this model.Our theoretical analysis is based on analytical treatment of pairing correlations, described in the basis of the exact single-particle eigenstates of the 3D Anderson model, which we combine with numerical data on eigenfunction correlations. Fractal nature of critical wavefunction's correlations is shown to be crucial for the physics of these systems.We identify three distinct phases: 'critical' superconductive state formed at E F = E c , superconducting state with a strong pseudogap, realized due to pairing of weakly localized electrons and insulating state realized at E F still deeper inside localized band. The 'critical' superconducting phase is characterized by the enhancement of the transition temperature with respect to BCS result, by the inhomogeneous spatial distribution of superconductive order parameter and local density of states. The major new feature of the pseudo-gaped state is the presence of two independent energy scales: superconducting gap ∆, that is due to many-body correlations and a new "pseudogap" energy scale ∆ P which characterizes typical binding energy of localized electron pairs and leads to the insulating behavior of the resistivity as a function of temperature above superconductive T c . Two gap nature of the pseudogapped superconductor is shown to lead to specific features seen in scanning tunneling spectroscopy and point-contact Andreev spectroscopy. We predict that pseudogaped superconducting state demonstrates anomalous behavior of the optical spectral weight. The insulating state is realized due to presence of local pairing gap but without superconducting correlations; it is characterized by a hard insulating gap in the density of single electrons and by purely activated low-temperature resistivity ln R(T ) ∼ 1/T .Based on these results we propose a new "pseudospin" scenario of superconductorinsulator transition and argue that it is realized in a particular class of disordered superconducting films. We conclude by the discussion of the experimental predictions of the theory and the theoretical issues that remain unsolved.

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“…Further investigations on the superconducting properties show that for low disorder NbN follows the conventional BCS behavior [11,12]. However, as we approach the SIT the superconducting state is characterized by a gradual loss in the coherence peak in the tunneling density of states and a linear variation of the superfluid density with temperature over a large temperature range.…”

Section: Figure 4 (A) Shows the ρ(Ht) As Function Of T For A Film Wimentioning

confidence: 79%

Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films

Mondal

Chand

Kamlapure

et al. 2010

J Supercond Nov Magn

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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 k F l→1 the superconducting transition temperature (T c ) and normal state conductivity in the limit T 0 (σ 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, k F l~1.

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Tunneling studies in a homogeneously disordereds-wave superconductor: NbN

Cited by 49 publications

References 32 publications

Anomalous gap-edge dissipation in disordered superconductors on the brink of localization

Anomalous gap-edge dissipation in disordered superconductors on the brink of localization

Fractal superconductivity near localization threshold

Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films

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