Tunneling Study of Superconductivity near the Metal-Insulator Transition

Dynes, R. C.; Garno, J. P.; Hertel, G.; Orlando, T. P.

doi:10.1103/physrevlett.53.2437

Cited by 415 publications

(280 citation statements)

References 8 publications

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“…The calculation for the thickest film fits the data well but for the thinner film the peak is very much attenuated, perhaps indicating a new effect outside standard Eliashberg theory. In their tunneling study of the metal-insulator transition in aluminum films, Dynes and coworkers [16] found a similar effect, namely, a broadening of the density of states coherence peak with increased sheet resistance. The lower panel of Fig.…”

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confidence: 73%

Unusual thickness dependence of the superconducting transition ofα-MoGethin films

et al. 2008

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Thin films of α-MoGe show progressively reduced Tc's as the thickness is decreased below 30 nm and the sheet resistance exceeds 100 Ω/ . We have performed far-infrared transmission and reflection measurements for a set of α-MoGe films to characterize this weakened superconducting state. Our results show the presence of an energy gap with ratio 2∆0/kBTc = 3.8 ± 0.1 in all films studied, slightly higher than the BCS value, even though the transition temperatures decrease significantly as film thickness is reduced. The material properties follow BCS-Eliashberg theory with a large residual scattering rate except that the coherence peak seen in the optical scattering rate is found to be strongly smeared out in the thinner superconducting samples. A peak in the optical mass renormalization at 2∆0 is predicted and observed for the first time.PACS numbers: 74.81.Bd, Disorder and reduced dimensionality affect the physical properties of metallic systems in a number of ways. Anomalous diffusion leads to localization of electrons and a related enhancement of the Coulomb interaction via reduced screening[1, 2], seen as an increase in µ * , the renormalized Coulomb interaction parameter. In a system of lower dimensions, the coupling to disorder increases, and pronounced effects are expected. Disorder-driven localization and the related enhancement of the Coulomb interaction inherently compete with the attractive interaction in superconducting metals [3,4], described by the electron-phonon spectral density α 2 F (ω) [5]. This competition reduces the transition temperature. Of particular interest are two-dimensional (2D) superconductors in which the degree of disorder can be adjusted by varying the appropriate parameters. In an ideal 2D system, the relevant parameter is normally considered to be the sheet resistance, R . The sheet resistance is determined by two factors: the (possibly thickness dependent) conductivity σ and the film thickness d.Amorphous MoGe (α-MoGe) thin films are thought to be a model system for studying the interplay between superconductivity and disorder. Several transport experiments have revealed a sharp reduction in the superconducting transition temperature T c with increasing R , even in the weakly localized regime [6,7,8,9]. The suppression of T c has been attributed to localization and an increase in the Coulomb interaction [3]. In this Letter, we explore the T c suppression in α-MoGe thin films with different thickness via temperature-dependent far-infrared transmittance and reflectance. A strong suppression of T c with increasing R is observed. The superconducting energy gap is also reduced, but the ratio of gap energy to transition temperature and the normal-state conductivity, both of which could be dependent on the disorderdriven Coulomb interaction, are not affected at all.Our films were prepared by co-magnetron sputtering

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confidence: 73%

Unusual thickness dependence of the superconducting transition ofα-MoGethin films

et al. 2008

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“…The terms in the right hand sides of the master equations (15) and (21) are proportional to the square of the tunneling amplitude via the tunneling conductance g T ∝t 2 ; this proportionality is a consequence of the lowest order perturbative treatment of the tunneling Hamiltonian [Eq. (8)], which enables us to neglect higher order (int) terms when evaluating certain correlation functions involving qubit and quasiparticle operators [see Appendix A for details].…”

Section: Qubit Phase Relaxation: the Master Equationmentioning

confidence: 99%

Decoherence of superconducting qubits caused by quasiparticle tunneling

et al. 2012

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In superconducting qubits, the interaction of the qubit degree of freedom with quasiparticles defines a fundamental limitation for the qubit coherence. We develop a theory of the pure dephasing rate Γ φ caused by quasiparticles tunneling through a Josephson junction and of the inhomogeneous broadening due to changes in the occupations of Andreev states in the junction. To estimate Γ φ , we derive a master equation for the qubit dynamics. The tunneling rate of free quasiparticles is enhanced by their large density of states at energies close to the superconducting gap. Nevertheless, we find that Γ φ is small compared to the rates determined by extrinsic factors in most of the current qubit designs (phase and flux qubits, transmon, fluxonium). The split transmon, in which a single junction is replaced by a SQUID loop, represents an exception that could make possible the measurement of Γ φ . Fluctuations of the qubit frequency leading to inhomogeneous broadening may be caused by the fluctuations in the occupation numbers of the Andreev states associated with a phase-biased Josephson junction. This mechanism may be revealed in qubits with small-area junctions, since the smallest relative change in frequency it causes is of the order of the inverse number of transmission channels in the junction.

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“…In this model, two parameters are introduced to describe the necessary physical quantities, i.e., the effective potential barrier (Z) and the superconducting energy gap (∆). As an extension, the quasiparticle energy E is replaced by E − iΓ, where Γ is the broadening parameter characterizing the finite lifetime of the quasiparticles due to inelastic scattering near the N/S micro-constriction [34,35]. In a real N-I-S junction configuration, total tunneling conductance spectrum includes the integration over the solid angle.…”

Section: B Theoretical Model -S-wave Btk Theorymentioning

confidence: 99%

Distinct pairing symmetries inNd1.85Ce0.15CuO4−yandLa1.89

et al. 2005

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We used point-contact tunnelling spectroscopy to study the superconducting pairing symmetry of electron-doped N d1.85Ce0.15CuO4−y (NCCO) and hole-doped La1.89Sr0.11CuO4 (LSCO). Nearly identical spectra without zero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented surfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast, LSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave superconductor. Detailed analysis reveals an s-wave component in the pairing symmetry of the NCCO sample with ∆/kBTc = 1.66, a value remarkable close to that of a weakly coupled BCS superconductor. We argue that this s-wave component is formed at the Fermi surface pockets centered at (±π,0) and (0,±π) although a d-wave component may also exist.

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Tunneling Study of Superconductivity near the Metal-Insulator Transition

Cited by 415 publications

References 8 publications

Unusual thickness dependence of the superconducting transition ofα-MoGethin films

Unusual thickness dependence of the superconducting transition ofα-MoGethin films

Decoherence of superconducting qubits caused by quasiparticle tunneling

Distinct pairing symmetries inNd1.85Ce0.15CuO4−yandLa1.89

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