Density of States of a Short-Mean-Free-Path Superconductor in a Magnetic Field by Electron Tunneling

Levine, James L.

doi:10.1103/physrev.155.373

Cited by 39 publications

(15 citation statements)

References 13 publications

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“…S7 (22), where the in-gap spectrum is featureless under the same magnetic field, displaying a hard gap with only minimal changes to the coherence peaks. Moreover, these observations contrast sharply with the tunneling spectra of conventional superconductors such as aluminum or lead, where magnetic field also causes the filling of the superconducting gap in a featureless manner (7), as do magnetic impurities (23). To understand the microscopic origin of the observed tunneling spectra, we perform theoretical calculations of the density of states at various field-induced Cooper pair momenta.…”

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confidence: 86%

Discovery of segmented Fermi surface induced by Cooper pair momentum

Zhu

Papaj

Nie

et al. 2021

Science

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A sufficiently large supercurrent can close the energy gap in a superconductor and create gapless quasiparticles through the Doppler shift of quasiparticle energy caused by finite Cooper pair momentum. In this gapless superconducting state, zero-energy quasiparticles reside on a segment of the normal state Fermi surface, whereas the remaining Fermi surface is still gapped. Here we use quasiparticle interference to image the field-controlled Fermi surface of Bi2Te3 thin films proximitized by the superconductor NbSe2. A small applied in-plane magnetic field induces a screening supercurrent, which leads to finite-momentum pairing on the topological surface states of Bi2Te3. We identify distinct interference patterns that indicate a gapless superconducting state with segmented Fermi surface. Our results reveal the strong impact of finite Cooper pair momentum on the quasiparticle spectrum.

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confidence: 86%

Discovery of segmented Fermi surface induced by Cooper pair momentum

Zhu

Papaj

Nie

et al. 2021

Science

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“…∆ 0 i describes the intrinsic gap within each band i, that is generated by the electron-phonon coupling and by the scattering rates of quasiparticles between the bands Γ ij . The extension of the two-band model to include Abrikosov-Gor'kov depairing [26][27][28][29] -via the terms with Γ AG i -was done by Kaiser and Zuckermann [30]. Here, depairing is due to magnetic field; thus, Γ AG i are set to 0 when no magnetic field is applied.…”

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confidence: 99%

Spectroscopy of bulk and few-layer superconducting NbSe2 with van der Waals tunnel junctions

Dvir¹,

Massee²,

Attias³

et al. 2018

Nat Commun

122

121

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Tunnel junctions, an established platform for high resolution spectroscopy of superconductors, require defect-free insulating barriers; however, oxides, the most common barrier, can only grow on a limited selection of materials. We show that van der Waals tunnel barriers, fabricated by exfoliation and transfer of layered semiconductors, sustain stable currents with strong suppression of sub-gap tunneling. This allows us to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) NbSe2 devices at 70 mK. These exhibit two distinct superconducting gaps, the larger of which decreases monotonically with thickness and critical temperature. The spectra are analyzed using a two-band model incorporating depairing. In the bulk, the smaller gap exhibits strong depairing in in-plane magnetic fields, consistent with high out-of-plane Fermi velocity. In the few-layer devices, the large gap exhibits negligible depairing, consistent with out-of-plane spin locking due to Ising spin–orbit coupling. In the 3-layer device, the large gap persists beyond the Pauli limit.

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“…The Usadel equations are now at the basis of the understanding of mesoscopic superconductivity in diffusive conductors [5,6]. Experimentally, measurements of the density of states (DOS) in a thin superconductor placed in an in-plane magnetic field were well accounted for by the concept of depairing energy [7]. In contrast, the effect of a supercurrent has been partly addressed in a single experiment, focused on the reduction of the superconducting gap close to the critical temperature [8].…”

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confidence: 99%

Density of States in a Superconductor Carrying a Supercurrent

2003

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We have measured the tunneling density of states (DOS) in a superconductor carrying a supercurrent or exposed to an external magnetic field. The pair correlations are weakened by the supercurrent, leading to a modification of the DOS and to a reduction of the gap. As predicted by the theory of superconductivity in diffusive metals, we find that this effect is similar to that of an external magnetic field.PACS numbers: PACS numbers: 74.78. Na, 74.20.Fg, 74.25.Sv How is the superconducting order modified by a supercurrent? The superconducting order is based on pairing electronic states which transform into one another by time reversal [1]. The ground-state wavefunction corresponds to a coherent superposition of doubly empty and doubly occupied time-reversed states, in an energy range around the Fermi level given by the BCS gap energy. When an external magnetic field B = curl A is applied, time-reversed states are dephased differently, resulting in a weakening of superconductivity. In presence of a supercurrent, the superconducting order no longer corresponds to the pairing of time-reversed states, which results in a kinetic energy cost, and again in a weakening of superconductivity. In the early stages of the theory of superconductivity, it was found that, in diffusive superconductors (in which the electron mean-free-path is short compared to the BCS coherence length) and in homogeneous situations, the modification of the superconducting order by a magnetic field, by a current and by paramagnetic impurities could be described by a single parameter, the depairing energy Γ [2]. Later on, the reformulation of the theory by Usadel [3,4] in the diffusive limit extended this equivalence to inhomogeneous situations, where the modulus of the order parameter may vary in space. In the Usadel equations, all physical quantities only involve the intrinsic combination − → ∇ϕ − (2e/ ) A, where the gradient − → ∇ϕ in the phase of the superconducting order parameter is associated with the supercurrent, revealing the equivalence of a supercurrent and of an applied magnetic field. The Usadel equations are now at the basis of the understanding of mesoscopic superconductivity in diffusive conductors [5,6]. Experimentally, measurements of the density of states (DOS) in a thin superconductor placed in an in-plane magnetic field were well accounted for by the concept of depairing energy [7]. In contrast, the effect of a supercurrent has been partly addressed in a single experiment, focused on the reduction of the superconducting gap close to the critical temperature [8]. A complication of the experiments with a supercurrent is that, if the sample width exceeds the London penetration length λ L , the current distribution given by the non-local equations of electrodynamics [9] is not homogeneous. In the experiment reported here, the superconductor is wire-shaped, with thickness and width smaller than λ L , so that the current flow is homogeneous and the magnetic field penetrates completely. Moreover, the effect of the magnetic field ind...

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Density of States of a Short-Mean-Free-Path Superconductor in a Magnetic Field by Electron Tunneling

Cited by 39 publications

References 13 publications

Discovery of segmented Fermi surface induced by Cooper pair momentum

Discovery of segmented Fermi surface induced by Cooper pair momentum

Spectroscopy of bulk and few-layer superconducting NbSe2 with van der Waals tunnel junctions

Density of States in a Superconductor Carrying a Supercurrent

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