Density of states in SF bilayers with arbitrary strength of magnetic scattering

Gusakova, D.; Golubov, Alexandre Avraamovitch; Kupriyanov, M. Yu.; Buzdin, Alexandre I.

doi:10.1134/s0021364006080066

Cited by 13 publications

(22 citation statements)

References 23 publications

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“…In the Flayer, /τ e ≈ 400 meV (from resistivity data), which is much greater than either the estimated exchange field, E ex = 78 meV (estimated from the Curie Temperature [16]), or the superconducting gap, ∆ = 1.3 meV. In this limit, the superconducting order parameter should obey the Usadel equation [17,18],…”

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confidence: 87%

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

et al. 2008

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We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers as a function of Ni thickness, dF . Our thinnest samples show the usual DOS peak at ±∆0, whereas intermediate-thickness samples have an anomalous "double-peak" structure. For thicker samples (dF ≥ 3.5 nm), we see an "inverted" DOS which has previously only been reported in superconductor/weak-ferromagnet structures. We analyze the data using the self-consistent nonlinear Usadel equation and find that we are able to quantitatively fit the features at ±∆0 if we include a large amount of spin-orbit scattering in the model. Interestingly, we are unable to reproduce the sub-gap structure through the addition of any parameter(s). Therefore, the observed anomalous sub-gap structure represents new physics beyond that contained in the present Usadel theory.The co-existence of superconductivity and ferromagnetism was first proposed by Fulde and Ferrell [1] and Larkin and Ovchinnikov [2] more than forty years ago. While some unusual materials have since been found with both superconducting and ferromagnetic transitions (e.g. Qualitative evidence for the first two of these effects is convincing, but definitive quantitative agreement with theory has been problematic. The evidence for triplet superconductivity is less certain, although a recent report by Keizer et al [7] provides tantalizing evidence for such an effect. One reason for the difficulty in achieving quantitative agreement with theory is the proliferation of physical effects that now have been incorporated into the theory, leading to a concomitant proliferation of fitting parameters, which makes discriminating fits to limited data sets nearly impossible.In order to obtain more discriminating data sets and to further explore the SF proximity effect in the case of strong ferromagnets, we have undertaken superconducting tunneling densities of state (DOS) measurements on Nb/Ni thin-film bilayers. By varying the Ni thickness, d F , we can track the spatial evolution of the behavior of the Cooper pairs diffusing into the ferromagnet. This approach gives us much more information per sample (the entire DOS spectrum) than T c or J c measurements, and is less sensitive to variations in boundary parameters. Analyzing these results with the most complete forms of the Usadel theory available has allowed us to discriminate critically for the first time the relative importance of the various physical effects now incorporated into the theory. We find that by far the most important parameter beyond the exchange field, E ex , is the degree of spin-orbit scattering (first suggested by Demler et al [8]). In addition, we find an anomalous double-peak structure in the DOS that has not been reported previously and that we have been unable to account for theoretically.We use planar tunnel junctions of the form normalinsulator-ferromagnet-superconductor. A schematic of our sample geometry is shown in the inset of Fig. 2. The deposition of our samples and characterization of the tunnel junctions has be...

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confidence: 87%

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

et al. 2008

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“…Using the θ parameterizations of the normal and anomalous Green's functions, G = cos θ , F = sin θ , we can write the Usadel equations in the F layer as [35][36][37] …”

Section: Model and Basic Equationsmentioning

confidence: 99%

“…37,[45][46][47] In the first step we calculate the pair potential coordinate dependence (x) using the self-consistency equation in the S layer, Eq. (6).…”

Section: Density Of States In the F/s Bilayermentioning

confidence: 99%

Current-voltage characteristics of tunnel Josephson junctions with a ferromagnetic interlayer

et al. 2011

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We present a quantitative study of the current-voltage characteristics (CVC) of diffusive superconductor/ insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions. In order to obtain the CVC we calculate the density of states (DOS) in the F/S bilayer for arbitrary length of the ferromagnetic layer, using quasiclassical theory. For a ferromagnetic layer thickness larger than the characteristic penetration depth of the superconducting condensate into the F layer, we find an analytical expression which agrees with the DOS obtained from a self-consistent numerical method. We discuss general properties of the DOS and its dependence on the parameters of the ferromagnetic layer. In particular we focus our analysis on the DOS oscillations at the Fermi energy. Using the numerically obtained DOS we calculate the corresponding CVC and discuss their properties. Finally, we use CVC to calculate the macroscopic quantum tunneling (MQT) escape rate for the current biased SIFS junctions by taking into account the dissipative correction due to the quasiparticle tunneling. We show that the influence of the quasiparticle dissipation on the macroscopic quantum dynamics of SIFS junctions is small, which is an advantage of SIFS junctions for superconducting qubits applications.

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“…Thus, competing mechanisms are in play and a clear winner may not be ascertained a priori. Earlier works have investigated the influence of spinflip on the critical current [20] and critical temperature [20,21], the spatial and energy dependences of the DOS [22][23][24][25], the anomalous Green's function [26], and quasiparticles distribution [27,28] in a spin-split S.…”

Section: Introductionmentioning

confidence: 99%

Spin-flip enhanced thermoelectricity in superconductor-ferromagnet bilayers

et al. 2018

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We study the effects of spin-splitting and spin-flip scattering in a superconductor (S) on the thermoelectric (TE) properties of a tunneling contact to a metallic ferromagnet (F) using the Green's function method. A giant thermopower has been theoretically predicted and experimentally observed in such structures. This is attributed to the spin-dependent particle-hole asymmetry in the tunneling density of states (DOS) in the S/F heterostructure. Here, we evaluate the S DOS and thermopower for a range of temperatures, Zeeman-splitting, and spin-flip scattering. In contrast to the naive expectation based on the negative effect of spin-flip scattering on Cooper pairing, we find that the spin-flip scattering strongly enhances the TE performance of the system in the low-field and lowtemperature regime. This is attributed to a complex interplay between the charge and spin conductances caused by the softening of the spin-dependent superconducting gaps. The maximal value of the thermopower exceeds k B /e by a factor of ≈5 and has a non-monotonic dependence on spin-splitting and spin-flip rate.

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Density of states in SF bilayers with arbitrary strength of magnetic scattering

Cited by 13 publications

References 23 publications

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

Anomalous Double Peak Structure in Superconductor/Ferromagnet Tunneling Density of States

Current-voltage characteristics of tunnel Josephson junctions with a ferromagnetic interlayer

Spin-flip enhanced thermoelectricity in superconductor-ferromagnet bilayers

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