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Register a new userTheoretical Field Limits for Multi-Layer Superconductors
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The SIS structure---a thin superconducting film on a bulk superconductor separated by a thin insulating film---was propsed as a method to protect alternative SRF materials from flux penetration by enhancing the first critical field $B_{c1}$. In this work, we show that in fact $B_{c1}$ = 0 for a SIS structure. We calculate the superheating field $B_{sh}$, and we show that it can be enhanced slightly using the SIS structure, but only for a small range of film thicknesses and only if the film and the bulk are different materials. We also show that using a multilayer instead of a single thick layer is detrimental, as this decreases $B_{sh}$ of the film. We calculate the dissipation due to vortex penetration above the $B_{sh}$ of the film, and find that it is unmanageable for SRF applications. However, we find that if a gradient in the phase of the order parameter is introduced, SIS structures may be able to shield large DC and low frequency fields. We argue that the SIS structure is not beneficial for SRF cavities, but due to recent experiments showing low-surface-resistance performance above $B_{c1}$ in cavities made of superconductors with small coherence lengths, we argue that enhancement of $B_{c1}$ is not necessary, and that bulk films of alternative materials show great promise.
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A comment to the authors SRF Conference pre-print [1] was submitted by A. Gurevich to the arXiv [2]. In this response, we show that the arguments used in the comment are not valid. [1] arXiv:1309.3239 [2] arXiv:1309.5626
A multilayered structure with a single superconductor layer and a single insulator layer formed on a bulk superconductor is studied. General formulae for the vortex-penetration field of the superconductor layer and the magnetic field on the bulk superconductor, which is shielded by the superconductor and insulator layers, are derived with a rigorous calculation of the magnetic field attenuation in the multilayered structure. The achievable peak surface field depends on the thickness and its material of the superconductor layer, the thickness of the insulator layer and material of the bulk superconductor. The calculation shows a good agreement with an experimental result. A combination of the thicknesses of superconductor and insulator layers to enhance the field limit can be given by the formulae for any given materials.
Determining the optimal arrangement of superconducting layers to withstand large amplitude AC magnetic fields is important for certain applications such as superconducting radiofrequency cavities. In this paper, we evaluate the shielding potential of the superconducting film/insulating film/superconductor (SIS) structure, a configuration that could provide benefits in screening large AC magnetic fields. After establishing that for high frequency magnetic fields, flux penetration must be avoided, the superheating field of the structure is calculated in the London limit both numerically and, for thin films, analytically. For intermediate film thicknesses and realistic material parameters we also solve numerically the Ginzburg-Landau equations. It is shown that a small enhancement of the superheating field is possible, on the order of a few percent, for the SIS structure relative to a bulk superconductor of the film material, if the materials and thicknesses are chosen appropriately.
We develop a lower critical field (Hc1) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. Parameter Hc1 is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor-Insulator-Superconductor multilayer structure (S-I-S structure). As a validation test, we measure the temperature dependence of Hc1 of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4-5 K without any problem of heat generation of the coil. This field value is higher than those reported in previous works and makes it possible to more accurately estimate Hc1 at lower temperatures.
Superconducting nickelates appear to be difficult to synthesize. Since the chemical reduction of ABO3 (A: rare earth; B transition metal) with CaH2 may result in both, ABO2 and ABO2H, we calculate the topotactic H binding energy by density functional theory (DFT). We find intercalating H is energetically favorable for LaNiO2 but not for Sr-doped NdNiO2. This has dramatic consequences for the electronic structure as determined by DFT+dynamical mean field theory: that of 3d9 LaNiO2 is similar to (doped) cuprates, 3d8 LaNiO2H is a two-orbital Mott insulator. Topotactical H might hence explain why some nickelates are superconducting and others are not.
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