Change of surface critical current in the surface superconductivity and mixed states of superconducting niobium

Abstract

A systematic study of irreversible magnetization was performed in bulk Niobium after different surface treatments. Starting with smooth surfaces and abrading them, a strong increase of the critical current is observed up an apparent limiting value. An impressive change of the critical current is also observed in the surface superconductivity (SSC) state, reaching values of the same order of magnitude as in the mixed state. We explain also the observation of strong SSC for magnetic field perpendicular to larges facets in terms of nucleation of SC along bumps of a corrugated surface.

Surface Superconductivity Changes of Niobium Sheets by Femtosecond Laser-Induced Periodic Nanostructures

“…Nevertheless, this value is not always easy to derive from magnetization curves, due to the appearance of a tail at high magnetic fields in some conditions, as in this case [ 41 ]. With this aim, heat capacity, which is essentially a bulk property, could better allow the determination of μ 0 H c2 , as marked by the arrow in Figure 6 b [ 12 , 13 ]. It must be noted that the C ( H ) curve for the FS_N sample (not shown here for clarity reasons) is very similar to that of the reference sample, indicative of similar bulk properties (and μ 0 H c2 ) between both samples.…”

“…Changes in the Δ M (or i c ) at fields above H c2 for different surface treatments have been reported by several groups. For example, Scola et al [ 13 ] observed an increase of i c values after irradiating the Nb surface with low-energy Ar + ions; Aburas et al [ 12 ] observed changes by polishing the surface using different processes (sandpaper, diamond, colloidal silica and chemical polishing), with lower i c values for smoother surfaces; Casalbuoni et al [ 10 ] also reported differences in i c values of Nb cylinders subjected to buffered chemical polishing or to electropolishing; and van Gurp [ 41 ] observed higher i c values for cold rolled Nb foils compared to electrolytic Nb foils.…”

“…These surface current vortices can be pinned, resulting in a surface critical current ( i c ), which depends on the surface characteristics, such as roughness and morphology. These can be changed, for example, by different polishing procedures [ 10 , 11 , 12 ], or by low-energy Ar + ion irradiation of the surface [ 13 ].…”

“…In SRF applications, very clean surfaces are required to achieve a high quality factor, Q 0 , which is inversely proportional to the surface resistance [ 11 ]. Surface treatment procedures including thermal etching, electropolishing and buffered chemical polishing are usually used to achieve the former requirement [ 10 , 12 ]. Grassallino et al [ 14 ] also found that the annealing in a partial pressure of nitrogen or argon gas, followed by the electropolishing of the niobium cavity, yields very low values of the microwave surface resistance and, therefore, more efficient accelerating structures.…”

Enhancement of the critical current by surface irregularities in Fe-based superconductors

“…In addition to bulk pinning, surface irregularities have also been shown to be effective at enhancing the critical current in conventional low-T c superconductors [36,37]. Among the procedures used to create surface defects in these materials are sandblasting [37], mechanical abrasion [38], buffered chemical or electrolytic polishing [39], and low temperature bakeout [40]. More recent work in metallic niobium sheets has shown that laser-induced periodic structuration can make the irreversibility field H irr increase [41], or decrease when using a femtosecond-pulsed laser [42].…”