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

Álvaro Cubero,Herbert Legall,Jörg Krüger,Rafael Navarro,Luis A Angurel,Elena Martínez,Germán F De La Fuente,Jörn Bonse

doi:10.3390/nano10122525

Abstract

Irradiation with ultra-short (femtosecond) laser beams enables the generation of sub-wavelength laser-induced periodic surface structures (LIPSS) over large areas with controlled spatial periodicity, orientation, and depths affecting only a material layer on the sub-micrometer scale. This study reports on how fs-laser irradiation of commercially available Nb foil samples affects their superconducting behavior. DC magnetization and AC susceptibility measurements at cryogenic temperatures and with magnetic fields of different amplitude and orientation are thus analyzed and reported. This study pays special attention to the surface superconducting layer that persists above the upper critical magnetic field strength Hc2, and disappears at a higher nucleation field strength Hc3. Characteristic changes were distinguished between the surface properties of the laser-irradiated samples, as compared to the corresponding reference samples (non-irradiated). Clear correlations have been observed between the surface nanostructures and the nucleation field Hc3, which depends on the relative orientation of the magnetic field and the surface patterns developed by the laser irradiation.

Highlights

IntroductionIt is well established for type II superconductors that both a lack of material’s extended lattice periodicity (grain boundaries, stacking faults, etc.) and local crystallographic defects (vacancies, substitutions) interact with magnetic vortices and act as effective pinning centers of the magnetic flux lines
It is well established for type II superconductors that both a lack of material’s extended lattice periodicity and local crystallographic defects interact with magnetic vortices and act as effective pinning centers of the magnetic flux lines
Irradiation with ultra-short laser beams enables the generation of sub-wavelength laser-induced periodic surface structures (LIPSS) over large areas with controlled spatial periodicity, orientation, and depths affecting only a material layer on the sub-micrometer scale

Summary

Introduction

It is well established for type II superconductors that both a lack of material’s extended lattice periodicity (grain boundaries, stacking faults, etc.) and local crystallographic defects (vacancies, substitutions) interact with magnetic vortices and act as effective pinning centers of the magnetic flux lines. Type II superconductors with negligible bulk pinning may present hysteresis effects, due to Bean–Livingston surface barriers [2] or due to geometrical edge barriers (specimen-shape dependent) [3] Both types have been observed in low- and high-temperature superconductors (LTS and HTS, respectively) [2,3,4,5,6,7,8]. Saint James and De Gennes [9] predicted a superconducting layer of a thickness approximately equal to the superconducting coherence length, up to a field Hc3 ≈ 1.69 × Hc2 when the field is applied parallel to the superconductor’s surface These surface current vortices can be pinned, resulting in a surface critical current (ic), 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]

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