Electronic stopping of slow protons in ZnO, VO2 (metal and semiconductor phases), HfO2 and Ta2O5 was investigated experimentally. As a comparison of the resulting stopping cross sections (SCS) to data for Al2O3 and SiO2 reveals, electronic stopping of slow protons does not correlate with electronic properties of the specific material such as band gap energies.Instead, the oxygen 2p states are decisive, as corroborated by DFT calculations of the electronic densities of states. Hence, at low ion velocities the SCS of an oxide primarily scales with its oxygen density.

Square arrays of sub-micrometer columnar defects in thin YBa2Cu3O 7−δ (YBCO) films with spacings down to 300 nm have been fabricated by a He ion beam projection technique. Pronounced peaks in the critical current and corresponding minima in the resistance demonstrate the commensurate arrangement of flux quanta with the artificial pinning landscape, despite the strong intrinsic pinning in epitaxial YBCO films. Whereas these vortex matching signatures are exactly at predicted values in field-cooled experiments, they are displaced in zero-field cooled, magnetic-field ramped experiments, conserving the equidistance of the matching peaks and minima. These observations reveal an unconventional critical state in a cuprate superconductor with an artificial, periodic pinning array. The long-term stability of such out-of-equilibrium vortex arrangements paves the way for electronic applications employing fluxons.

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Magnetic fields penetrate a type-II superconductor as magnetic flux quanta, called vortices. In a clean superconductor they arrange in a hexagonal lattice, while by adding periodic artificial pinning centers many other arrangements can be realized. Using the focused beam of a helium ion microscope we have fabricated periodic patterns of dense pinning centers with spacings as small as 70 nm in thin films of the cuprate superconductor YBa 2 Cu 3 O 7−δ . In these ultradense kagomé-like patterns, the voids lead to magnetic caging of vortices, resulting in unconventional commensurability effects that manifest themselves as peaks in the critical current and minima in the resistance vs applied magnetic field up to ∼ 0.4 T. The various vortex patterns at different magnetic fields are analyzed by molecular-dynamics simulations of vortex motion, and the magnetic-field dependence of the critical current is confirmed. These findings open the way for a controlled manipulation of vortices in cuprate superconductors by artificial sub-100 nm pinning landscapes.