Control of superconductivity in MgB₂ by ion bombardment

The superconducting properties of MgB₂ can be modified via controlled introduction of lattice defects by ion bombardment. Here, magnetometry and Raman spectroscopy are used to study  ∼160 nm-thick MgB₂ films bombarded at room temperature with megaelectronvolt-energy He, Ar, or Xe ions. Ion bombardment leads to a monotonic reduction in the critical superconducting transition temperature, transition width, and critical current density. There is a clear collision cascade density effect, whereby radiation-induced changes, once normalized to the number of atomic displacements generated, are greater for heavier ions. The sharpening of the superconducting transition with increasing ion dose suggests improved homogeneity of irradiated films. The suppression of superconducting properties correlates with the growth of defect signatures revealed by Raman spectroscopy. Superconducting properties can be recovered by annealing at relatively low temperatures of 500 °C, even for films exposed to high doses of heavy ions. These results have direct implications for the performance of MgB₂ in radiation environments and understanding its defect-mediated superconductivity. (paper)