Effects of Ion Milling on the Microwave Properties of MgB2 Films

TL;DR

This study investigates how ion milling affects the microwave properties of MgB2 superconducting films, revealing that controlled ion milling can enhance low-temperature microwave performance by modifying interband scattering.

Contribution

It demonstrates that ion milling can improve MgB2 films' microwave properties by increasing interband scattering, a novel approach to optimizing superconductor performance.

Findings

Ion milling reduces surface resistance at low temperatures.

Ion milling increases interband scattering, affecting the small gap.

The Tc decreases slightly after ion milling.

Abstract

The new superconductor MgB2 may prove useful for microwave applications at intermediate temperatures. MgB2 films with the thickness of 300 - 400 nm have surface resistance Rs less than 0.01 mOhms at a frequency (f) of 8.5 GHz and 7 K,and ~ 1.5 mOhms at 87 GHz and 4.2 K. The critical temperature (Tc) of these films is ~ 39 K when they are prepared under optimum conditions. The Rs appears to scale as f^2 up to 30 K. After surface ion-milling, a reduction of the Tc and an enhanced resistivity rho(Tc) are observed consistently at 8.5 GHz and 87 GHz along with a reduced Rs at low temperatures. The observed rho(Tc) - Tc relation and the uncorrelated behavior between rho(Tc) and Rs values measured at low temperatures are well explained in terms of the two-gap model, with the observed rho(Tc) - Tc relation attributed to the properties of the large gap, and the Rs at lower temperatures reflecting the properties of the small gap, with an enhanced small gap energy due to increased interband scattering. This study suggests that the interband scattering should be enhanced to improve the low temperature microwave properties of MgB2 films and that the ion-milling process must be performed with great care to preserve the high quality of MgB2 films.