Microwave surface resistance of thick MgB₂films onc-plane sapphire: a study on the depth profile of the surface resistance

Abstract

A depth profile of the surface resistance (RS) is obtained at 8.6 GHz for a thick MgB2 film both in the superconducting state and in the normal state, for which 1 µm thick MgB2 films are grown in situ on a c-plane sapphire substrate using the hybrid physical–chemical vapor deposition (HPCVD). A critical temperature TC of 40.4 K, a value higher than that of 39 K for MgB2 single crystals, is observed for the pristine 1 µm thick MgB2 film due to the thermal strain caused by its epitaxial nature. The depth profile of the effective RS for the 1 μm thick MgB2 film shows a drastic increase in the effective RS at low temperatures for films having thicknesses of 600 nm or less. The results for the x-ray diffraction and the normal-state resistivity provide evidence for the existence of Mg-rich phases in the lower part of the pristine MgB2 film. Our results show that thick MgB2 films grown using HPCVD could have very low RS despite the existence of Mg-rich phases in the lower part of the MgB2 films and that microwave technique could provide a sensitive method for investigating the homogeneity of thick MgB2 films.

Depth profiling of the microwave surface resistance of high-JCGdBa2Cu3O7−δcoated conductors grown using the RCE-DR process

“…Considering that both R S and J C reflect the intrinsic transport properties of superconductors, our results for the R S imply possible inhomogeneity of the J C over the thickness of GdBCO CCs. We note that the thickness dependence of the R S of our GdBCO CCs is totally different from that of thick MgB 2 films grown using the hybrid physical-chemical deposition, for which existence of Mg-rich phase at the lower part of the MgB 2 film was the reason for the R S of the lower part being higher than that of the upper part [23]. We also note that different R S behaviors observed among the MgB 2 films and the GdBCO CCs are not due to the differences in the kinds of substrates used for growing the films and CCs but solely attributed to differences in the microwave properties of the corresponding superconductive layers.…”

“…For the GdBCO CCs Ar-ionmilled from the top side, the thicknesses are measured using scanning electron microscopy. We note that this Ar-ion-milling procedure has been proved useful for obtaining the depth profiling of the R S of thick MgB 2 films [23].…”