LAUSR

Thanks for Dr Chris Stevens’s Viewpoint on our letter and his question where the large equivalent series resistance (ESR) of the proposed variable high-temperature superconducting (HTS) electrode capacitor bank arises (up to 2.29 Ω) in contrast to the estimated equivalent resistance (about 0.5 Ω) from the loss tangents of SrTiO3. Our answer is as follows: Besides the contacts made to the measuring device as surmised, the extra losses come from the conductive components of two capacitor elements including electrode plates, lead wires, and joints. The ESR parameter which represents the total losses caused by a capacitor at high frequency is mainly dependent on electrode plates, lead wires, and joints, besides the dielectric substrate, since the resistances of these conductive components at high frequency (e.g. in kHz or MHz level) would get much larger than those at power frequency. For more details about the equivalent series circuit model and ESR derivation of an HTS electrode capacitor, see our early paper [17]. Especially limited to the hot brittleness of SrTiO3, we have had to use silver conductive adhesive to adhere copper lead wires onto YBCO–SrTiO3–YBCO thin films, which would bring large additional losses to capacitor elements. As pointed out in the Viewpoint, this kind of SrTiO3 capacitor would provide a tuning functionality for tunable HTS devices indeed at the expense of large losses. However, there are several potential solutions to promote the variable HTS electrode capacitor from experimental prototype to practical product, as follows: