Characterization of Field Penetration in Superconducting Multilayers Samples

The best rf accelerating cavities are made of bulk niobium. This technology is now close to it full development, with any further improvement limited by the superconducting properties of Nb itself. In the best performing cavities the rf equatorial magnetic field H is close to the Nb thermodynamic critical field (H_C ≈ 200 mT). In 2006 Gurevich proposed the use of nanoscale layers of superconducting materials having H_C higher than Nb. This multilayer structure is expected to shield bulk niobium and therefore to increase the cavity breakdown field. In order to explore this pioneering idea, we have studied the penetration of the magnetic field by mean of nanometric sized multi layer structures deposited on flat single crystal substrates. We have deposited high quality "model" samples by dc magnetron reactive sputtering on R-plane cut sapphire substrates. A 250 nm layer of niobium represents the bulk material as in rf cavities. Such Nb layers were coated with a single or multiple stacks of NbN layers (25 nm or 12 nm) separated by 15 nm MgO barriers, and characterized by X-ray reflectivity and dc transport measurements, dc magnetization curves have been measured by conventional super conducting quantum interference device (SQUID) to determine the first penetration field B_C1. For comparison, B_C1 was also mea sured with a local probe method based on 3rd harmonic analysis. The Nb samples coated with NbN multi-layers clearly exhibit a higher first penetration field.