Evidence for current suppression in superconductor–superconductor bilayers

Abstract

Abstract Superconducting radio frequency (SRF) cavities, which are critical components in many particle accelerators, need to be operated in the Meissner state to avoid strong dissipation from magnetic vortices. For a defect-free superconductor, the maximum attainable magnetic field for operation is set by the superheating field, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> <mml:mi mathvariant="normal">h</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> , which directly depends on the surface current. In heterostructures composed of different superconductors, the current in each layer depends not only on the properties of the individual material, but also on the electromagnetic response of the adjacent layers through boundary conditions at the interfaces. Three prototypical bilayers [ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:mi mathvariant="normal">b</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">T</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> <mml:mtext> </mml:mtext> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mn>50</mml:mn> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">n</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:mi mathvariant="normal">b</mml:mi> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:msub> <mml:mi mathvariant="normal">b</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">T</mml:mi> <mml:msub> <mml:mi mathvariant="normal">i</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> <mml:mtext> </mml:mtext> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mn>80</mml:mn> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">n</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:mi mathvariant="normal">b</mml:mi> </mml:mrow> </mml:math> , and, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:msub> <mml:mi mathvariant="normal">b</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">T</mml:mi> <mml:msub> <mml:mi mathvariant="normal">i</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> <mml:mtext> </mml:mtext> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mn>160</mml:mn> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">n</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:mi mathvariant="normal">b</mml:mi> </mml:mrow> </mml:math> ] are investigated here by depth-resolved measurements of their Meissner screening profiles using low energy muon spin rotation (LE- µ SR). From fits to a model based on London theory (with appropriate boundary and continuity conditions), a magnetic penetration depth for the thin <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:msub> <mml:mi mathvariant="normal">b</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">T</mml:mi> <mml:msub> <mml:mi mathvariant="normal">i</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> </mml:math> layers of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msu