Enhancing Properties of High‐Temperature Superconducting Step‐Edge Josephson Junctions by Nano‐Multilayers with a Small Mismatch

Abstract

activated motion in these superconducting structures. [ 6 ] One of the approaches to minimize the 1/f noise caused by vortex motion is improvement of pinning properties and, hence, the critical current density ( J c ) of the superconducting fi lm. Recently, the noise performance of HTS SQUID has been improved in a magnetic environment by using YBCO fi lm with a large thickness and high J c value. [ 8 ] We have also demonstrated that fi lms with better structural integrity (smooth, no holes) minimize fl ux creep and vortex hopping in alternating magnetic fi elds and have lower 1/f noise. [ 9 ] Thus, the materials technology needs for qubits are similar to the requirements of SQUID applications, also having Josephson junctions as their basic element. High quality YBCO Josephson junctions play a key-role in the successful implementation of HTS superconducting circuits at 77 K. [ 6 ] This is mainly due to YBCO material properties, resulting from the complexity of structural characteristics and having strong infl uence on the transport properties of YBCO. [ 10 ] A range of different types of YBCO junctions have been engineered. [ 10 ] Among them, stepedge junctions are singled out due to their low cost, relatively high characteristic voltage (the so-called I R c n value of 100 to 400 μV at 77 K, [ 7,11 ] where I c is the critical current and R n is the resistance in the normal state), and excellent integrability and the scalability of the junctions. These features enable miniaturization and high effi ciency of electronic circuits, as well as applications in magnetic sensing and terahertz (THz) imaging. [ 12,13 ] Therefore, the combined enhancement of fl ux pinning, critical current and microstructure in the YBCO junctions is a crucial task for obtaining tailored junction properties towards the well-defi ned requirements. [ 1 ] In this work, we demonstrate a new straightforward, but effective multilayering approach for fabrication of HTS step-edge junctions with signifi cantly enhanced characteristics. The alternation of main YBCO and intermediate NdBa 2 Cu 3 O 7 (NdBCO) superconducting layers was shown to dramatically improve surface morphology of 1 μm thick fi lms. [ 14 ] Furthermore, this architecture resulted in considerable enhancement of fl ux pinning and current-carrying ability over a wide range of magnetic fi elds and temperatures. We have successfully managed to apply this technology to YBCO step-edge junctions and to demonstrate signifi cant enhancement of transport and structural properties in these junctions. This approach is preferred over the fabrication of thick fi lm, [ 8 ] because it enables structural tailoring with simultaneous adjustment of transport properties required for junction performance. Thus, the multilayered deposition is offering a new promising approach for applications in HTS-based electronics.Recent signifi cant progress in superconducting qubits (based on Josephson junctions) has achieved coherence times approaching to the practically required for quantum computers. [ 1 ] At the same...

Critical current variation in YBa2Cu3O7−xstep-edge junction arrays on MgO substrates

“…Due to the growth of individual grains, meandering along the width of the junction causes a variation of supercurrent flow direction [5,32], depending on the local meandering direction against the individual grain [33]. Furthermore, TEM images reveal meandering also happens along the grain boundary from the bottom of the MgO substrate to the YBCO top surface [17,32,34]. These TEM images also show there is a variation of barrier thickness on the nanometre scale.…”

“…Low-energy ion beam etching has been used to clean the MgO step-edge surface before YBCO deposition [17,32,35] and was found to reduce the grain growth faceting along the width of the step [32], resulting in an improvement in the I c R n value. A multilayer approach has been studied to reduce the lattice mismatch between MgO and YBCO [34], and is likely to reduce the meandering both along the width and height of the step.…”

Large artificial ferromagnetic dot arrays for the critical current enhancement in superconducting YBa2Cu3O$_{7-\delta}$ thin films

“…Flux pinning of a quantized magnetic vortex is arguably one of the most crucial parameters controlling the superconducting critical current density (J c ) at both self-field and applied magnetic fields for many practical applications. Tremendous effort has been devoted to pinning enhancement by using different approaches, such as by introducing artificial pinning centres [1][2][3][4][5][6] and multilayering [7][8][9]. However, intrinsic (structural) defects face an abrupt decrease in the vortex pinning energy as temperature increases U cp ∼ (H 2 c /8π)πξ 2 ≈ Φ 0 /8πλ L ) due to increasing London penetration depth λ L as λ L (T) = λ L (0)(1-T/T c ) −1/2 , (λ L (0) ∼ 160 nm for YBCO [10,11]), thus very small, or even negligible, potentials pinning at temperature close to T c [12,13].…”

Enhancing the critical current of YBa2Cu3O7 thin films by substrate nanoengineering

“…A similar technology is employed to create step-edge Josephson Junctions (SEJs) in YBCO. 16 This process involves etching a step or a ramp into a substrate to produce SEJs caused by a nanoscale thick, artificially created weak-link. [16][17][18] This weak-link separates the top and the ramp part of the step.…”