Room-temperature optical absorption data in the 1.5–2.5-eV range are reported for indium nitride thin films prepared by reactive radio-frequency sputtering. The fundamental absorption edge in high-purity material is located at 1.89±0.01 eV and corresponds to a direct transition at k=0, in agreement with band-structure calculations. A significant Moss-Burstein shift is noted for carrier concentrations in excess of 1019 cm−3 and obeys the empirical relationship EG =1.89+2.1×10−8 n1/3 eV.

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Step-edge junctions represent one type of grain boundary Josephson junction employed in high-temperature superconducting junction technology. To date, the majority of results published in the literature focus on [001]-tilt grain boundary junctions (GBJs) produced using bicrystal substrates. We investigate the step morphology and YBCO (yttrium barium copper oxide) film structure of YBCO-based step-edge junctions on MgO [001] substrates which structurally resemble [100]-tilt junctions. High-resolution electron microscopy reveals a clean GBJ interface of width ∼1 nm and a single junction at the top edge. The dependence of the transport properties on the MgO step-edge and junction morphology is examined at 4.2 K, to enable direct comparison with results for other junction studies such as [001]-tilt and [100]-tilt junctions and building on previously published 77 K data. MgO step-edge junctions show a slower reduction in critical current density with step angle compared with [001]-tilt junctions. For optimized step parameters, transport measurements revealed large critical current and normal resistance (I c R N ) products (∼3-5 mV), comparable with the best results obtained in other kinds of [100]-tilt GBJs in YBCO at 4.2 K. Junction-based devices such as SQUIDs (superconducting quantum interference devices) and THz imagers show excellent performance when MgO-based step-edge junctions are used.

Nano-scale superconducting interference devices, known as nanoSQUIDs, are an emerging research area that has been attracting a lot of attention in recent years. This is an introduction to this special edition of Superconductor Science and Technology to place the following papers into context by briefly outlining the various methods of fabrication and the wide range of potential applications.