Wednesday 29 July 2015- Nadia Mason - Engineering Interactions in Arrays of Superconducting Islands

Superconductivity in artificially engineered, completely tunable arrays of superconducting islands has been a dream for experimentalists working in the area of mesoscopic superconductivity. Nadya’s talk focussed on her work in such tunable systems grown in her lab. She started by explaining how electron beam lithography is used to pattern films consisting of arrays of Nb islands of typical size of 90-150 nm on a 10 nm thick Au film which has underlying leads to carry transport measurements. Both the island size which is much larger than the superconducting coherence length (~ 27 nm) as well as the spacing between them which is larger than the normal metal coherence length can be tuned (~270/√T, T being the temperature). The Nb islands itself consisted of small Nb grains (~ 2-3 nm diameter) grown by e-beam evaporation. From the transport measurements they could observe two transitions. The transition at higher temperature (T1) corresponded to the individual Nb islands becoming superconducting while the transition at lower temperature (T2) related to the temperature at which the islands become Josephson coupled to give global superconductivity. T2 decreased with increasing spacing between the islands confirming this scenario. However, interestingly T1 also decreased with increased island spacing which formed the motivation for the second part of her talk.

Nadya also briefly mentioned that for very dilute systems with large island spacing, they could see the transition to the low temperature metallic state. 

Nadya also showed some of her very recent experiments on individual Nb islands grown on lithographically patterned substrates with Au contact pads. One of the intriguing results of these experiments was the observation of a strong suppression (~ almost 4 times) in Tc for individual Nb islands with relatively large sizes (~200-1000 nm). Nadya argued at these length scales neither of the usual culprits of proximity effect, finite size effects or charging effects could lead to the observed suppression of Tc. Moreover, different measurements on same sized islands showed larger Tc fluctuations in small islands as compared to larger ones. According to Nadya this was due to the large grain size distribution in these islands observed from transmission electron microscopy images. A simple calculation taking an exponential size distribution could explain the suppression in Tc of the micro-islands. Nadya termed this as the “rare-grain effect” where the onset of the transition is influenced by the largest grain in the island, belonging to the exponential tail of the grain size distribution. 

Nadya’s experiments opens up several questions, one of them being; How will Tc get influenced if two islands with same size showing the “rare grain effect” are brought in proximity to each other and will it be able to explain the decrease in T1 observed in the arrays with island spacing? This leaves room for further exploration in these superconducting islands and tunable arrays.

Blogged by Sangita Bose