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Superconductor Science and Technology

Out of the loop—a superconducting tunnel junction that behaves like a SQUID

Edward J. Romans

Published Apr 27, 2018• 1 citation
Publisher

Abstract

Recently there has been a trend to develop nanoscale superconducting quantum interference devices (nanoSQUIDs) with the aim to reach magnetic flux sensitivities sufficient to detect the flip of a single electron spin in close proximity (see refs [1], [2]
DOI
10.1088/1361-6668/aac0e6

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Nanoscale Superconducting QUantum Interference Devices (SQUIDs) have spin sensitivities approaching that required to detect the flip of a single spin in close proximity. There is considerable interest in developing them for measuring the properties of small spin populations in magnetic systems. It is desirable that such measurements can be realised at sub-kelvin temperatures thereby allowing the study of magnetic systems that undergo phase changes at such temperatures. However, most nanoscale SQUIDs use nanobridges as the Josephson elements which limits the operating range to temperatures close to the transition temperature of the device. Well below this, the current-phase relation can become non-sinusoidal, and hot spots arising from the large critical current lead to hysteretic I-V characteristics. To extend the temperature range downwards, we have developed a range of nanoSQUIDs fabricated from alternative materials with lower transition temperatures including Ti/Au and Al/Ag bilayers patterned using lift-off and e-beam lithography (EBL). We report on their I-V characteristics, noise performance and behaviour in applied magnetic fields at temperatures down to 60 mK. We discuss theoretical analysis and computer modelling of the heat flow in the nanobridge structure, and consider the effects of bank geometry and kinetic inductance on the overall device performance. Finally, we present measurements of several magnetic systems: control lines, superconducting islands and ground planes and discuss the feasibility of magnetic measurements of novel materials of interest, including the heterointerface between lanthanium aluminate and strontium titanate.
Science and industry demand ever more sensitive measurements on ever smaller systems, as exemplified by spintronics, nanoelectromechanical system, and spin-based quantum information processing, where single electronic spin detection poses a grand challenge. Superconducting quantum interference devices (SQUIDs) have yet to be effectively applied to nanoscale measurements. Here, we show that a simple bilayer deposition route, combining photolithography with focused ion beam patterning, produces high performance nanoscale SQUIDs. We present results of noise measurements on these nanoSQUIDs which correspond to a magnetic flux sensitivity of around 0.2μΦ0∕Hz1∕2. This represents one of the lowest noise values achieved for a SQUID device operating above 1K.