Super SQUID

Superconductivity is a quantum phenomenon, which is ultra-cool – literally. It is only when certain materials are cooled to extremely low temperatures that the “magic” starts to happen: they lose all resistance to the flow of electricity and expel the magnetic fields within them. These bizarre properties have enabled trains to “float” in air, particles to be steered at nearly the speed of light inside particle accelerators, as well as their use in MRI scanners for diagnostic imaging of the human body, to name a few. And, as it happens, superconducting materials are also used in the fabrication of the very SQUIDs used to measure superconducting properties – as evidenced by their name: Superconducting QUantum Interference Device. Although discovered over 100 years ago, scientists still do not fully understand the physics that underlies the behavior of superconductors.

As opposed to traditional optical microscopy that uses light and lenses to magnify images of small samples, scanning probe microscopy is a technique that uses a probe – in this case, a nano-SQUID – to scan and measure some property (e.g., magnetic field) at different points of a sample, forming an image of the entire surface – a bit like creating a heat map of a hand by taking a thermometer and measuring the temperature at individual points on the hand.

Apart from having very sensitive SQUIDs, there are also geometrical challenges when it comes to using them as scanning probes: They need to be as small as possible to attain the highest image resolution, and they need to get as close as possible to the sample to enable the imaging of ever smaller magnetic features. Postdoctoral fellows Yonathan Anahory and Denis Vasyukov, and Ph.D. student Lior Embon, along with other colleagues in the lab of Prof. Eli Zeldov of the Condensed Matter Physics Department, have risen to the challenge – as reported in Nature Nanotechnology – thanks to a unique setup: They took a hollow quartz tube and pulled it into a very sharp point – the ideal geometry for a scanning probe microscope. They then succeeded in fabricating a SQUID encircling the ring of the tip, which measures a mere 46 nm in diameter, making it the smallest SQUID to date. They proceeded in gluing the tube to a quartz tuning fork and constructing a scanning microscope, which has enabled them to achieve magnetic imaging at distances as small as a few nanometers from the sample. In contrast, current SQUIDs are usually made using lithography on flat silicon chips, limiting their size and their ability to get very close to the surface. “In fact, we have the opposite problem of having to prevent the probe from ‘crashing’ into the sample,” says Embon. “While there exist SQUIDs with higher sensitivities to uniform magnetic fields, it is the combination of high sensitivity, proximity of the probe to the sample and its minute dimension that makes the overall resolution, accuracy and sensitivity of the device record-breaking.” So much so that if the right conditions can be achieved, this so-called nano-SQUID-on-tip is heralded to hold the potential to measure the magnetic field due to the spin of a single electron – the Holy Grail of magnetic imaging.

This novel instrument is already proving a powerful tool: Zeldov's lab is currently using the device to investigate vortex dynamics and quantum magnetism on the nanoscale. This will hopefully not only lead to a better understanding of superconductivity and of vortex flow that is required for the effective application of superconductor technology, but it will aid in gaining insights into novel physics phenomena. As a surprising, added bonus, the new SQUID has turned out to be so versatile, it is able to measure many materials other than superconductors. Embon: “Queues are already being formed by scientists from both Weizmann and abroad in order to study the nanoscale magnetic properties of their samples.”

 

Prof. Eli Zeldov is the incumbent of the David and Inez Myers Professorial Chair.