November 07, 2019 by Alessandro Mascellino
École Polytechnique Fédérale de Lausanne (EPFL) has revealed a metallic microdevice that allows for superconductivity patterns to be defined and tuned by varying strain on the material.
The new research was published in Science earlier this month and demonstrates disorder-free control on the micrometre scale, particularly over the superconducting state in samples of the heavy-fermion superconductor CeIrIn5.
Superconductivity is the set of physical properties related to certain materials, namely those that allow electrical resistance to vanish from its surface—causing magnetic flux fields to be expelled.
Materials become superconductors at a slightly different temperature (known as its critical temperature, aka its Tc), but only within a few degrees of absolute zero.
At the moment, zero-resistance superconductivity materials are mostly used in magnetic resonance imaging machines, mass spectrometers, particle accelerators, and plasma-confining magnets in some tokamaks (i.e. toroidal apparatus used for producing controlled fusion reactions in hot plasma).
The superconductors used in these machines are traditionally robust and hard to influence; however, in the near future, the necessity of higher-speed calculations in quantum computers will increasingly demand superconducting materials that, conversely, can be easily manipulated.
Tuning and defining superconductivity demonstrated in a material. Image Credit: École Polytechnique Fédérale de Lausanne.
As touched on, CeIrIn5, the said superconductor mentioned in the EPFL study belongs to a specific group of unconventional superconductors called ‘heavy fermion materials’.
Such metal super-conducts at 0.4°C above absolute zero (around −273°C), and according to the new findings, it could be produced with superconducting regions that coexist alongside regions in a normal metallic state.
The team, led by Dr Maja D. Bachmann (who was the first PhD student to graduate from EPFL’s Laboratory of Quantum Materials), tested the above by creating a model that allows researchers to design complex conducting patterns and, by manually varying the temperature, manipulate those patterns within the material (and in a highly controlled way).
As Dr. Toni Helm, who contributed to the study told Electronic Point:
“It all started with a relatively old long-standing puzzle about how this material can exhibit different critical superconducting temperatures for different current directions.”
The team’s preliminary studies revealed that bulk crystals cut along different directions with the conventional, rather rough “first-polishing-then wire-saw-cutting” technique had Case temperature different by a factor of two from each other.
Following this discovery, the researchers sliced thin layers of CeIrIn5 to then join them to a sapphire substrate.
This way, when the microdevice was exposed to cold temperatures. the material contracted a great deal, whereas the sapphire contracted very little—putting stress on the material in all directions and thus slightly distorting the atomic bonds between the two.
“This was unexplainable with existing knowledge about the superconducting order parameter in CeIrIn5,” Dr. Helm admits. “We, however, knew that the response of Tc for the two perpendicular directions was exactly opposite, but had now proof for this to be the driving force in our devices.”
To test the new findings, after the bulk sample had been prepared and characterised, the researchers threw them into acid in order to gently etch the surfaces between the contacts. “A new measurement of the very same, now-etched device showed that the two current directions suddenly exhibited the same Tc.”
Temperature evolution of the spatially-modulated superconducting state. Image Credit: Ecole Polytechnique Fédérale de Lausanne.
To further the scope of their experiments, the team at EPFL then contacted scientists who may be able to resolve the spatial distribution of SC in their micron-scale devices.
“Katja Novack’s group [from Cornell University] provided us with the key experiment, scanning squid, which was able to do exactly that,” Dr. Helm explains. “Not only were they able to resolve the SC locally and provide proof for the anisotropic Tc distribution, [but] our simulations can also exactly predict this for any desired device design with unprecedented accuracy.”
“We [were] able to induce different states of matter in different sections of the device without the need of introducing doping that comes with impurities,” Dr Helm told Electronics Point.
According to Dr Helm, this would represent a notable advance in the field of condensed matter research, where studies of complex phase diagrams (of a variety of topical materials) are a fundamental task that is required to understand and control new materials and physical properties
“This approach may be helpful to understand and think about circuitry made from all kinds of exciting materials that so far had belonged to the category ‘exciting but too complex for applications’, such as high Tc cuprate or pnictide materials,” Dr Helm says.
A figure displaying the chemical structure of the CeIrIn5 heavy fermion material. Image Credit: Journal of the Physical Society of Japan.
Helm also believes the new findings will allow researchers to design superconducting circuitry on a single crystal bar that will also inevitably prove fundamental in developing quantum applications.
“For the field of Quantum computing, this may hold the key for SC-based Q-bit infrastructure working at much higher temperatures and critical currents or speed levels. For example, Josephson junctions made from all kinds of exciting superconductors—based on non-local SC distributions induced by strain—are absolutely possible with those results,” he explains.
While the new findings represent a major step forward in controlling superconductivity in heavy fermion materials for quantum computers, their implications might be on a larger scale still for electronics engineering.“Essentially, the research has shown that the technical limitations of applying strain on a material can be exceeded by several orders of magnitude once you translate those approached into the micro or nano regime,” Dr Helm says.
Ultimately, this could altogether revolutionise many engineering applications—both in research and across the industry.
