An international team of researchers has shown how a heterostructure consisting of superconductors and magnets can be used to create a current that only flows in one direction, like that found in semiconductor diodes. In their study supported by the EU-funded SUPERTED, TERASEC, SuperGate and SuperCONtacts projects, the scientists demonstrated that the novel superconductor diodes operate at significantly lower temperatures than their semiconductor counterparts, making them useful in quantum technologies. Diodes are electronic components that allow an electric current to flow only in one direction, while blocking it in the reverse direction. They can be found in many devices we use today, such as radio receivers, temperature sensors and photovoltaic cells. Such diodes rely on the electronic properties of semiconductor systems. However, because of their large energy gap, semiconductors do not work at the extremely low sub-Kelvin temperatures required for cryogenic electronics and ultrasensitive detection, in other words, for future quantum technologies. The energy gap of a semiconductor is the minimum energy needed to excite an electron stuck in its bound state (that is, bound to an atom) into a free state, so that it can conduct an electric current. Compared to semiconductors, superconductors have a smaller energy gap. This, and their intrinsically low impedance, makes superconductors ideal candidates for use in cryogenic diodes.
Breaking the symmetry
However, as reported in a news item posted on ‘Mirage News’, whereas scientists have known about the existence of such a gap for decades, they had not previously observed “the diode-like feature.” This is because creating a superconducting diode requires breaking something called the electron-hole symmetry – the usually robust symmetry characterising the current-voltage characteristics of the superconductor’s metal contact. The researchers now show how this symmetry can be broken using a ferromagnetic insulator suitably placed in the junction. This paves the way for quantum technologies based on superconducting materials operating at ultra-low temperatures. “I believe this finding is promising for several tasks in quantum technology, such as current rectification or current limiting,” observes study senior author Dr Francesco Giazotto of TERASEC and SuperCONtacts projects’ host and coordinator and SUPERTED and SuperGate projects’ partner Consiglio Nazionale delle Ricerche (CNR), Italy. Dr Giazotto is a researcher at CNR’s Istituto Nanoscienze (NANO). First author Dr Elia Strambini, also of CNR NANO, describes discovering the diode functionality as “a pleasant surprise, a consequence of the thorough characterization of SUPERTED samples.” The research scientist was the one who made the initial discovery. “This finding showed the power of collaboration between different types of researchers, from materials science to superconducting electronics and theory,” remarks co-author Prof. Tero Heikkilä of SUPERTED (Thermoelectric detector based on superconductor-ferromagnet heterostructures) project coordinator University of Jyväskylä, Finland. “Without European support such collaboration would not take place.” Although mainly supported by SUPERTED, the research was also partially funded by TERASEC (THz imaging technology for public security), SuperGate (Gate Tuneable Superconducting Quantum Electronics.) and SuperCONtacts (Solid state diffusion for atomically sharp interfaces in semiconductor-superconductor hybrid structures). The study has been published in the journal ‘Nature Communications’. For more information, please see: SUPERTED project website TERASEC project SuperGate project SuperCONtacts project
SUPERTED, TERASEC, SuperGate, SuperCONtacts, superconductor, diode, quantum, semiconductor, current