Raphael Thys
A superconducting nanowire (blue) with three gate electrodes (red) placed on a silicon substrate (gray). Application of a gate voltage to the electrode Vg1 results in a transition of the nanowire from superconducting to resistive state. Application of a voltage difference between the two remote electrodes Vg2 and Vg3 results in a similar effect, but mediated by the silicon substrate. On the top right: the critical current (large means superconducting, zero means resistive) of the nanowire as a function of Vg1. At Vg1=0 the nanowire is superconducting, at Vg1<-5V and Vg1>5V the nanowire is resistive. On the bottom right: the current flowing from the gate Vg1 to the nanowire in a logarithmic scale. The flat region around Vg1=0 is the noise floor of our measurement. Credit: IBM, Ritter et al (2022)
Superconductors are materials that can enter a state of no electrical resistance, through which magnetic fields cannot penetrate. Due to their interesting properties, many material scientists and engineers have been exploring the potential of these materials for a wide range of electronics applications.
A key advantage of superconductors is that they can transport electrical signals while preventing their dissipation, which is particularly useful when developing quantum computers. Controlling their states, as is commonly done with semiconductor technology, however, has so far proved to be challenging.
A few years ago, a study suggested the superconductivity of superconducting materials could be switched on and off. Researchers at IBM Research in Zurich have been investigating these results further, in the hope of explaining the switching mechanism unveiled by this previous study. Their findings were recently outlined in a paper published in Nature Electronics.
"Superconductors are, first of all, metals, and metals screen external electric fields very effectively," Andreas Fuhrer and Fabrizio Nichele, two of the researchers who carried out the study, told Phys.org. "This fundamental concept, found in all physics textbooks, was put into question by a 2018 publication. In that work, the authors claimed to have turned on and off the superconductivity in a titanium nanowire via moderate electric fields applied by a nearby gate electrode."
If confirmed, the findings gathered in 2018 by NEST and SPIN-CNR in Italy would enable the development of entirely new types of electronic and quantum computing devices based on superconductors. A few years ago, they thus set out to unveil the microscopic, physical mechanism occurring in nanometer-sized superconductors when electric fields are present.
In an initial paper published in 2021, the researchers outlined some initial hints about the possible origin of the observed suppressed superconductivity in titanium nanowires. Their new study builds on this paper, offering a more detailed explanation for the findings gathered by the team at NEST and SPIN-CNR.
"Our previous work showed that the suppression of superconductivity always went hand in hand with small leakage currents flowing from the gate electrode to the nanowire," Fuhrer and Nichele explained. "Such currents were very small (a few pA or 0.000,000,000,001 Ampere), so that they might have gone unnoticed in previous work. For us, it was reasonable to assume that such a current would be responsible for disrupting superconductivity, as the energy of each electron carried by the current was quite large (about 100,000 larger than the binding energy keeping the electrons in a metal in the superconducting state)."
While their previous study allowed Fuhrer, Nichele and their colleagues to get a sense of the possible mechanism underpinning the observed suppression of superconductivity, it still lacked a number of key ingredients. The key goal of their recent paper was to offer a solid and satisfactory explanation for the phenomenon.
"Our new experiments are completely consistent with our first work, in the sense that we show again that currents leaking from the gates (not electric fields) are needed to suppress superconductivity in metallic nanowires," Fuhrer and Nichele said. "However, we now also showed that the current does not have to flow necessarily from the gate to the nanowire."
A device similar to that presented above, but with a 500 nm deep trench in the substrate. The trench shields the nanowire from the phonons. Credit: IBM, Ritter et al (2022)
The researchers attained similar results when the current of high-energy electrons flowed out of the wire and when it flowed between two electrodes placed in the vicinity of the nanowire (without any electrons reaching the nanowire itself). These results highlight the crucial role of the material's substrate in the suppression of superconductivity.
The devices that the researchers used in their experiments are based on a crystalline silicon wafer. This is the substrate where the currents of high energy electrons flow when high voltages are applied between electrodes.
"As electrons, accelerated to high energy by the large voltages, move in the silicon, they kick silicon atoms continuously, transferring their energy to vibrations in the crystal lattice (what physicists call 'phonons')," Fuhrer and Nichele explained. "Differently from electrons, phonons travel very long distances in the silicon lattice (several micrometers) and can easily perturb the superconducting state in the metallic nanowire."
The recent work by Fuhrer, Nichele, and their colleagues shows that, in contrast with photons, phonons act as mediators. Based on this finding, the team created a switching device that consists of a deep trench etched into a silicon substrate.
"The trench reflects the phonons generated on one side and shields the nanowire, which persists longer in the superconducting state," Fuhrer and Nichele said. "Vibrations are always present in a crystal, the higher the temperature the more the crystal vibrates. However, the phonons we produce in our devices have totally different energies than those resulting from a temperature increase."
When the researchers carried out their experiments at temperatures below 4 Kelvin, they found that the photons produced had a temperature of above 100 Kelvin. This finding explains why switching devices like the one they developed have very low power-requirements compared to more conventional switches.
Overall, the recent work by Fuhrer, Nichele and their colleagues at IBM Research offers a coherent and convincing explanation for the experimental results published by the team at NEST and SPIN-CNR in 2018, which were previously unexplained. In the future, their explanation could help to understand superconductors further, potentially enabling their use for the development of new types of devices.
"Our study also contributes to a new generation of superconducting devices where a metallic element can be switched from superconducting to resistive in a very fast and power-efficient manner," Fuhrer and Nichele said. "This might find immediate application in the field of quantum computation, for example in the area that involves the control electronics interfacing quantum bits to classical computers."
In their paper, Fuhrer, Nichele and their colleagues also introduced an approach to generate high-energy electrons and phonons on demand. High-energy particles, such as cosmic rays that strike the Earth from outer space, are known to adversely impact the functioning of quantum computers. In the future, therefore, their approach could also be used to study the effects of high-energy excitations on quantum technology further.
"Our main activity is the realization of quantum bits," added. "In our next papers, we would like to combine our switching element with a qubit and investigate how close the switch can be placed so that new functionalities are introduced without the drawbacks associated to phonons."
More information: M. F. Ritter et al, Out-of-equilibrium phonons in gated superconducting switches, Nature Electronics (2022). DOI: 10.1038/s41928-022-00721-1
Giorgio De Simoni et al, Metallic supercurrent field-effect transistor, Nature Nanotechnology (2018). DOI: 10.1038/s41565-018-0190-3
M. F. Ritter et al, A superconducting switch actuated by injection of high-energy electrons, Nature Communications (2021). DOI: 10.1038/s41467-021-21231-2
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