Scientists at MIT, along with their colleagues, have developed a simple superconducting device that could significantly improve the efficiency of current transfer in electronic devices. This new diode, acting as a switch, has the potential to substantially reduce energy consumption in high-power computing systems, which is a critical issue expected to worsen in the future.
Despite being in the early stages of development, this diode already outperforms similar ones created by others, boasting over twice the efficiency. Its capabilities could even be vital for the advancement of emerging quantum computing technologies. The research, featured in the July 13 online edition of Physical Review Letters and Physics Magazine, has garnered attention for its significant engineering achievement.
Philip Moll, the Director of the Max Planck Institute for the Structure and Dynamics of Matter, praised the work, stating that the team achieved remarkable efficiencies without even optimizing their structures yet.
Jagadeesh Moodera, leader of the project and a senior research scientist at MIT’s Department of Physics, emphasized the diode’s robustness and its ability to function across a wide temperature range. He believes that their creation has the potential to pave the way for groundbreaking technologies.
The nanoscopic rectangular diode is incredibly thin, approximately 1,000 times thinner than a human hair, and can be easily scaled up. It is feasible to produce millions of these diodes on a single silicon wafer.
Toward a superconducting switch
Diodes are essential components in computing systems, allowing current to flow in one direction while blocking it in the reverse direction. In modern computer chips, there are billions of diode-like transistors, but they can generate excessive heat due to electrical resistance, necessitating significant energy for cooling in data centers supporting technologies like cloud computing.
According to a 2018 Nature news feature, these systems could consume nearly 20% of the world’s power within a decade.
To address this energy challenge, researchers have been exploring diodes made of superconductors. Superconductors carry current with zero resistance below a critical temperature, making them much more efficient than semiconductors, which lose energy as heat.
The approach taken by MIT scientists in creating superconducting diodes is refreshingly simple, relying on a fundamental property of superconductors. It is a departure from more complex physics-based solutions explored previously.
Philip Moll from the Max Planck Institute highlights that the significance of this work lies in revealing that superconducting diodes can be common and widespread, arising from certain broken symmetries in classical materials, rather than being tied to exotic physics like finite-momentum pairing states.
A somewhat serendipitous discovery
In 2020, Moodera and his team made a remarkable discovery by observing exotic particle pairs called Majorana fermions. These pairs hold promise for developing a new type of topological qubits, which are fundamental to quantum computers. As they explored ways to create superconducting diodes, they realized that the material platform used for studying Majorana fermions could also be adapted for solving the diode problem.
Their intuition proved correct. Utilizing the versatile platform, they designed various versions of superconducting diodes, each more efficient than the last. The first iteration involved a nanoscopically thin layer of vanadium, a superconductor, patterned into a typical electronic structure known as the Hall bar. When applying a small magnetic field comparable to the Earth’s magnetic field, they observed the diode effect—a significant polarity-dependent current flow.
Building upon this success, they developed another diode by combining a superconductor with a ferromagnet, specifically a ferromagnetic insulator in their case. The ferromagnet generates its own tiny magnetic field, and by applying a slight magnetic field to magnetize it, they achieved an even larger diode effect that remained stable even after turning off the initial magnetic field.
Continuing their investigation, the team delved into the underlying mechanisms of their superconducting diodes. Besides their well-known zero resistance, superconductors possess another important property: the Meissner screening effect, which shields them from external magnetic fields by generating an internal supercurrent. This effect is akin to our immune system fighting off bacteria and pathogens. However, it has its limits, as large magnetic fields can’t be entirely kept out.
The diodes crafted by the team harnessed this Meissner screening effect. The minute magnetic field they applied, directly or through the adjacent ferromagnetic layer, triggered the material’s screening current, maintaining superconductivity by expelling the external magnetic field.
Moreover, the researchers discovered that tiny differences in the diode’s edges played a crucial role. By engineering specific edge asymmetries, such as sawtooth features on one side and no alterations on the other, they boosted efficiency from 20% to over 50%. This finding opens up possibilities for “tuning” diode edges for even higher efficiencies.
Overall, the interplay of edge asymmetries, the Meissner screening effect, and another property called vortex pinning led to the observed diode effect. The work offers a straightforward approach with immense potential to further enhance efficiency.
Christoph Strunk, a professor at the University of Regensburg, praised the research, noting the ability to achieve non-reciprocal supercurrent in simple strips and maintain the diode effect even without an external magnetic field when combined with a ferromagnetic insulator.
Remarkably, two high school researchers, Ourania Glezakou-Elbert and Amith Varambally, who worked in Moodera’s lab during the summer, played a significant role in creating the engineered edges, highlighting the accessibility and potential impact of this work.