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Stepping up with Josephson junctions
Cnr Nano researchers observed half-integer Shapiro steps in highly transmissive InSb nanoflag Josephson junctions. This intriguing finding suggests the presence of new and unexplored phenomena within such devices.

A team of researchers from Cnr Nano conducted a study on an InSb nanoflag-based Josephson junction featuring Nb superconducting contacts. Their investigation led to the observation of half-integer Shapiro steps, which are voltage plateaus occurring in Josephson junctions when they are subjected to microwave radiation. Traditionally, these plateaus are found at integer values of the Josephson frequency, but this study’s breakthrough reveals their occurrence at half-integer values, which was unexpected. This intriguing finding suggests the presence of new and unexplored phenomena within these devices.


The results of the research, published in an article in the journal Physical Review Research, were obtained by a team of researchers from the NEST laboratory of Cnr Nano and Scuola Normale Superiore, led by Stefan Heun and Francesco Giazotto, with Andrea Iorio as the first author.


The study focuses on quantum transport in hybrid superconductor-semiconductor devices. “These devices consist of both superconducting and semiconducting materials and operate on a nanometer to micrometer scale. By cooling them to near absolute zero temperature, they exhibit intriguing quantum mechanical behavior, which can be investigated by means of electrical measurements. This area of research at the forefront of physics holds great promise for various quantum technologies” explains Andrea Iorio.


The study stands out in several ways, researchers say. “Firstly, it leveraged a relatively new and distinctive material platform, the InSb nanoflag. Secondly, it marks the first time Shapiro steps have been observed in a Josephson junction composed of this particular material. Thirdly, the study unveiled an unusual half-integer relationship. However, understanding the origin of this unique relation calls for further investigation”, says Stefan Heun.


The research holds implications for quantum devices based on hybrid superconductor-semiconductor systems. One potential application lies in the development of novel qubits, like Andreev qubits, which are integral to the advancement of quantum computers. Additionally, the study offers valuable insights into the limiting factors affecting these devices, contributing to their overall understanding and optimization.


Moreover, the InSb nanoflag material exhibits intriguing exotic quantum properties, and this research showcases its potential for future applications.


Beyond quantum computing, the study’s findings have broader applications, particularly in metrology. The use of Shapiro steps, already employed for precise voltage standards, could be further enhanced by the research’s discoveries.


The work comprised multiple phases. Initially, the InSb nanoflags were grown using an advanced Chemical Beam Epitaxy facility. Subsequently, the team fabricated the Josephson junction in the clean room of Laboratorio NEST. The device was then cooled to a temperature near absolute zero. Finally, the experiment was conducted, and data was collected. The team utilized state of the art instrumentation of the SQEL group led by Francesco Giazotto, including a cryogenic dilution refrigerator and advanced electronic tools, which facilitated the successful completion of their research.


The team, composed by Andrea Iorio, Alessandro Crippa, Bianca Turini, Sedighe Salimian, Luca Chirolli, Valentina Zannier, Lucia Sorba, Elia Strambini, Francesco Giazotto, and Stefan Heun from Cnr Nano and by Matteo Carrega from Cnr Spin Institute, has just been awarded a national PRIN grant to further their research. with a focus on in-depth studying InSb nanoflags and exploring new quantum computing applications, like Andreev qubits.


This research received support from the AndQC project (https://www.andqc.eu/).



Iorio, A. Crippa, B. Turini, S. Salimian, M. Carrega, L. Chirolli, V. Zannier, L. Sorba, E. Strambini, F. Giazotto, and S. Heun, Half-integer Shapiro steps in highly transmissive InSb nanoflag Josephson junctions, Phys. Rev. Research 5, 033015 – Published 7 July 2023. https://doi.org/10.1103/PhysRevResearch.5.033015

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