Supercurrent Diode Effect in High Mobility Free-Standing InSb Nanoflags

High-quality III–V narrow bandgap semiconductor materials with strong spin–orbit coupling and large Landé g-factor provide a promising platform for next-generation applications in the field of high-speed electronics, spintronics, and quantum computing. InSb offers a narrow bandgap, high carrier mobility, and small effective mass and, thus, is very appealing in this context. In fact, this material has attracted tremendous attention in recent years for the implementation of topological superconducting states. An attractive pathway to obtain two-dimensional (2D) InSb layers is the growth of freestanding single-crystalline InSb nanoflags. We have demonstrated fabrication of ballistic Josephson-junction devices based on these InSb nanoflags with Ti/Nb contacts that show a gate-tunable proximity-induced supercurrent and a sizable excess current. The devices show clear signatures of subharmonic gap structures, indicating phase-coherent transport in the junction and a high transparency of the interfaces. In an applied in-plane magnetic field, the device is driven into a non-reciprocal transport regime, where we observe an asymmetry between the critical current in opposite directions, modulated by the angle between in-plane field and current direction. This supercurrent diode effect is promising for superconducting electronic circuits and places InSb nanoflags in the spotlight as a versatile and convenient 2D platform for advanced quantum technologies.

Figure 1: Behavior of the Josephson dode effect with in-plane magnetic field and perpendicular component of the in-plane field. Left: Asymmetry in switching current ΔI_sw versus in-plane magnetic field for different orientations of the devices. The angle θ is measured between the direction of the magnetic field and the current direction. Right: Asymmetry versus the component of the magnetic field perpendicular to the current flow. The maximum of ΔI_sw is observed for B_ip,⟂ = -6 mT for each curve. The amplitude of the effect is maximum when θ is close to ± 90°, i.e., when the in–plane magnetic field is perpendicular to the current vector. Note that the polarity of the curve at θ = -152° on the right is reversed with respect to left due to the sign of sin(θ). The curves are shifted by 5 nA each for clarity.

We use the difference in the switching currents ΔI_sw = I_sw⁺ – |I_sw^–| to quantify the Josephson Diode Effect (JDE). The dependence of ΔI_sw on magnetic field is presented in Figure 1. The curves are obtained as switching current differences between two consecutive bias sweeps, one in the positive direction, the other in the negative direction. To consider the asymmetry beyond the fluctuations due to stochastic switching, we performed a gentle smoothing procedure. The experiment was repeated for different relative orientations of the magnetic field, to collect information about the angular dependence of the JDE. The first information to note from Figure 1 is that all measurements show antisymmetric curves. Furthermore, the curve for θ = -152° is flipped with respect to the others, so that the sign of the point in magnetic field of maximum value in ΔI_sw is opposite. This is consistent with the different orientations of the devices with respect to the field direction, i.e., the polarity of the ΔI_sw curves reflects the sign of the angle θ. Secondly, we observe that ΔI_sw varies smoothly from a linear regime around zero field via a smooth rounded maximum at intermediate field values to the high magnetic field region, in which the effect is completely suppressed. To highlight the linear regime around zero field, we have added linear best fits at the origin of each curve. A similar and consistent behavior was also observed for the retrappig current.

To disentangle the contributions of the parallel and perpendicular (B_ip,⟂) components of the field, computed with respect to the direction of current flow, we mapped the measured data on the effective B_ip,⟂. This is shown in Figure 1 in the right panel. Note that, in case of θ < 0, the change in polarity is due to sign of sin(θ). In all data sets, the maximum value of the asymmetry is observed for B_ip,⟂ = -6 mT, while the magnitude of the effect depends on the specific orientation. This shows that the main contribution to the effect is given by the perpendicular component of the field.

Publications:

1. Bianca Turini: Josephson diode effect in high-mobility InSb nanoflags, Master Thesis, University of Pisa, Italy, 2022.
2. Bianca Turini, Sedighe Salimian, Matteo Carrega, Andrea Iorio, Elia Strambini, Francesco Giazotto, Valentina Zannier, Lucia Sorba, Stefan Heun: Josephson Diode Effect in High Mobility InSb Nanoflags, arXiv:2207.08772 [cond-mat.supr-con].
3. B. Turini, S. Salimian, M. Carrega, A. Iorio, E. Strambini, F. Giazotto, V. Zannier, L. Sorba, and S. Heun: Josephson diode effect in high-mobility InSb nanoflags, Nano Letters 22 (2022) 8502 – 8508.

Presented at:

1. Stefan Heun, Bianca Turini, Andrea Iorio, Sedighe Salimian, Matteo Carrega, Elia Strambini, Francesco Giazotto, Valentina Zannier, and Lucia Sorba: Supercurrent Diode Effect in High Mobility Free-Standing InSb Nanoflags, SuperTop Workshop, Budapest, Hungary, 9-11 May 2022 (invited). [Abstract] [Talk]
2. Bianca Turini, Andrea Iorio, Sedighe Salimian, Matteo Carrega, Elia Strambini, Francesco Giazotto, Valentina Zannier, Lucia Sorba, and Stefan Heun: Unconventional superconductivity in InSb nanoflag-based planar Josephson junctions, International School of Physics “Enrico Fermi” – Quantum Fluids of Light and Matter, Varenna, Lake Como, Italy, 2 – 7 July 2022. [Poster]
3. Bianca Turini: Josephson diode effect in high-mobility InSb nanoflags, ICFO, Barcelona, Spain, 1 August 2022. [Talk]
4. Bianca Turini: Josephson Diode Effect in High-Mobility InSb Nanoflags, Master thesis defense, University of Pisa, Italy, 30 September 2022. [Abstract] [Talk]
5. Stefan Heun: Josephson Diode Effect in High-Mobility InSb Nanoflags, Super Gate Project Meeting, Pisa, Italy, 5-6 October 2022. [Talk]
6. Stefan Heun: Josephson Diode Effect in High-Mobility InSb Nanoflags, Seminar at Scuola Normale Superiore, Pisa, Italy, 26 October 2022. [Talk]
7. B. Turini, S. Salimian, M. Carrega, A. Iorio, E. Strambini, F. Giazotto, V. Zannier, L. Sorba, and S. Heun: Josephson Diode Effect in High-Mobility InSb Nanoflags, CMD 30 Fismat 2023, Milan, Italy, September 4 – 8, 2023. [Abstract] [Talk]
8. Bianca Turini: Josephson Diode Effect in High-Mobility InSb Nanoflags, ‘Licenza’ Seminar at Scuola Normale Superiore, Pisa, Italy, 29 November 2023. [Talk]