邀请人: 沈洁 研究员

报告摘要:
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 stands out due to its narrow bandgap, high carrier mobility, and small effective mass, making it 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 [1]. We demonstrated fabrication of ballistic Josephson-junction devices based on these InSb nanoflags with Nb contacts that show gate-tunable proximity-induced supercurrent and sizable excess current [2]. The devices show clear signatures of subharmonic gap structures, indicating phase-coherent transport in the junction and a high transparency of the interfaces. The high quality of the devices enabled the observation of the Josephson diode effect [3]. When subjected to an in-plane magnetic field, the devices enter a non-reciprocal transport regime,  manifesting an asymmetry between positive and negative critical currents. The degree of asymmetry varies with the angle between the in-plane field and the current direction, exhibiting a pronounced temperature dependence. Our data demonstrate that these devices can work as Josephson diodes, with dissipation–less current flowing in only one direction. Furthermore, we investigated superconducting quantum interference devices (SQUIDs) based on InSb nanoflag Josephson junctions [4]. We measured interference patterns in both symmetric and asymmetric geometries. The interference patterns in both configurations can be modulated by a back gate voltage, a feature well reproduced through numerical simulations. The observed behavior aligns with the skewed current-phase relations of the Josephson junctions, demonstrating significant contributions from higher harmonics.Finally, we report Scanning Gate Microscopy (SGM) experiments on these InSb nanoflag-based Josephson junctions [5]. Our results demonstrate the possibility of manipulating the supercurrent flow across a semiconductor-superconductor heterostructure at a local level. The experimental findings are consistent with theoretical predictions and establish a way of investigating the behavior of superconducting weak links. This raises the perspective of locally imaging the supercurrent flow. Our results demonstrate the potential of ballistic InSb nanoflag Josephson junctions as a valuable platform for understanding the physics of hybrid devices and investigating their nonequilibrium dynamics.

报告人介绍:
Stefan Heun is Research Director at the Istituto Nanoscienze of the Italian National Research Council CNR. He received his diploma in physics at the University of Hannover (Germany), where he investigated in the group of Prof. M. Henzler the initial stages of epitaxial growth of silicon on Si(100). In 1993 he received his Ph.D. in the same group, working on the magneto conductivity of monolayer-thin films of Ag, Pb, and Au deposited on Si(111). In 1993, he joined the Interdisciplinary Research Laboratories of the Japanese Telecom NTT in Tokyo (Japan), where he studied the surface passivation of III-V semiconductors using synchrotron radiation. In 1995, he moved as a Marie Curie Fellow to the Materials Division of the TASC (now IOM) lab in Trieste (Italy), where he worked on the engineering of heterostructure interfaces.In 1997, he became a beam line scientist at the Sincrotrone Trieste, where he was appointed responsible of the Nanospectroscopy Beamline, at which he also performed his research on lowdimensional systems.Finally, in 2004 he joined the CNR, first in Trieste and since October 2006 at Istituto Nanoscienze in Pisa (Italy). Recently, his work focuses on the study of two-dimensional semiconductors and graphene nanostructures, using low-temperature magneto-transport and scanning probe microscopies, in particular scanning gate microscopy (SGM) and scanning tunneling microscopy (STM).Stefan Heun has co-authored more than 210 papers and has given more than 130 invited presentations and seminars. To date (October 2026) his papers have been cited over 4700 times. H-index: 35. He is Adjunct Professor at Scuola Normale Superiore in Pisa and at the University of Pisa, where heteaches on Nanostructured Materials.