Long-range crossed Andreev reflection in a topological insulator nanowire proximitized by a superconductor
Crossed Andreev reflection is a non-local transport phenomenon that creates and detects Cooper pair correlations between distant locations. It is also the basis of Cooper pair splitting to generate remote entanglement. Although crossed Andreev reflection has been extensively studied in semiconductors proximity-coupled to a superconductor, observing it in a topological insulator has been very difficult. In this work, we report the observation of this effect in a proximitized topological insulator nanowire. We perform local and non-local conductance spectroscopy on mesoscopic devices in which superconducting niobium and metallic contacts are connected to a bulk-insulating nanowire. In our local conductance measurements we detect a hard gap and the appearance of Andreev bound states that can reach zero bias. We also occasionally observe a negative non-local conductance when sweeping the chemical potential, providing evidence of crossed Andreev reflection. This signal is detected even over length scales much longer than the expected superconducting coherence length of either niobium or the proximitized nanowire. We suggest that this long-range effect is due to the intricate role of disorder in proximitized nanowires.
Individual Assembly of Radical Molecules on Superconductors: Demonstrating Quantum Spin Behavior and Bistable Charge Rearrangement
Magnetic impurities on superconductors present a viable platform for building advanced applications in quantum technologies. In this work, we show the manipulation of magnetic states in the radical molecule 4,5,9,10-tetrabromo-1,3,6,8-tetraazapyrene on a Pb(111) superconducting surface using low-temperature scanning tunneling microscopy. Tunneling spectra reveal Yu-Shiba-Rusinov (YSR) states near the Fermi energy in isolated molecules. The presence of a second TBTAP molecule allows tuning of the YSR state position by altering the relative distance and can induce splitting of the YSR states for certain orientations. The construction of molecular chains up to pentamers shows periodic arrangements of charged and neutral molecules, with even-numbered chains forming a charged dimer structure at one end. Information can be encoded in these chains by switching the dimer position. These findings elucidate interactions between molecular assemblies and superconducting substrates, paving the way for advanced quantum-state engineering.
Spin excitations of high spin iron(II) in metal–organic chains on metal and superconductor
Using surface coordination chemistry with pyrene-4,5,9,10-tetraone (PTO) precursors as ligands, Jung-Ching Liu and coworkers have demonstrated the synthesis of an iron-based spin chain on Ag(111) and Pb(111). Low temperature tunneling spectroscopy (white spectrum) shows low-energy spin excitations (orange arrows) from coordinated Fe atoms with high S = 2 spin-state.
On-surface synthesis and characterization of radical spins in kagome graphene
Flat bands in Kagome graphene might host strong electron correlations and frustrated magnetism upon electronic doping. However, the porous nature of Kagome graphene opens a semiconducting gap due to quantum confinement, preventing its fine-tuning by electrostatic gates. Here we induce zero-energy states into a semiconducting Kagome graphene by inserting π- radicals at selected locations. We utilize the on-surface reaction of tribromotrioxoazatriangulene molecules to synthesize carbonyl-functionalized Kagome graphene on Au(111), thereafter modified in situ by exposure to atomic hydrogen. Atomic force microscopy and tunneling spectroscopy unveil the stepwise chemical transformation of the carbonyl groups into radicals, which creates local magnetic defects of spin state S = 1/2 and zero-energy states as confirmed by density functional theory.
