Design of low-dimensional electron spin structures on superconductors
The molQ centre aims the experimental realization of topological superconductors and topological qubits using radical molecules adsorbed on a superconducting substrate. Radical molecules are designed in collaboration with the University of Bern and characterized at the University of Basel by low temperature scanning tunnleing microscopy (STM) and atomic force microscopy (AFM).
We use the toolbox of suface reations well-established at metallic surfaces (i.e; supramolecular chemistry, coordination chemistry and on-surface chemistry) to synthesize one-dimensional and two-dimensional lattices of charge and spins on a series of superconducting surfaces. In 2D lattices, radical molecules host a single electron (spin) inherited from the substrate, thus forming as a lattice of spin 1/2. In agreement with theory, zero-energy edge modes might emerge around the molecular island (see figure) which in principle flow without dissipation. They can be used as an information storage or information-processing element (topological qubit), enabling information processing without energy loss possible. As compared to conventional wires or transistors, topological superconductors or topological qubit have the advantage that the electrical currents do not scatter at defects.
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Probing charge and spins on superconductors
The interaction of localized spins of atoms or molecules with a metallic substrate leads to the screening by the itinerant conduction electrons. In tunneling spectra, the signature of this coupling is observed as a Kondo-resonance at zero energy or spin-flip excitations.We detect this feature arising from localized magnetic moment in single molecules and molecular structures with dI/dV spectroscopy.
On superconductor, local magnetic moments interact with the Cooper pairs of the superconducting condensate. This gives rise to the so-called Yu-Shiba-Rusinov (YSR) states, which emerge as sub-gap states inside the superconducting gap and reflect the interaction strength with the substrate. Using a STM operated at 1K, we now explore the spectral fingerprint of these molecular spins with µeV resolution.
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Chemical structure resolution using atomic force microscopy
To disentangle the structure of the supramolecular assemblies, we employ highly resolved AFM images with functionalized tips. These AFM experiments are possible thank to the small amplitudes of tuning fork sensors operated at low temperature. In such conditions, a single carbon monoxide (CO) molecule is attached to the the very end of the AFM tip, which drastically enhances the image contrast by probing Pauli repulsive force between tip and sample. Real-space investigations can adress various on-surface chemical reactions allowing an unambiguous visualization of the atom/molecule organizations, chemical reaction and their biproducts using our low temperature tuning fork STM/AFM.
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Miniaturization and device integration for low temperature measurements
One goal of the MolQ centre is the miniaturisation of a qubit architecture and its integration into a transport device. Thank to the small size of the radical molecules (about 1 nm), a topological island consisting of 20 to 100 molecules should have a size of only approx. 20 to 100 nanometres. Within the project, we will also investigate both theoretically (group Klinovaja/Loss) and experimentally how large the critical size required for a topological qubit is as well as the distance between these islands needed to form a qubit.
Further points are the investigation of different superconducting substrates (such as Pb, Nb, Ag/Nb, TiN, Ta and high-temperature superconductors) in order to facilitate the integration into already existing superconduting architecture. The Ag/Nb system is an interesting example of proximity-induced superconductivity, which has also the advantage to enable on-surface chemical reactions. This is possible for silver films up to a thickness of 10-20nm. When selecting the superconductor, the boundary conditions conditions in clean rooms must also be taken into account, which is important for technology transfer.
Energy dissipation of superconducting devices with pAFM
Energy dissipation of only few aW down to the nanoscale can be measured using an atomic force microscope (AFM) with single-electron precision. In past years, we demonstrated using the low temperature STM/AFM that the the self-organisation of pyrenelene molecules on a silver substrate leads to porous networks (see Figure). The pores confine electrons of the silver surface state and forms an array of artificial quantum dots. They have a discrete energy spectrum similar to the electrons bound in an atom. The local field of the probing tip interacts with them as a function of gate voltage opening conductance channels, which manifest as a strong increase of energy dissipation.
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We also explore dissipation mechanism using an low-temperature AFM in the pendulum geometry equipped with ultrasoft cantilever (pAFM), which allows to probe the tiniest tip-sample interactions forces as well as aW dissipation. For instance, we demonstrated the suppression of electronic friction on Nb films in the superconducting state as compared to its metallic state. This system is further compatible with the characterization of quantum materials with back-gate allowing a fine tuning of their electronic properties. Such measurements will be explored on the proximitized molecular islands into superconducting devices.
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