Design of low-dimensional electron spin structures on superconductors

 The topological qubit.

Our concept of topological qubit. The edge currents shown by blue and red lines are the elements for a topological qubit with the states (|0> and |1>)

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. 

For more details, see Reference: 

Rémy Pawlak, Jung-Ching Liu, Chao Li, Richard Hess, Hongyan Chen, Carl Drechsel, Ping Zhou, Robert Häner, Ulrich Aschauer, Thilo Glatzel, Silvio Decurtins, Daniel Loss, Jelena Klinovaja, Shi-Xia Liu, Wulf Wulfhekel, Ernst Meyer. Gate-tunable topological superconductivity in a supramolecular electron spin lattice. Arxiv, 2024

Chao Li, Vladislav Pokorný, Martin Žonda, Jung-Ching Liu, Ping Zhou, Outhmane Chahib, Thilo Glatzel, Robert Häner, Silvio Decurtins, Shi-Xia Liu, Rémy Pawlak, Ernst Meyer. Individual Assembly of Radical Molecules on Superconductors: Demonstrating Quantum Spin Behavior and Bistable Charge Rearrangement. ACS Nano, in press.

 

Probing charge and spins on superconductors

figure

dI/dV spectra of the superconducting gap of Pb(111) probed with a superconducting Pb tip

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.

See examples here:

Jung-Ching Liu, Rémy Pawlak et al. Proximity-Induced Superconductivity in Atomically Precise Nanographene on Ag/Nb(110). ACS Mater.  Lett.  2023, 5 1083-90

Jung‐Ching Liu, Chao Li, Outhmane Chahib, Xing Wang, Simon Rothenbühler, Robert Häner, Silvio Decurtins, Ulrich Aschauer, Shi‐Xia Liu, Ernst Meyer, Rémy Pawlak. Spin Excitations of High Spin Iron (II) in Metal–Organic Chains on Metal and Superconductor. Adv. Sci, 2024, 2412351

Chemical structure resolution using atomic force microscopy

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AFM image with a CO-terminated tip showing the chemical structure of organic molecules

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.

See details here: 

Rémy Pawlak, Khalid N. Anindya, Outhmane Chahib, Jung-Ching Liu, Paul Hiret, Laurent Marot, Vincent Luzet, Frank Palmino, Frédéric Chérioux, Alain Rochefort, and Ernst Meyer. On-Surface Synthesis and Characterization of Radical Spins in Kagome Graphene, ACS Nano 2025, in press.

 

 

Miniaturization and device integration for low temperature measurements

upscaling

The size of a molecule is about 1 nanometre and a topological qubit consists of approx. 20 to 100 molecules. Miniaturisation in the range of a few nanometres is therefore feasible, which will be integrated into a transport device.

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

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Energy Dissipation from Confined States in Nanoporous Molecular Networks detected by AFM

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. 

See Reference here:

P. D’Astolfo, X. Wang, X. Liu, M. Kisiel, C. Drechsel, A. Baratoff, U. Aschauer, S. Decurtins, S.-X. Liu, R. Pawlak, E. Meyer, Energy Dissipation from Confined States in Nanoporous Molecular Networks, ACS Nano  2022, 16 16314-16321

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.

See References here:

Marcin Kisiel, Enrico Gnecco, Urs Gysin, Laurent Marot, Simon Rast, Ernst Meyer. Suppression of electronic friction on Nb films in the superconducting state. Nat. Mater.  2011, 10 119-122

Dilek Yildiz, Marcin Kisiel, Urs Gysin, Oguzhan Gürlü, Ernst Meyer. Mechanical dissipation via image potential states on a topological insulator surface. Nat. Mater.  2019, 18 1201-1206

Alexina Ollier, Marcin Kisiel, Xiaobo Lu, Urs Gysin, Martino Poggio, Dmitri K Efetov, Ernst Meyer Energy dissipation on magic angle twisted bilayer graphene. Comm. Phys.  2023, 6 344