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Farinacci Group

Research

Magnetic bound states in superconductors

Exchange scattering at magnetic impurities creates bound states inside the gap. With increasing coupling strength J, the system undergoes a quantum phase transition from a free-spin state to the screened-spin state.

Exchange scattering between a magnetic impurity and the Cooper pairs of a superconductor create a bound state, called Yu-Shiba-Rusinov state, inside the superconducting gap. With STM we can study these states, tune their energy, map their spatial location and study their hybridization. These states are particularly relevant as potential for topological superconductivity. In our group we want to investigate and control how these states couple to their surroundings.

Review Article: B.W. Heinrich, J.I. Pascual, K.J. Franke. Single magnetic adsorbates on s-wave superconductors. Progress in Surface Science, 93, 1 (2018). Link

Electric control of magnetism

We use the very large electric fields of the STM junction to tune the properties of magnetic atoms and molecules.

„Magnetic fields are the natural choice to tune the properties of atomic and molecular spin systems, as these couple directly to the magnetic field via the Zeeman effect. However, electric fields are much easier to control and confine at the atomic scale, and they are intrinsically enhanced by the geometry of the STM junction. We therefore want to explore several avenues to use electric fields in order to control magnetism at the atomic scale.

State-of-the-art techniques to enhance energy and time resolution of STM

In ESR-STM, a RF voltage is added to the DC bias in order to detect transitions between spin states of single atoms and molecules.

„While STM has traditionally a poor energy resolution due to Fermi broadening of the tip DOS and limited time resolution because of the finite bandwidth of transimpedance amplifiers we are using several state-of-the-art techniques to enhance these two resolution. First, we combine STM with electron spin resonance (ESR-STM) to bring the energy resolution down to the neV regime [1]. Second, we tackle the time resolution by a combination of DC-pump-probe spectroscopy [2] and stochastic resonance [3] to cover a large frequency domain and access both incoherent and coherent dynamics.

References:
[1] S. Baumann, W. Paul, T. Choi, C.P. Lutz, A. Ardavan, A.J. Heinrich. Electron paramagnetic resonance of individual atoms on a surface. Science 350, 417-420 (2015). link

[2] S. Loth, M. Etzkorn, C.P. Lutz, D.M. Eigler, A.J. Heinrich. Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution. Science 329, 1628-1630 (2010). link

[3] N. Betz, G. McMurtrie, M. Hänze, V.K. Rajathilakam, L. Farinacci, S.N. Coppersmith, S. Baumann, S. Loth. Stochastic Resonance Spectroscopy: Characterizing Fast Dynamics with Slow Measurements. arXiv:2412.12647. link