Construction of a THz-STM

The combination of high spatial and temporal resolution is still a limiting factor in the field of surface science. Only very recently, a novel experimental approach has been presented that allows versatile access to this regime by combining ultrafast Terahertz (THz) pulses with the extraordinary spatial resolution of scanning tunneling microscopy (STM): The so-called THz-STM. In this device, the THz pulses act as ultrafast transient bias voltage and hence allow to unravel the ultrafast physics of highly localized electronic states via pump-probe experiments. In this thesis, we describe the process of establishing this novel technique in our group. We firstly discuss the new configuration of the designated lab, the design and alignment of our THz-source as well as its characterization using electro-optic sampling. Subsequently we describe the modifications performed on the preexisting STM to facilitate the coupling of THz radiation. The THz-STM was thoroughly tested in different scenarios showcasing its capabilities. We initially present data originating from optical-pump, THz-probe measurements based on photoelectron emission, confirming the successful coupling of ultrashort THz pulses into the STM. Subsequent measurements using field emission resonances show the stability of our system under cryo conditions and using junctions at tunneling distances. Experiments involving Kondo resonances of point defects in MoS2 prove the sensitivity of our THz-STM when used with small nonlinearities and are used to perform exemplary spatially resolved measurements. Finally we present preliminary time-resolved data as part of an outlook

Mapping the perturbation potential of metallic and dipolar tips in tunneling spectroscopy on MoS2

Scanning tunneling spectroscopy requires the application of a potential difference between the sample and a tip. In metal-vacuum-metal junctions, one can safely assume that the potential is constant along the metallic substrate. Here, we show that the inhomogeneous shape of the electric potential has to be taken into account when probing spatially extended molecules on a decoupling layer. To this end, oligothiophene-based molecules were deposited on a monolayer of molybdenum disulfide (MoS2) on a Au(111) surface. By probing the delocalized molecular orbital along the thiophene backbone, we found an apparent intramolecular shift of the positive ion resonance, which can be ascribed to a perturbation potential caused by the tip. Using a simple model for the electrostatic landscape, we show that such a perturbation is caused by the inhomogeneity of the applied bias potential in the junction and may be further modified by an electric dipole of a functionalized tip. The two effects can be disentangled in tunneling spectra by probing the apparent energy shift of vibronic resonances along the molecular backbone. We suggest that extended molecules on MoS2 can be used as a sensor for the shape of the electrostatic potential of arbitrary tips

Quantum Phase Transition and Transport

The exchange scattering at magnetic adsorbates on superconductors gives rise to Yu-Shiba-Rusinov (YSR) bound states. Depending on the strength of the exchange coupling, the magnetic moment perturbs the Cooper pair condensate only weakly, resulting in a free-spin ground state, or binds a quasiparticle in its vicinity, leading to a (partially) screened spin state. Here, we use the flexibility of Fe-porphin (FeP) molecules adsorbed on a Pb(111) surface to reversibly and continuously tune between these distinct ground states. We find that the FeP moment is screened in the pristine adsorption state. Approaching the tip of a scanning tunneling microscope, we exert a sufficiently strong attractive force to tune the molecule through the quantum phase transition into the free-spin state. We ascertain and characterize the transition by investigating the transport processes as function of tip-molecule distance, exciting the YSR states by single-electron tunneling as well as (multiple) Andreev reflections

Photon-assisted tunneling at the atomic scale: Probing resonant Andreev reflections from Yu-Shiba-Rusinov states

Tunneling across superconducting junctions proceeds by a rich variety of processes, which transfer single electrons, Cooper pairs, or even larger numbers of electrons by multiple Andreev reflections. Photon-assisted tunneling combined with the venerable Tien-Gordon model has long been a powerful tool to identify tunneling processes between superconductors. Here, we probe superconducting tunnel junctions including an impurity-induced Yu-Shiba-Rusinov (YSR) state by exposing a scanning tunneling microscope with a superconducting tip to microwave radiation. We find that a simple Tien-Gordon description describes tunneling of single electrons and Cooper pairs into the bare substrate, but breaks down for tunneling via YSR states by resonant Andreev reflections. We develop an improved theoretical description which is in excellent agreement with the data. Our results establish photon-assisted tunneling as a powerful tool to analyze tunneling processes at the atomic scale which should be particularly informative for unconventional and topological superconductors

Diode effect in Josephson junctions with a single magnetic atom

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes and known as non-reciprocal charge transport. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems. Probing the ultimate limits of miniaturization, we have created atomic-scale Pb--Pb Josephson junctions in a scanning tunneling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behavior, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing via Yu-Shiba-Rusinov (YSR) states inside the superconducting energy gap. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation

Diode effect in Josephson junctions with a single magnetic atom

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes1 and known as non-reciprocal charge transport2. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems3,4,5,6,7,8,9,10. Here we investigate the ultimate limits of miniaturization by creating atomic-scale Pb–Pb Josephson junctions in a scanning tunnelling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behaviour, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing by means of electron–hole asymmetric Yu–Shiba–Rusinov states inside the superconducting energy gap and identify a new mechanism for diode behaviour in Josephson junctions. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation

Moiré Tuning of Spin Excitations: Individual Fe Atoms on MoS2/Au(111)

Magnetic adatoms on properly designed surfaces constitute exquisite systems for addressing, controlling, and manipulating single quantum spins. Here, we show that monolayers of MoS2 on a Au(111) surface provide a versatile platform for controllably tuning the coupling between adatom spins and substrate electrons. Even for equivalent adsorption sites with respect to the atomic MoS2 lattice, we observe that Fe adatoms exhibit behaviors ranging from pure spin excitations, characteristic of negligible exchange and dominant single-ion anisotropy, to a fully developed Kondo resonance, indicating strong exchange and negligible single-ion anisotropy. This tunability emerges from a moiré structure of MoS2 on Au(111) in conjunction with pronounced many-body renormalizations. We also find striking spectral variations in the immediate vicinity of the Fe atoms, which we explain by quantum interference reflecting the formation of Fe-S hybrid states despite the nominally inert nature of the substrate. Our work establishes monolayer MoS2 as a tuning layer for adjusting the quantum spin properties over an extraordinarily broad parameter range. The considerable variability can be exploited for quantum spin manipulations

Manuskript-Titel: "Photon-assisted tunneling at the atomic scale: Probing resonant Andreev reflections from Yu-Shiba-Rusinov states"

Tunnelling across superconducting junctions proceeds by a rich variety of processes, which transfer single electrons, Cooper pairs or even larger numbers of electrons by multiple Andreev reflections. Photon-assisted tunnelling combined with the venerable Tien–Gordon model has long been a powerful tool to identify tunnelling processes between superconductors. Here, we probe superconducting tunnel junctions including an impurity-induced Yu–Shiba–Rusinov (YSR) state by exposing a scanning tunnelling microscope with a superconducting tip to microwave radiation. We find that a simple Tien–Gordon description describes tunnelling of single electrons and Cooper pairs into the bare substrate, but breaks down for tunnelling via YSR states by resonant Andreev reflections. We develop an improved theoretical description that is in excellent agreement with the data. Our results establish photon-assisted tunnelling as a powerful tool to analyse tunnelling processes at the atomic scale, which should be particularly informative for unconventional and topological superconductors