arXiv subjects

Klaus Kern

Publications and source records attributed to Klaus Kern.

At least 37 records · Page 2Linked to original sources

arXiv2022

Portrait of locally driven quantum phase transition cascades in a molecular monolayer

Strongly interacting electrons in layered materials give rise to a plethora of emergent phenomena, such as unconventional superconductivity. heavy fermions, and spin textures with non-trivial topology. Similar effects can also be observed in bulk materials, but the advantage of two dimensional (2D) systems is the combination of local accessibility by microscopic techniques and tuneability. In stacks of 2D materials, for example, the twist angle can be employed to tune their properties. However, while material choice and twist angle are global parameters, the full complexity and potential of such correlated 2D electronic lattices will only reveal itself when tuning their parameters becomes possible on the level of individual lattice sites. Here, we discover a lattice of strongly correlated electrons in a perfectly ordered 2D supramolecular network by driving this system through a cascade of quantum phase transitions using a movable atomically sharp electrostatic gate. As the gate field is increased, the molecular building blocks change from a Kondo-screened to a paramagnetic phase one-by-one, enabling us to reconstruct their complex interactions in detail. We anticipate that the supramolecular nature of the system will in future allow to engineer quantum correlations in arbitrary patterned structures.

cond-mat.mes-hall↗

arXiv2021

Extracting the Transport Channel Transmissions in Scanning Tunneling Microscopy using the Superconducting Excess Current

Transport through quantum coherent conductors, like atomic junctions, is described by the distribution of conduction channels. Information about the number of channels and their transmission can be extracted from various sources, such as multiple Andreev reflections, dynamical Coulomb blockade, or shot noise. We complement this set of methods by introducing the superconducting excess current as a new tool to continuously extract the transport channel transmissions of an atomic scale junction in a scanning tunneling microscope. In conjunction with ab initio simulations, we employ this technique in atomic aluminum junctions to determine the influence of the structure adjacent to the contact atoms on the transport properties.

cond-mat.supr-con↗

arXiv2021

Berry curvature-induced local spin polarisation in gated graphene/WTe$_2$ heterostructures

Full experimental control of local spin-charge interconversion is of primary interest for spintronics. Heterostructures combining graphene with a strongly spin-orbit coupled two-dimensional (2D) material enable such functionality by design. Electric spin valve experiments have provided so far global information on such devices, while leaving the local interplay between symmetry breaking, charge flow across the heterointerface and aspects of topology unexplored. Here, we utilize magneto-optical Kerr microscopy to resolve the gate-tunable, local current-induced spin polarisation in graphene/WTe$_2$ van der Waals (vdW) heterostructures. It turns out that even for a nominal in-plane transport, substantial out-of-plane spin accumulation is induced by a corresponding out-of-plane current flow. We develop a theoretical model which explains the gate- and bias-dependent onset and spatial distribution of the massive Kerr signal on the basis of interlayer tunnelling, along with the reduced point group symmetry and inherent Berry curvature of the heterostructure. Our findings unravel the potential of 2D heterostructure engineering for harnessing topological phenomena for spintronics, and constitute an important further step toward electrical spin control on the nanoscale.

cond-mat.mes-hall↗

arXiv2021

Atomic-scale structural fluctuations of a plasmonic cavity

We study fluctuations in plasmonic electroluminescence at the single-atom limit profiting from the precision of a low-temperature scanning tunneling microscope. First, we investigate the influence of a controlled single-atom transfer on the plasmonic properties of the junction. Next, we form a well-defined atomic contact of several quanta of conductance. In contact, we observe changes of the electroluminescence intensity that can be assigned to spontaneous modifications of electronic conductance, plasmonic excitation and optical antenna properties all originating from minute atomic rearrangements at or near the contact. The observations are relevant for the understanding of processes leading to spontaneous intensity variations in plasmon-enhanced atomic-scale spectroscopies.

cond-mat.mes-hall↗

arXiv2020

Gigahertz frame rate imaging of charge-injection dynamics in a molecular light source

Light sources on the scale of single molecules can be addressed and characterized on their proper sub-nanometer scale by scanning tunneling microscopy induced luminescence (STML). Such a source can be driven by defined short charge pulses while the luminescence is detected with sub-nanosecond resolution. We introduce an approach to concurrently image the molecular emitter, which is based on an individual defect, with its local environment along with its luminescence dynamics at a resolution of a billion frames per second. The observed dynamics can be assigned to the single electron capture occurring in the low-nanosecond regime. While the emitter's location on the surface remains fixed, the scanning of the tip modifies the energy landscape for charge injection to the defect. The principle of measurement is extendable to fundamental processes beyond charge transfer like exciton diffusion.

cond-mat.mes-hall↗

arXiv2020

Spin-dependent tunneling between individual superconducting bound states

Magnetic impurities on superconductors induce discrete bound levels inside the superconducting gap, known as Yu-Shiba-Rusinov (YSR) states. YSR levels are fully spin-polarized such that the tunneling between YSR states depends on their relative spin orientation. Here, we use scanning tunneling spectroscopy to resolve the spin dynamics in the tunneling process between two YSR states by experimentally extracting the angle between the spins. To this end, we exploit the ratio of thermally activated and direct spectral features in the measurement to directly extract the relative spin orientation between the two YSR states. We find freely rotating spins down to 7mK, indicating a purely paramagnetic nature of the impurities. Such a non-collinear spin alignment is essential not only for producing Majorana bound states but also as an outlook manipulating and moving the Majorana state onto the tip.

cond-mat.supr-con↗

arXiv2020

Fast Molecular Compression by a Hyperthermal Collision Gives Bond-Selective Mechanochemistry

Using electrospray ion beam deposition, we collide the complex molecule Reichardt's Dye (C41H30NO+) at low, hyperthermal translational energy (2-50 eV) with a Cu(100) surface and image the outcome at single-molecule level by scanning tunneling microscopy. We observe bond-selective reaction induced by the translational kinetic energy. The collision impulse compresses the molecule and bends specific bonds, prompting them to react selectively. This dynamics drives the system to seek thermally inaccessible reactive pathways, since the compression timescale (sub-ps) is much shorter than the thermalization timescale (ns), thereby yielding reaction products that are unobtainable thermally.

physics.chem-ph↗

arXiv2020

Visualizing the multifractal wavefunctions of a disordered two-dimensional electron gas

The wavefunctions of a disordered two-dimensional electron gas at the quantum-critical Anderson transition are predicted to exhibit multifractal scaling in their real space amplitude. We experimentally investigate the appearance of these characteristics in the spatially resolved local density of states of a two-dimensional mixed surface alloy Bi_xPb_{1-x}/Ag(111), by combining high-resolution scanning tunneling microscopy with spin and angle-resolved inverse-photoemission experiments. Our detailed knowledge of the surface alloy electronic band structure, the exact lattice structure and the atomically resolved local density of states enables us to construct a realistic Anderson tight binding model of the mixed surface alloy, and to directly compare the measured local density of states characteristics with those from our model calculations. The statistical analyses of these two-dimensional local density of states maps reveal their log-normal distributions and multifractal scaling characteristics of the underlying wavefunctions with a finite anomalous scaling exponent. Finally, our experimental results confirm theoretical predictions of an exact scaling symmetry for Anderson quantum phase transitions in the Wigner-Dyson classes.

cond-mat.mes-hall↗

arXiv2020

Symmetry mediated tunable molecular magnetism on a 2D material

The induction of unconventional superconductivity by twisting two layers of graphene a small angle was groundbreaking1, and since then has attracted widespread attention to novel phenomena caused by lattice or angle mismatch between two-dimensional (2D) materials2. While many studies address the influence of angle mismatch between layered 2D materials3-5 , the impact of the absorption alignment on the physical properties of planar molecules on 2D substrates has not been studied in detail. Using scanning probe microscopy (SPM) we show that individual cobalt phthalocyanine (CoPc) molecules adsorbed on the layered superconductor 2H-NbSe2 change drastically their charge and spin state when the symmetry axes of the molecule and the substrate are twisted with respect to each other. The CoPc changes from an effective spin-1/2 as found in gas-phase6 to a molecule with non-magnetic ground-state. On the latter we observe a singlet-triplet transition originating from an antiferromagnetic interaction between the central-ion spin and a distributed magnetic moment on the molecular ligands. Because the Ising superconductor 2H-NbSe2 lacks inversion symmetry and has large spin-orbit coupling7 this intramolecular magnetic exchange has significant non-collinear Dzyaloshinskii-Moriya (DM)8, 9 contribution.

cond-mat.mes-hall↗

arXiv2019

Tunneling dynamics between superconducting bound states at the atomic limit

Despite plenty of room at the bottom, there is a limit to the miniaturization of every process. For charge transport this is realized by the coupling of single discrete energy levels at the atomic scale. Here, we demonstrate sequential tunneling between parity protected Yu-Shiba-Rusinov (YSR) states bound to magnetic impurities located on the superconducting tip and sample of a scanning tunneling microscope at 10 mK. We reduce the relaxation of the excited YSR state to the bare minimum and find an enhanced lifetime for single quasiparticle levels. Our work offers a way to characterize and to manipulate coupled superconducting bound states, such as Andreev levels, YSR states, or Majorana bound states.

cond-mat.supr-con↗

arXiv2019

Magnetic Impurities on Superconducting Surfaces: Phase Transitions and the Role of Impurity-Substrate Hybridization

Spin-dependent scattering from magnetic impurities inside a superconductor gives rise to Yu-Shiba-Rusinov (YSR) states within the superconducting gap. As such, YSR states have been very successfully modeled with an effective scattering potential (Kondo impurity model). Using a scanning tunneling microscope, we exploit the proximity of the tip to a surface impurity and its influence on the YSR state to make a quantitative connection between the YSR state energy and the impurity-substrate hybridization. We corroborate the coupling between impurity and substrate as a key energy scale for surface derived YSR states using the Anderson impurity model in the mean field approximation, which accurately explains our observations. The model allows to decide on which side of the quantum phase transition the system resides based on additional conductance measurements. We propose that the Anderson impurity model is much more appropriate to describe YSR states from impurities on a superconducting surface than the Kondo impurity model, which is more appropriate for impurities inside a superconductor. We thus provide a first step towards a more quantitative comparison of experimental data with fully correlated calculations based on the Anderson impurity model.

cond-mat.supr-con↗

arXiv2019

Dynamical Coulomb Blockade as a Local Probe for Quantum Transport

Quantum fluctuations are imprinted with valuable information about transport processes. Experimental access to this information is possible, but challenging. We introduce the dynamical Coulomb blockade (DCB) as a local probe for fluctuations in a scanning tunneling microscope (STM) and show that it provides information about the conduction channels. In agreement with theoretical predictions, we find that the DCB disappears in a single-channel junction with increasing transmission following the Fano factor, analogous to what happens with shot noise. Furthermore we demonstrate local differences in the DCB expected from changes in the conduction channel configuration. Our experimental results are complemented by ab initio transport calculations that elucidate the microscopic nature of the conduction channels in our atomic-scale contacts. We conclude that probing the DCB by STM provides a technique complementary to shot noise measurements for locally resolving quantum transport characteristics.

cond-mat.mes-hall↗

arXiv2019

Single Photon Emission from a Plasmonic Light Source Driven by a Local Field-Induced Coulomb Blockade

A hallmark of quantum control is the ability to manipulate quantum emission at the nanoscale. Through scanning tunneling microscopy induced luminescence (STML) we are able to generate plasmonic light originating from inelastic tunneling processes that occur in a few-nanometer thick molecular film of C$_{60}$ deposited on Ag(111). Single photon emission, not of excitonic origin, occurs with a 1/$e$ lifetime of a tenth of a nanosecond or less, as shown through Hanbury Brown and Twiss photon intensity interferometry. We have performed tight-binding calculations of the electronic structure for the combined Ag-C$_{60}$-tip system and obtained good agreement with experiment. The tunneling happens through electric field induced split-off states below the C$_{60}$ LUMO band, which leads to a Coulomb blockade effect and single photon emission. The use of split-off states is shown to be a general technique that has special relevance for narrowband materials with a large bandgap.

cond-mat.mes-hall↗

arXiv2019

Robustness of Yu-Shiba-Rusinov resonances in presence of a complex superconducting order parameter

Robust quantum systems rely on having a protective environment with minimized relaxation channels. Superconducting gaps play an important role in the design of such environments. The interaction of localized single spins with a conventional superconductor generally leads to intrinsically extremely narrow Yu-Shiba-Rusinov (YSR) resonances protected inside the superconducting gap. However, this may not apply to superconductors with nontrivial, energy dependent order parameters. Exploiting the Fe-doped two-band superconductor NbSe$_2$, we show that due to the nontrivial relation between its complex valued and energy dependent order parameters, YSR states are no longer restricted to be inside the gap. They can appear outside the gap (i. e. inside the coherence peaks), where they can also acquire a substantial intrinsic lifetime broadening. T-matrix scattering calculations show excellent agreement with the experimental data and relate the intrinsic YSR state broadening to the imaginary part of the host's order parameters. Our results suggest that non-thermal relaxation mechanisms contribute to the finite lifetime of the YSR states, even within the superconducting gap, making them less protected against residual interactions than previously assumed. YSR states may serve as valuable probes for nontrivial order parameters promoting a judicious selection of protective superconductors.

cond-mat.supr-con↗

arXiv2019

Spin-split band hybridization in graphene proximitized with $α$-RuCl$_3$ nanosheets

Proximity effects induced in the 2D Dirac material graphene potentially open access to novel and intriguing physical phenomena. Thus far, the coupling between graphene and ferromagnetic insulators has been experimentally established. However, only very little is known about graphene's interaction with antiferromagnetic insulators. Here, we report a low temperature study of the electronic properties of high quality van der Waals heterostructures composed of a single graphene layer proximitized with $α$-RuCl$_3$. The latter is known to become antiferromagnetically ordered below 10 K. Shubnikov de Haas oscillations in the longitudinal resistance together with Hall resistance measurements provide clear evidence for a band realignment that is accompanied by a transfer of electrons originally occupying the graphene's spin degenerate Dirac cones into $α$-RuCl$_3$ band states with in-plane spin polarization. Left behind are holes in two separate Fermi pockets, only the dispersion of one of which is distorted near the Fermi energy due to spin selective hybridization with these spin polarized $α$-RuCl$_3$ band states. This interpretation is supported by our DFT calculations. An unexpected damping of the quantum oscillations as well as a zero field resistance upturn close to the N$é$el temperature of $α$-RuCl$_3$ suggests the onset of additional spin scattering due to spin fluctuations in the $α$-RuCl$_3$.

cond-mat.mtrl-sci↗

arXiv2018

A single hydrogen molecule as an intensity chopper in an electrically-driven plasmonic nanocavity

Photon statistics is a powerful tool for characterizing the emission dynamics of nanoscopic systems and their photophysics. Recent advances that combine correlation spectroscopy with scanning tunneling microscopy-induced luminescence (STML) have allowed measuring the emission dynamics from individual molecules and defects demonstrating their nature as single photon emitters. The application of correlation spectroscopy to the analysis of the dynamics of a well-characterized adsorbate system in ultrahigh vacuum remained to be shown. Here we combine single photon time correlations with STML to measure the dynamics of individual $H_2$ molecules between a gold tip and a Au(111) surface. An adsorbed $H_2$ molecule performs recurrent excursions below the tip apex. We use the fact that the presence of the $H_2$ molecule in the junction modifies plasmon emission to study the adsorbate dynamics. Using the $H_2$ molecule as a chopper for STM-induced optical emission intensity we demonstrate bunching in the plasmonic photon train in a single measurement over six orders of magnitude in the time domain (from microseconds to seconds) that takes only a few seconds. Our findings illustrate the power of using photon statistics to measure the diffusion dynamics of adsorbates with STML.

cond-mat.mes-hall↗

arXiv2018

Single Channel Josephson Effect in a High Transmission Atomic Contact

The Josephson effect in scanning tunneling microscopy (STM) is an excellent tool to probe the properties of the superconducting order parameter on a local scale through the Ambegaokar-Baratoff (AB) relation. Using single atomic contacts created by means of atom manipulation, we demonstrate that in the extreme case of a single transport channel through the atomic junction modifications of the current-phase relation lead to significant deviations from the linear AB formula relating the critical current to the involved gap parameters. Using the full current-phase relation for arbitrary channel transmission, we model the Josephson effect in the dynamical Coulomb blockade regime because the charging energy of the junction capacitance cannot be neglected. We find excellent agreement with the experimental data. Projecting the current-phase relation onto the charge transfer operator shows that at high transmission multiple Cooper pair tunneling may occur. These deviations become non-negligible in Josephson-STM, for example, when scanning across single adatoms.

cond-mat.supr-con↗

arXiv2018

Mapping of Yu-Shiba-Rusinov states from an extended scatterer

We investigate the spatial evolution of Yu-Shiba-Rusinov (YSR) states resulting from the interaction between Copper phthalocyanine molecules and a superconducting Vanadium (100) surface with submolecular resolution. Each molecule creates several YSR states at different energies showing distinctly different spatial intensity patterns. Surprisingly, on the molecules the largest YSR intensities are found not at the metal center, but close to one of the pyrrolic N-atoms, demonstrating strong molecular substrate interactions via the organic ligands. Energy resolved YSR maps reveal that the different YSR states originate from different molecular orbitals. We also follow the YSR states well-beyond the extent of the molecules and find clear oscillations of the YSR intensities without strong particle hole scattering phase differences. This is in contrast to expectations from a point scattering model and recent experimental findings on atomic impurities on superconductors. Our results can be explained by treating the molecular system as extended scatterer. Our findings provide new insights that are crucial to interpret the effects of a variety of magnetic systems on superconductors, in particular also those discussed in the context of Majorana bound states, which because of their size can not be considered point like as well.

cond-mat.supr-con↗