arXiv subjects

Felix Küster

Publications and source records attributed to Felix Küster.

11 recordsLinked to original sources

arXiv2026

Nonmonotonic Scaling of the Anomalous Hall Effect in a Bicollinear Antiferromagnet

An anomalous Hall effect (AHE) in antiferromagnetic (AF) systems with no net magnetization is of considerable interest for both fundamental physics and spintronic applications. Of particular interest is the two-dimensional van der Waals antiferromagnet FeTe that has an unusual fully magnetically compensated bicollinear AF structure and exhibits pronounced Kondo interaction leading to strong band renormalization. Here, we investigate the AHE in epitaxial FeTe thin films grown by molecular beam epitaxy. A large anomalous Hall conductivity is exhibited below the Neel temperature (T_N ~ 60 K) and, strikingly, becomes nonlinear at high fields within a narrow temperature window around 49 K, deviating from conventional AHE scaling behavior versus its longitudinal conductivity. Linear fits reveal a pronounced negative peak in the intercept, accompanied by a field-induced canted magnetic moment. The AHE responses are related to the Berry curvature derived from FeTe's topological band structure, highlighting the intricate interplay between topology, magnetism, and electronic transport.

cond-mat.mtrl-sci↗

arXiv2024

Two-dimensional Shiba lattices as possible platform for crystalline topological superconductivity

Localized or propagating Majorana boundary modes are the key feature of topological superconductors. They are rare in naturally-occurring compounds, but the tailored manipulation of quantum matter offers opportunities for their realization. Specifically, lattices of Yu-Shiba-Rusinov bound states $-$ Shiba lattices $-$ that arise when magnetic adatoms are placed on the surface of a conventional superconductor can be used to create topological bands within the superconducting gap of the substrate. Here, using scanning tunnelling microscopy to create and probe adatom lattices with single atom precision we reveal two signatures consistent with the realization of two types of mirror symmetry protected topological superconductors. The first has edge modes as well as higher-order corner states, and the second has symmetry-protected bulk nodal points. In principle, their topological character and boundary modes should be protected by the spatial symmetries of the adatom lattice. Our results highlight the potential of Shiba lattices as a platform to design the topology and sample geometry of 2D superconductors.

cond-mat.mes-hall↗

arXiv2023

Yu-Shiba-Rusinov tips: imaging spins at the atomic scale with full magnetic sensitivity

Measurements of magnetic properties at the atomic scale require probes capable of combining high spatial resolution with spin sensitivity. Spin-polarized scanning tunneling microscopy (SP-STM) fulfills these conditions by using atomically sharp magnetic tips. The imaging of spin structures results from the tunneling magneto-conductance that depends on the imbalance in the local density of spin-up and spin-down electrons. Spin-sensitive tips are generally formed from bulk materials or by coating non-magnetic tips with a thin magnetic layer. However, ferromagnetic materials generate stray magnetic fields which can influence the magnetic structure of the probed system, while the magnetization of antiferromagnetic materials is difficult to set tip by externally applied magnetic fields. Here, we use functionalized Yu-Shiba-Rusinov (YSR) tips prepared by attaching magnetic adatoms at the apex of a superconducting cluster to image magnetic interactions at the atomic scale. We demonstrate that YSR tips are capable of sensing different magnetization directions, conferring them full magnetic sensitivity. We additionally show that the finite size of the tip superconducting cluster makes it robust against relatively strong magnetic fields, making YSR tips capable of visualizing magnetic field driven transitions of the spin texture.

cond-mat.supr-con↗

arXiv2022

Interaction effects in a 1D flat band at a topological crystalline step edge

Step edges of topological crystalline insulators can be viewed as predecessors of higher-order topology, as they embody one-dimensional edge channels embedded in an effective three-dimensional electronic vacuum emanating from the topological crystalline insulator. Using scanning tunneling microscopy and spectroscopy we investigate the behaviour of such edge channels in Pb$_{1-x}$Sn$_{x}$Se under doping. Once the energy position of the step edge is brought close to the Fermi level, we observe the opening of a correlation gap. The experimental results are rationalized in terms of interaction effects which are enhanced since the electronic density is collapsed to a one-dimensional channel. This constitutes a unique system to study how topology and many-body electronic effects intertwine, which we model theoretically through a Hartree-Fock analysis.

cond-mat.mes-hall↗

arXiv2022

Non-Majorana modes in diluted spin chains proximitized to a superconductor

Spin chains proximitized with superconducting condensates have emerged as one of the most promising platforms for the realization of Majorana modes. Here, we craft diluted spin chains atom-by-atom following seminal theoretical proposal suggesting indirect coupling mechanisms as a viable route to trigger topological superconductivity. Starting from single adatoms hosting deep Shiba states, we use the highly anisotropic Fermi surface of the substrate to create spin chains characterized by different magnetic configurations along distinct crystallographic directions. By scrutinizing a large set of parameters we reveal the ubiquitous emergence of boundary modes. Although mimicking signatures of Majorana modes, the end modes are identified as topologically trivial Shiba states. Our work demonstrates that zero-energy modes in spin chains proximitized to superconductors are not necessarily a link to Majorana modes while simultaneously identifying new experimental platforms, driving mechanisms, and test protocols for the determination of topologically non-trivial superconducting phases.

cond-mat.supr-con↗

arXiv2022

Anomalous excitations of atomically crafted quantum magnets

High energy resolution spectroscopic studies of quantum magnets have proven to be extremely valuable in directly accessing magnetodynamics quantities, such as energy barriers, magnetic interactions, lifetime of excited states and fluctuations at the most fundamental level. Here, we explore the existence of a new flavor of low-energy spin-excitations for quantum spins coupled to an electron bath. In sharp contrast to the usual tunneling signature of two steps symmetrically centered around the Fermi level, we find a single step in the conductance. Combining time-dependent and many-body perturbation theories, magnetic field-dependent tunneling spectra are explained to be the result of an interplay between weak magnetic anisotropy energy, magnetic interactions and Stoner-like electron-hole excitations that are strongly dependent on the magnetic states of the studied nanostructures. We additionally map the evolution of the conductance peak in artificial nanostructures crafted atom-by-atom, which show clear evidence of spin-coupled behavior. The results are rationalized in terms of a non-collinear magnetic ground state and the dominance of ferro- and antiferromagnetic interactions. The atomically crafted nanomagnets offer an appealing model for the exploration of electrically pumped spin systems.

cond-mat.mes-hall↗

arXiv2021

Long range and highly tunable interaction between local spins coupled to a superconducting condensate

Interfacing magnetism with superconducting condensates is rapidly emerging as a viable route for the development of innovative quantum technologies. In this context, the development of rational design strategies to controllably tune the interaction between magnetic moments is crucial. In the metallic regime, the indirect interaction mediated by conduction electrons, the so-called RKKY coupling, has been proven to be remarkably fertile in creating and controlling magnetic phenomena. However, despite its potential, the possibility of using superconductivity to control the sign and the strength of indirect interactions between magnet moments remains largely unexplored. Here we address this problem at its ultimate limit, demonstrating the possibility of maximally tuning the interaction between local spins coupled through a superconducting condensate with atomic scale precision. By using Cr atoms coupled to superconducting Nb as a prototypical system, we use atomic manipulation techniques to precisely control the relative distance between local spins along different crystallographic directions while simultaneously sensing their coupling by scanning tunneling spectroscopy. Our results reveal the existence of highly anisotropic superconductor-mediated indirect couplings between the local spins, lasting up to very long distances, up to 12 times the lattice constant of Nb. Moreover, we demonstrate the possibility of controllably crossing a quantum phase transition by acting on the direction and interatomic distance between spins. The extremely high tunability provides novel opportunities for the realization of exotic phenomena such as topological superconductivity and the rational design of magneto-superconducting interfaces.

cond-mat.supr-con↗

arXiv2021

Correlating Josephson supercurrents and Shiba states in quantum spins unconventionally coupled to superconductors

Local spins coupled to superconductors give rise to several emerging phenomena directly linked to the competition between Cooper pair formation and magnetic exchange. These effects are generally scrutinized using a spectroscopic approach which relies on detecting the in-gap bound modes arising from Cooper pair breaking, the so-called Yu-Shiba-Rusinov (YSR) states. However, the impact of local magnetic impurities on the superconducting order parameter remains largely unexplored. Here, we use scanning Josephson spectroscopy to directly visualize the effect of magnetic perturbations on Cooper pair tunneling between superconducting electrodes at the atomic scale. By increasing the magnetic impurity orbital occupation by adding one electron at a time, we reveal the existence of a direct correlation between Josephson supercurrent suppression and YSR states. Moreover, in the metallic regime, we detect zero bias anomalies which break the existing framework based on competing Kondo and Cooper pair singlet formation mechanisms. Based on first-principle calculations, these results are rationalized in terms of unconventional spin-excitations induced by the finite magnetic anisotropy energy. Our findings have far reaching implications for phenomena that rely on the interplay between quantum spins and superconductivity.

cond-mat.supr-con↗

arXiv2020

Competing energy scales in topological superconducting heterostructures

Artificially engineered topological superconductivity has emerged as a viable route to create Majorana modes, exotic quasiparticles which have raised great expectations for storing and manipulating information in topological quantum computational schemes. The essential ingredients for their realization are spin non-degenerate metallic states proximitized to an s-wave superconductor. In this context, proximity-induced superconductivity in materials with a sizable spin-orbit coupling has been heavily investigated in recent years. Although there is convincing evidence that superconductivity may indeed be induced, it has been difficult to elucidate its topological nature. In this work, we systematically engineer an artificial topological superconductor by progressively introducing superconductivity (Nb) into metals with strong spin-orbital coupling (Pt) and 3D topological surface states (Bi2Te3). Through a longitudinal study of the character of superconducting vortices within s-wave superconducting Nb and proximity-coupled Nb/Pt and Nb/Bi2Te3, we detect the emergence of a zero-bias peak that is directly linked to the presence of topological surface states. Supported by a detailed theoretical model, our results are rationalized in terms of competing energy trends which are found to impose an upper limit to the size of the minigap separating Majorana and trivial modes, its size being ultimately linked to fundamental materials properties.

cond-mat.supr-con↗

arXiv2020

Microscopic manipulation of ferroelectric domains in SnSe monolayers at room temperature

Two-dimensional (2D) van der Waals ferroelectrics provide an unprecedented architectural freedom for the creation of artificial multiferroics and non-volatile electronic devices based on vertical and co-planar heterojunctions of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with in-plane polarization. Here we report the discovery of robust ferroelectricity with a critical temperature close to 400 K in SnSe monolayer plates grown on graphene, and the demonstration of controlled room temperature ferroelectric domain manipulation by applying appropriate bias voltage pulses to the tip of a scanning tunneling microscope (STM). This study shows that STM is a powerful tool for detecting and manipulating the microscopic domain structures in 2D ferroelectric monolayers, which is difficult for conventional approaches such as piezoresponse force microscopy, thus facilitating the hunt for other 2D ferroelectric monolayers with in-plane polarization with important technological applications.

cond-mat.mtrl-sci↗

arXiv2020

Direct observation of handedness-dependent quasiparticle interference in the two enantiomers of topological chiral semimetal PdGa

It has recently been proposed that combining chirality with topological band theory may result in a totally new class of fermions. These particles have distinct properties: they appear at high symmetry points of the reciprocal lattice, they are connected by helicoidal surface Fermi arcs spanning the entire Brillouin zone, and they are expected to exist over a large energy range. Additionally, they are expected to give rise to totally new effects forbidden in other topological classes. Understanding how these unconventional quasiparticles propagate and interact is crucial for exploiting their potential in innovative chirality-driven device architectures. These aspects necessarily rely on the detection of handedness-dependent effects in the two enantiomers and remain largely unexplored so far. Here, we use scanning tunnelling microscopy to visualize the electronic properties of both enantiomers of the prototypical chiral topological semimetal PdGa at the atomic scale. We reveal that the surface-bulk connectivity goes beyond ensuring the existence of topological Fermi arcs, but also determines how quasiparticles propagate and scatter at impurities, giving rise to chiral quantum interference patterns of opposite handedness and opposite spiralling direction for the two different enantiomers, a direct manifestation of the change of sign of their Chern number. Additionally, we demonstrate that PdGa remains topologically non-trivial over a large energy range, experimentally detecting Fermi arcs in an energy window of more than 1.6 eV symmetrically centerd around the Fermi level. These results are rationalized in terms of the deep connection between chirality in real and reciprocal space in this class of materials, and they allow to identify PdGa as an ideal topological chiral semimetal.

cond-mat.mes-hall↗