arXiv subjects

Roland Wiesendanger

Publications and source records attributed to Roland Wiesendanger.

At least 37 records · Page 2Linked to original sources

arXiv2024

Strain-driven domain wall network with chiral junctions in an antiferromagnet

Materials with antiferromagnetic order have recently emerged as promising candidates in spintronics based on their beneficial characteristics such as vanishing stray fields and ultra-fast dynamics. At the same time more complex localized non-coplanar magnetic states as for instance skyrmions are in the focus of applications due to their intriguing properties such as the topological Hall effect. Recently a conceptual shift has occurred to envision the use of such magnetic defects not only in one-dimensional race track devices but to exploit their unique properties in two-dimensional networks. Here we use local strain in a collinear antiferromagnet to induce non-coplanar domain wall junctions, which connect in a very specific way to form extended networks. We combine spin-polarized scanning tunneling microscopy with density functional theory to characterize the different building blocks of the network, and unravel the origin of the handedness of triple-junctions and the size of the arising topological orbital moments.

cond-mat.mes-hall↗

arXiv2024

Kicking Co and Rh atoms on a row-wise antiferromagnet

Diffusion on surfaces is a fundamental process in surface science, governing nanostructure and film growth, molecular self-assembly, and chemical reactions. Atom motion on non-magnetic surfaces has been studied extensively both theoretically and by real-space imaging techniques. For magnetic surfaces density functional theory (DFT) calculations have predicted strong effects of the magnetic state onto adatom diffusion, but to date no corresponding experimental data exists. Here, we investigate Co and Rh atoms on a hexagonal magnetic layer, using scanning tunneling microscopy (STM) and DFT calculations. Experimentally, we "kick" atoms by local voltage pulses and thereby initiate strictly one-dimensional motion which is dictated by the row-wise antiferromagnetic (AFM) state. Our calculations show that the one-dimensional motion of Co and Rh atoms results from conserving the Co spin direction during movement and avoiding high induced Rh spin moments, respectively. These findings demonstrate that magnetism can be a means to control adatom mobility.

cond-mat.mtrl-sci↗

arXiv2024

Antiferromagnetic order of topological orbital moments in atomic-scale skyrmion lattices

Topological orbital moments can arise in non-coplanar spin structures even in the absence of spin-orbit coupling and a net topological orbital magnetization occurs for the triple-Q state and for isolated skyrmions. For atomic-scale skyrmion lattices, a significant effect can also be expected, however, no studies have been reported yet. Here, we observe via spin-polarized scanning tunneling microscopy a non-coplanar atomic-scale spin structure with a nearly square magnetic unit cell for a pseudomorphic Fe monolayer on three atomic Ir layers on the Re(0001) surface. Employing density functional theory (DFT) calculations we consider different skyrmionic lattices to find the magnetic ground state. By mapping the DFT total energies to an atomistic spin model we demonstrate that these spin textures are stabilized by the interplay of the Dzyaloshinskii-Moriya and four-spin interactions. We evaluate the emerging phenomena of the different non-coplanar magnetic states and find significant local topological orbital moments oriented perpendicular to the surface, which order in an antiferromagnetic fashion.

cond-mat.mtrl-sci↗

arXiv2024

SP-STM study of the multi-Q phases in GdRu2Si2

The two stable surfaces of GdRu2Si2 are studied using spin-polarized scanning tunneling microscopy (SP-STM). Depending on the applied magnetic field different magnetic phases have been found and the presented measurements are in agreement with the respective previously proposed multi-Q spin textures. In particular the multi-Q nature of the zero magnetic field state, for which previous experiments could not rule out the coexistence of single-Q states, can be confirmed by our spin-resolved measurements on the Si-terminated surface. The surfaces of GdRu2Si2 exhibit strong magnetism-induced modulations of the spin-averaged density of states. We find that while the magnetic contribution to the tunnel signal can be clearly identified for the Si-terminated surface this proves to be much more difficult for the Gd-terminated surface. However, the magnetic field dependent spatial modulations on the Gd-terminated surface demonstrate that additional magnetic phase transitions occur for the surface layer compared to those identified for bulk GdRu2Si2 and possible spin textures are presented.

cond-mat.mtrl-sci↗

arXiv2024

The Non-collinear Path to Topological Superconductivity

Combining spin textures in ultra-thin films with conventional superconductors has emerged as a powerful and versatile platform for designing topologically non-trivial superconducting phases as well as spin-triplet Cooper pairs. As a consequence, two-dimensional magnet-superconductor hybrids (2D MSHs) are promising candidate systems to realize devices for topology-based quantum technologies and superconducting spintronics. So far, studies have focused mostly on systems hosting collinear ferromagnets or antiferromagnets. However, topologically non-trivial phases have been predicted to emerge in MSH systems with non-collinear spin textures as well. In this article, we present the experimental discovery of topological superconductivity in the MSH system Fe/Ta(110) where a magnetic spiral is realized in the Fe monolayer on the surface of the s-wave superconductor Ta. By combining low-temperature spin-polarized scanning tunneling microscopy measurements with theoretical modeling, we are able to conclude that the system is in a topological nodal-point superconducting phase with low-energy edge modes. Due to the non-collinear spin texture in our MSH system, these edge modes exhibit a magnetization direction-dependent dispersion. Furthermore, we identify direct signatures of Rashba spin-orbit coupling in the experimentally measured differential tunneling conductance. The present work realizes a non-collinear spin texture-based path to topological superconductivity.

cond-mat.supr-con↗

arXiv2024

High-resolution spectroscopy of proximity superconductivity in finite-size quantized surface states

Adding superconducting (SC) electron pairing via the proximity effect to pristinely non-superconducting materials can lead to a variety of interesting physical phenomena. Particular interest has recently focused on inducing SC into two-dimensional surface states (SSs), potentially also combined with non-trivial topology. We study the mechanism of proximity-induced SC into the Shockley-type SSs of the noble metals Ag(111) and Cu(111) grown on the elemental SC Nb(110) using scanning tunneling spectroscopy. The tunneling spectra exhibit an intriguing multitude of sharp states at low energies. Their appearance can be explained by Andreev bound states (ABS) formed by the weakly proximitized SSs subject to lateral finite-size confinement. We study systematically how the proximity gap in the bulk states of both Ag(111) and Cu(111) persists up to island thicknesses of several times the bulk coherence length of Nb. We find that even for thick islands, the SSs acquire a gap, with the gap size for Cu being consistently larger than for Ag. Based on this, we argue that the SC in the SS is not provided through direct overlap of the SS wavefunction with the SC host but can be understood to be mediated by step edges inducing electronic coupling to the bulk. Our work provides important input for the microscopic understanding of induced superconductivity in heterostructures and its spectral manifestation. Moreover, it lays the foundation for more complex SC heterostructures based on noble metals.

cond-mat.supr-con↗

arXiv2023

Phase Coexistence of Mn Trimer Clusters and Antiferromagnetic Mn Islands on Ir(111)

Clusters supported by solid substrates are prime candidates for heterogeneous catalysis and can be prepared in various ways. While mass-selected soft-landing methods are often used for the generation of monodisperse particles, self-assembly typically leads to a range of different cluster sizes. Here we show by scanning tunneling microscopy measurements that in the initial stages of growth Mn forms trimers on a close-packed hexagonal Ir surface, providing a route for self-organized monodisperse cluster formation on an isotropic metallic surface. For an increasing amount of Mn, first a phase with reconstructed monolayer islands is formed, until at full coverage a pseudomorphic Mn phase evolves which is the most densely packed one of the three different observed Mn phases on Ir(111). The magnetic state of both the reconstructed islands and the pseudomorphic film is found to be the prototypical antiferromagnetic Néel state with 120° spin rotation between all nearest neighbors in the hexagonal layer.

cond-mat.mtrl-sci↗

arXiv2023

Coupling of the triple-Q state to the atomic lattice by anisotropic symmetric exchange

We identify the triple-Q (3Q) state as magnetic ground state in Pd/Mn and Rh/Mn bilayers on Re(0001) using spin-polarized scanning tunneling microscopy and density functional theory. An atomistic model reveals that in general the 3Q state with tetrahedral magnetic order and zero net spin moment is coupled to a hexagonal atomic lattice in a highly symmetric orientation via the anisotropic symmetric exchange interaction, whereas other spin-orbit coupling terms cancel due to symmetry. Our experiments are in agreement with the predicted orientation of the 3Q state. A distortion from the ideal tetrahedral angles leads to other orientations of the 3Q state which, however, results in a reduced topological orbital magnetization compared to the ideal 3Q state.

cond-mat.mtrl-sci↗

arXiv2023

Proximity-induced superconductivity in a 2D Kondo lattice of an f-electron-based surface alloy

Realizing hybrids of low-dimensional Kondo lattices and superconducting substrates leads to fascinating platforms for studying the exciting physics of strongly correlated electron systems with induced superconducting pairing. Here, we report a scanning tunneling microscopy and spectroscopy study of a new type of two-dimensional (2D) La-Ce alloy grown epitaxially on a superconducting Re(0001) substrate. We observe the characteristic spectroscopic signature of a hybridization gap evidencing the coherent spin screening in the 2D Kondo lattice realized by the ultrathin La-Ce alloy film on normal conducting Re(0001). Upon lowering the temperature below the critical temperature of rhenium, a superconducting gap is induced with an in-gap Shiba band arising from the interaction of residual unscreened magnetic moments with the superconducting substrate. A positive correlation between the Kondo hybridization gap and the binding energy of the subgap Shiba band maximum is found. Our results open up a promising route toward the design of artificial superconducting Kondo and heavy fermion systems.

cond-mat.str-el↗

arXiv2023

Increased localization of Majorana modes in antiferromagnetic chains on superconductors

Magnet-superconductor hybrid (MSH) systems are a key platform for custom-designed topological superconductors. Ideally, the ends of a one-dimensional MSH structure will host Majorana zero-modes (MZMs), the fundamental unit of topological quantum computing. However, some of the experiments with ferromagnetic chains show a more complicated picture. Due to tiny gap sizes and hence long coherence lengths MZMs might hybridize and lose their topological protection. Recent experiments on a niobium surface have shown that both ferromagnetic and antiferromagnetic chains may be engineered, with the magnetic order depending on the crystallographic direction of the chain. While ferromagnetic chains are well understood, antiferromagnetic chains are less so. Here we study two models inspired by the niobium surface: a minimal model to elucidate the general topological properties of antiferromagnetic chains, and an extended model to more closely simulate a real system by mimicking the proximity effect. We find that in general for antiferromagnetic chains the topological gap is larger than for ferromagnetic ones and thus coherence lengths are shorter for antiferromagnetic chains, yielding more pronounced localization of MZMs in these chains. While topological phases for both ferromagnetic and antiferromagnetic chains both depend on the magnetic moment of the adatoms and the chemical potential, we find that antiferromagnetic chains also have a strong dependence on the magnitude of Rashba spin-orbit coupling at the surface.

cond-mat.supr-con↗

arXiv2023

Preparation and STM study of clean Nb(111) surfaces

Niobium with its highest transition temperature among all elemental superconductors has become a favorable substrate for realizing well-defined low-dimensional magnet-superconductor hybrid systems exhibiting novel types of exotic electronic states such as Majorana zero-energy modes. While a preparation procedure for obtaining atomically clean Nb(110) substrates has previously been reported, a suitable preparation method for clean Nb(111) surfaces is still lacking. Here, we report a recipe for cleaning Nb(111) surfaces based on an atomic hydrogen treatment followed by short flashes to elevated temperatures. The atomic surface structure of clean non-reconstructed Nb(111) is investigated by high-resolution scanning tunneling microscopy (STM), as well as a surface reconstruction with a reduced atom density compared to the (111) plane of a bcc crystal resulting from a surface premelting at high annealing temperatures.

cond-mat.supr-con↗

arXiv2023

Topological Nodal Point Superconductivity in Checkerboard Magnet-Superconductor Hybrid Systems

We demonstrate that checkerboard magnet-superconductor hybrid systems possess a rich phase diagram exhibiting both strong topological superconducting (STSC) and topological nodal point superconducting (TNPSC) phases. We show that TNPSC phases exist both for ferromagnetic and antiferromagnetic systems, yielding a plethora of qualitatively different edge mode structures. Checkerboard MSH systems also facilitate the emergence of STSC phases which can be induced even in the limit of vanishing magnetization. Our results provide a new path for the quantum engineering of topological superconducting phases using atomic manipulation techniques.

cond-mat.supr-con↗

arXiv2023

Search for large topological gaps in atomic spin chains on proximitized superconducting heavy metal layers

One-dimensional systems comprising s-wave superconductivity with meticulously tuned magnetism and spin-orbit coupling can realize topologically gapped superconductors hosting Majorana edge modes whose stability is determined by the gap's size. The ongoing quest for larger topological gaps evolved into a material science issue. However, for atomic spin chains on superconductor surfaces, the effect of the substrate's spin-orbit coupling on the system's topological gap size is largely unexplored. Here, we introduce an atomic layer of the heavy metal Au on Nb(110) which combines strong spin-orbit coupling and a large superconducting gap with a high crystallographic quality enabling the assembly of defect-free Fe chains using a scanning tunneling microscope tip. Scanning tunneling spectroscopy experiments and density functional theory calculations reveal ferromagnetic coupling and ungapped YSR bands in the chain despite of the heavy substrate. By artificially imposing a spin spiral state our calculations indicate a minigap opening and zero-energy edge state formation. The presented methodology paves the way towards a material screening of heavy metal layers on elemental superconductors for ideal systems hosting Majorana edge modes protected by large topological gaps.

cond-mat.supr-con↗

arXiv2022

Testing the topological nature of end states in antiferromagnetic atomic chains on superconductors

Edge states forming at the boundaries of topologically non-trivial phases of matter are promising candidates for future device applications because of their stability against local perturbations. Magnetically ordered spin chains proximitized by an s-wave superconductor are predicted to enter a topologically non-trivial mini-gapped phase with zero-energy Majorana modes (MMs) localized at their ends. However, the presence of non-topological end states mimicking MM properties can spoil their unambiguous observation. Here, we report on a method to experimentally decide on the MM nature of end states observed for the first time in antiferromagnetic spin chains. Using scanning tunneling spectroscopy, we find end states at either finite or near-zero energy in Mn chains on Nb(110) or Ta(110), respectively, within a large minigap. By introducing a locally perturbing defect on one end of the chain, the end state on this side splits off from zero-energy while the one on the other side doesn't - ruling out their MM origin. A minimal model shows that, while wide trivial minigaps hosting such conventional end states are easily achieved in antiferromagnetic spin chains, unrealistically large spin-orbit couplings are required to drive the system into the topologically nontrivial phase with MMs. The methodology of perturbing chains by local defects is a powerful tool to probe the stability of future candidate topological edge modes against local disorder.

cond-mat.supr-con↗

arXiv2022

Majorana modes with side features in magnet-superconductor hybrid systems

Magnet-superconductor hybrid (MSH) systems represent promising platforms to host Majorana zero modes (MZMs), the elemental building blocks for fault-tolerant quantum computers. Theoretical description of such MSH structures is mostly based on simplified models, not accounting for the complexity of real materials. Here, based on density functional theory, we derive a superconducting 80-band model to study an MSH system consisting of a magnetic manganese chain on the s wave superconductor niobium. For a wide range of values of the superconducting order parameter, the system is a topological superconductor, with MZMs exhibiting non-universal spatial patterns and a drastic accumulation of spectral weight on both sides along the magnetic chain. These side feature states can be explained by an effective model which is guided by the ab initio results. Performing scanning tunneling spectroscopy experiments on the same system, we observe a spatial structure in the low-energy local density of states that is consistent with the theoretical findings. Our results open a first-principle approach to the discovery of topological superconductors.

cond-mat.supr-con↗

arXiv2022

Proximity superconductivity in atom-by-atom crafted quantum dots

Gapless materials in electronic contact with superconductors acquire proximity-induced superconductivity in a region near the interface. Numerous proposals build on this addition of electron pairing to originally non-superconducting systems like ferromagnets and predict intriguing quantum phases of matter, including topological-, odd-frequency-, or nodal-point superconductivity. However, atomic-scale experimental investigations of the microscopic mechanisms leading to proximity-induced Cooper pairing in surface or interface states are missing. Here, we investigate the most miniature example of the proximity effect on only a single quantum level of a surface state confined in a quantum corral on a superconducting substrate, built atom-by-atom by a scanning tunneling microscope. Whenever an eigenmode of the corral is pitched close to the Fermi energy by adjusting the corral's size, a pair of particle-hole symmetric states enters the superconductor's gap. We identify the in-gap states as scattering resonances theoretically predicted 50 years ago by Machida and Shibata, which had so far eluded detection. We further show that the observed anticrossings of the in-gap states indicate proximity-induced pairing in the quantum corral's eigenmodes. Our results have direct consequences on the interpretation of in-gap states in unconventional or topological superconductors, corroborate concepts to induce superconductivity into a single quantum level and further pave the way towards superconducting artificial lattices.

cond-mat.supr-con↗

arXiv2022

Creating arbitrary sequences of mobile magnetic skyrmions and antiskyrmions

Magnetic skyrmions and their anti-particles, the antiskyrmions, are stable magnetic solitons existing down to the nanometer scale. Their stability and size as well as the possibility to propel them by, e.g., electric currents make them promising candidates for the use in memory devices, such as racetrack memories. Skyrmions and antiskyrmions share those same advantages individually, but may annihilate each other when they coexist in the same device. Yet, combining them to represent logical bits of 0 (skyrmion) and 1 (antiskyrmion) in one device opens new possibilities to create new types of densely packed racetrack memory devices. For this, a controlled creation and annihilation procedure, i.e., a writing or deletion operation, is necessary. Here, we propose a method to create arbitrary sequences of coexisting skyrmions and antiskyrmions by rotations of the magnetic moments at the edge of a rectangular slab. The skyrmions and antiskyrmions remain stable and do not annihilate each other.

cond-mat.mes-hall↗

arXiv2022

Antiferromagnetism-driven two-dimensional topological nodal-point superconductivity

Magnet$/$superconductor hybrids (MSHs) hold the promise to host emergent topological superconducting phases. Both one-dimensional (1D) and two-dimensional (2D) magnetic systems in proximity to s-wave superconductors have shown evidence of gapped topological superconductivity with zero-energy end states and chiral edge modes. Recently, it was proposed that the bulk transition-metal dichalcogenide 4Hb$-$TaS$_2$ is a gapless topological nodal-point superconductor (TNPSC). However, there has been no experimental realization of a TNPSC in a MSH system yet. Here we present the discovery of TNPSC in antiferromagnetic (AFM) monolayers on top of an s-wave superconductor. Our calculations show that the topological phase is driven by the AFM order, resulting in the emergence of a gapless time-reversal invariant topological superconducting state. Using low-temperature scanning tunneling microscopy we observe a low-energy edge mode, which separates the topological phase from the trivial one, at the boundaries of antiferromagnetic islands. As predicted by the calculations, we find that the relative spectral weight of the edge mode depends on the edge's atomic configuration. Our results establish the combination of antiferromagnetism and superconductivity as a novel route to design 2D topological quantum phases.

cond-mat.supr-con↗