Fe Adatoms Research Articles

Merohedral disorder and impurity impacts on superconductivity of fullerenes

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb3C60 films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes.

Observation of magnetic adatom-induced Majorana vortex and its hybridization with field-induced Majorana vortex in an iron-based superconductor

Braiding Majorana zero modes is essential for fault-tolerant topological quantum computing. Iron-based superconductors with nontrivial band topology have recently emerged as a surprisingly promising platform for creating distinct Majorana zero modes in magnetic vortices in a single material and at relatively high temperatures. The magnetic field-induced Abrikosov vortex lattice makes it difficult to braid a set of Majorana zero modes or to study the coupling of a Majorana doublet due to overlapping wave functions. Here we report the observation of the proposed quantum anomalous vortex with integer quantized vortex core states and the Majorana zero mode induced by magnetic Fe adatoms deposited on the surface. We observe its hybridization with a nearby field-induced Majorana vortex in iron-based superconductor FeTe0.55Se0.45. We also observe vortex-free Yu-Shiba-Rusinov bound states at the Fe adatoms with a weaker coupling to the substrate, and discover a reversible transition between Yu-Shiba-Rusinov states and Majorana zero mode by manipulating the exchange coupling strength. The dual origin of the Majorana zero modes, from magnetic adatoms and external magnetic field, provides a new single-material platform for studying their interactions and braiding in superconductors bearing topological band structures.

Spin-Polarized Yu-Shiba-Rusinov States in an Iron-Based Superconductor.

Yu-Shiba-Rusinov (YSR) bound states appear when a magnetic atom interacts with a superconductor. Here, we report on spin-resolved spectroscopic studies of YSR states related with Fe atoms deposited on the surface of the topological superconductor FeTe_{0.55}Se_{0.45} using a spin-polarized scanning tunneling microscope. We clearly identify the spin signature of pairs of YSR bound states at finite energies within the superconducting gap having opposite spin polarization as theoretically predicted. In addition, we also observe zero-energy bound states for some of the adsorbed Fe atoms. In this case, a spin signature is found to be absent indicating the absence of Majorana bound states associated with Fe adatoms on FeTe_{0.55}Se_{0.45}.

Observation of tunable single-atom Yu-Shiba-Rusinov states

The coupling of a spin to an underlying substrate is the basis for a plethora of phenomena. In the case of a metallic substrate, Kondo screening of the adatom magnetic moment can occur. As the substrate turns superconducting, an intriguing situation emerges where the pair breaking due to the adatom spins leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo phenomena. Through scanning tunneling spectroscopy, we analyze the interdependence of Kondo screening and superconductivity. Our data obtained on single Fe adatoms on Nb(110) show that the coupling and the resulting YSR states are strongly adsorption site-dependent and reveal a quantum phase transition at a Kondo temperature comparable to the superconducting gap. The experimental signatures are rationalized by combined density functional theory and continuous-time quantum Monte-Carlo calculations to rigorously treat magnetic and hybridization effects on equal footing.

Prospecting chiral multisite interactions in prototypical magnetic systems

Atomistic spin models have found enormous success in addressing the properties of magnetic materials, grounded on the identification of the relevant underlying magnetic interactions. The huge development in the field of magnetic skyrmions and other noncollinear magnetic structures is largely due to our understanding of the chiral Dzyaloshinskii-Moriya interaction. Recently, various works have proposed new types of chiral interactions, with seemingly different forms, but the big picture is still missing. Here, we present a systematic construction of a generalized spin model containing isotropic and chiral multi-site interactions. These are motivated by a microscopic model that incorporates local spin moments and the spin-orbit interaction, and their symmetry properties are established. We show that the chiral interactions arise solely from the spin-orbit interaction and that the multi-site interactions do not have to follow Moriya's rules, unlike the Dzyaloshinskii-Moriya and chiral biquadratic interactions. The chiral multi-site interactions do not vanish due to inversion symmetry, and comply with a generalized Moriya rule: If all sites connected by the interaction lie in the same mirror plane, the chiral interaction vector must be perpendicular to this plane. We then illustrate our theoretical considerations with density functional theory calculations for prototypical magnetic systems. These are triangular trimers built out of Cr, Mn, Fe and Co adatoms on the Re(0001), Pt(111) and Au(111) surfaces, for which $C_\mathrm{3v}$ symmetry applies, and Cr and Fe square tetramers on Pt(001) with $C_\mathrm{4v}$ symmetry. The multi-site interactions are substantial in magnitude and cannot be neglected when comparing the energy of different magnetic structures. Finally, we discuss the recent literature in light of our findings, and clarify several unclear or confusing points.

The effects of transition metal adatoms on the electronic properties of stanene

Abstract The electronic characteristics of transition metal adatoms (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt and Au) adsorbed on monolayer stanene have been investigated by using the first-principles calculations. The results show that the transition metal atoms are energetically favorable at the hollow and top sites of the stanene. Stanene has good adsorption capacity to foreign adsorbed atoms, and the adsorption energy is up to 6.241 eV, which is stronger than that of phosphorene and graphene. For Ni-, Ru- and Pd-adatoms, a narrow bandgap is opened in the monolayer stanene with the bandgap of 109.4 meV, 69.6 meV and 32.4 meV, respectively. The adsorption of Fe, Co, Cu, Ag, and Au adatoms upshifts the Fermi level to the conduction band and causing n-type doping. For the Fe-, Co-, Ru-, Rh- and Os-adsorbed stanene, it can be found that the magnetic moments are 3.144 μ B , 2.024 μ B , 2.028 μ B , 1.007 μ B and 1.995 μ B , respectively. Interestingly, for Os-adsorbed stanene, the spin upward is a metal, and the spin down is a semiconductor with 215.5 meV, namely, it shows the half-metallicity. The remaining (Fe-, Co-, Cu-, Ag-, Pt-) adsorbed stanene all exhibit metallic character. Our results indicate that the transition metal adsorption may be pave the way for the study of nanoelectronics and spintronics of stanene.

Fe dopants and surface adatoms versus nontrivial topology of single-crystalline Bi2Se3

Both preserved gapless states and gapping of Dirac states due to broken time reversal symmetry in bismuth chalcogenide topological insulators with surface and bulk magnetic impurities have been observed and reported in the literature. In order to shed more light on the mechanism of such effects we have performed comprehensive element selective study of the impact of Fe impurity position in the Bi2Se3 lattice on its magnetism. The iron atoms were imbedded in the structure (volume dopants) or deposited on the surface (adatoms) and they revealed striking phenomena. Volume doping preserves non-trivial topology of Bi1.98Fe0.02Se3. Fe atoms not only substitute Bi, but also locate in van der Waals gap. The former are magnetically isotropic, while the latter reveal large magnetic moment (4.5 μB) with perpendicular anisotropy if located near the surface. Majority of Fe adatoms on the surface of Bi2Se3 exhibit weaker moment (3.5 μB) with in-plane anisotropy, as expected for non-interacting species. Negligible interaction between surface electronic states and magnetic adatoms is confirmed by identical vibration spectra of Fe deposited on TI surface of Bi2Se3 and non-TI surface of Bi2S3. The data gathered show how indispensable is the knowledge of the magnetic impurity distribution for applications of bismuth chalcogenide systems.

Stoichiometry and defect superstructures in epitaxial FeSe films on SrTiO3

Cryogenic scanning tunneling microscopy is employed to investigate the stoichiometry and defects of epitaxial FeSe thin films on ${\mathrm{SrTiO}}_{3}$(001) substrates under various postgrowth annealing conditions. Low-temperature annealing with an excess supply of Se leads to the formation of Fe vacancies and superstructures, accompanied by a superconductivity (metal)-to-insulator transition in FeSe films. By contrast, high-temperature annealing could eliminate the Fe vacancies and superstructures, and thus recover the high-temperature superconducting phase of monolayer FeSe films. We also observe multilayer FeSe during low-temperature annealing, which is revealed to link with Fe vacancy formation and adatom migration. Our results document the very special roles of film stoichiometry and help unravel several controversies in the properties of monolayer FeSe films.

First-principles study on the adsorption and diffusion properties of non-noble (Fe, Co, Ni and Cu) and noble (Ru, Rh, Pt and Pd) metal single atom on graphyne

Based on first-principles calculations, the adsorption and diffusion behavior of non-noble (Fe, Co, Ni and Cu) and noble (Ru, Rh, Pt and Pd) metal single atom on graphyne were performed. The hole formed by 12C-hexagon ring was found to be the most stable adsorption site. Strong interactions were also verified by bader charge analyses, charge difference density and density of states. Furthermore, the diffusion properties and growth modes of single metal adatoms were studied. With the adsorption on the most stable site getting stronger, the diffusion will get more difficult. The single adatoms diffuse difficultly at room temperature except Pd adatom. Moreover, results imply that Fe, Co, Cu, Ru, Rh, Pt and Pd adatoms present a tendency to grow 3D cluster while Ni adatom shows a tendency to grow 2D metal sheet at high concentration. Results imply graphyne could be a promise candidate substrate for most transition metal atoms.

Field-free platform for Majorana-like zero mode in superconductors with a topological surface state

Superconducting materials exhibiting topological properties are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. In this work, we explore the possibility of a field-free platform for generating Majorana zero energy excitations by depositing magnetic Fe impurities on the surface of candidate topological superconductors, LiFeAs and PbTaSe$_2$. We use scanning tunneling microscopy to probe localized states induced at the Fe adatoms on the atomic scale and at sub-Kelvin temperatures. We find that each Fe adatom generates a striking zero-energy bound state inside the superconducting gap, which do not split in magnetic fields up to 8T, underlining a nontrivial topological origin. Our findings point to magnetic Fe adatoms evaporated on bulk superconductors with topological surface states as a new platform for exploring Majorana zero modes and quantum information science under field-free conditions.

Tuning structural, electronic, and magnetic properties of black-AsP monolayer by adatom adsorptions: A first principles study

Black Arsenic-phosphorus (AsP) monolayer is a novel two-dimensional nanomaterial with the characteristics of modest direct bandgap and superhigh carrier mobility. However, little is known about how the surface adsorption affects the property of AsP monolayer. Motivated by this, we researched systematically the geometry, adsorption energy, magnetic moment and electronic structure of 11 different adatoms adsorbed on AsP monolayer using first-principles calculations. The adatoms used in this study include light nonmetallic (C, N, O) adatoms, period-3 metal (Na, Mg, Al) adatoms, and transition-metal (Ti, V, Cr, Mn, and Fe) adatoms. The adatoms cause an abundant variety of structural, magnetic and electronic properties. This study shows that AsP binds strongly with all adatoms under study and the adsorption energies in all systems are much stronger than that on graphene, SiC, BN, or MoS2. The semiconductor property of AsP is affected by the introduction of adsorbed atoms, which can induce mid-gap states or cause n-type doping. Moreover, the adatom adsorptions cause various spintronic characteristics: N-, Ti-, and Fe-adsorbed AsP become bipolar semiconductors, while the Mn-decorated AsP becomes a bipolar spin-gapless semiconductor. Our results suggest that atomic adsorption on AsP monolayers has potential application in the field of nanoelectronics and spintronics.

Spin, atomic, and interatomic orbital magnetism induced by 3d nanostructures deposited on transition metal surfaces

We present a first-principles study of the surface magnetism induced by Cr, Mn, Fe and Co adatoms on the (111) surfaces of Rh, Pd, Ag, Ir, Pt and Au. We first describe how the different contributions to the surface magnetism enter the magnetic stray field, with special attention paid to the induced orbital moments. Then we present results for the spin and orbital magnetic moments of the adatoms, and for the induced surface spin and orbital magnetic moments, the latter being further divided into atomic and inter-atomic contributions. We investigate how the surface magnetism is determined by the chemical nature of the elements involved, such as the filling of the magnetic d-orbitals of the adatoms and the properties of the itinerant electrons at the surface (whether they are sp- or d-like, and whether the spin-orbit interaction is relevant), and how it is modified if the magnetic adatoms are brought together to form a cluster, with Cr, Mn, Fe, and Co trimers on Pt(111) as an example. We also explore the impact of computational approximations, such as the distance between the adatom and the Pt(111) surface, or confinement effects due to the finite thickness of the slab used to model it. Our discussion of the magnetic stray field generated by a single adatom and its environment suggests a possible way of disentangling the induced surface magnetism from the adatom one, which could be feasible with scanning NV-center microscopy.

Observation of short-range Yu-Shiba-Rusinov states with threefold symmetry in layered superconductor 2H-NbSe2.

Yu-Shiba-Rusinov (YSR) states arise when magnetic impurities interact with superconductivity. The intricacy of coupling and the nature of the superconductivity determine the behavior of the YSR state, whose detailed correlations are not yet fully understood. Here, we study the YSR state of a single Fe adatom on the surface of 2H-NbSe2 with combined low temperature scanning tunneling microscopy/spectroscopy, density functional theory calculations and tight-binding modeling. It is found that the Fe adatom occupies the hollow site of the Se surface layer. A prominent YSR state close to the Fermi level is observed. The YSR state exhibits a threefold symmetry along the diagonal direction of the Se lattice. The spatial decay of the YSR state follows a behavior in three-dimensional superconductivity. This behavior contrasts with a previous study of imbedded Fe impurities, whose YSR state shows a six-fold symmetry and a two-dimensional long-range decay. According to our theoretical modeling, the coupling configurations affect the adatom-substrate hopping and the interlayer coupling of the substrate. Both factors are crucial for the consequent behavior of the YSR state.

Elastic interactions of Fe adatoms on Cu(111) at the mesoscale

An extended elastic eigenvector approach had earlier been developed to interpret ab-initio calculations of adatom interactions. It shows oscillating interactions as well as trio- and quarto (multisite) terms within clusters. It is now applied to the interaction of Fe adatoms on Cu(111).The extended approach differs from previous calculations by using a sharp cutoff - generating oscillating interactions - and by taking into account interacting dimers - generating strong anisotropies and multisite terms. Additional weak anisotropy stems from the substrate and from the surface Brillouin zone shape.This approach has 3 free parameters which have been fitted to first principles interactions of Fe adatoms on Cu(111). Elastic adatom pair interactions and dimer-dimer interactions at mesoscale separations show a reasonable good fit to the first principles interactions. At smaller separations elastic interaction values remain questionable. To enable a comparison with future first principle calculations also dimer-monomer interactions are shown.Some conclusions on initial adatom formation and diffusion are proposed and open questions are formulated.

Impact of single atomic defects and vacancies on the magnetic anisotropy energy of CoPt thin films

The impact of surface vacancies and single adatoms on the magnetic properties of tetragonal L10 CoPt thin films is investigated from first principles. We consider Co and Fe single adatoms deposited on a Pt-terminated thin film while a Pt adatom is assumed to be supported by a Co-terminated film. The vacancy is injected in the top-surface layer of the films with both types of termination. After finding the most stable location of the defects, we discuss their magnetic properties tied to those of the substrate and investigate the magnetocrystalline anisotropy energy (MAE). Previous simulations (Brahimi et al 2016 J. Phys.: Condens. Matter 28 496002) predicted a large out-of-plane surface MAE for the Pt-terminated CoPt films (4 meV per f.u.) in contrast to in-plane surface MAE for Co-terminated films (−1 meV per f.u.). Here, we find that the surface MAE is significantly modified upon the presence of the atomic defects. All investigated defects induce an in-plane MAE, which is large enough for Fe adatom and Pt vacancy to switch the surface MAE from out-of-plane to in-plane for the Pt-terminated films. Interestingly, among the investigated defects Pt vacancy has the largest effect on the MAE in contrast to Co vacancy, which induced the smallest but still significant effect. This behavior is explained in terms of the orbital moment anisotropy of the thin films.

Computational characterization of nitrogen-doped carbon nanotube functionalized by Fe adatom and Fe substituent for oxygen reduction reaction

Herein we investigated how Fe-coordinated, N-doped carbon nanotubes (CNT) enhance oxygen reduction reaction (ORR) activity by comparatively studying CNT with different pyridinic and graphitic nitrogen ratios, “Fe-CNT-PA” and “Fe-CNT-Py” in reported experimental results. Density functional theory (DFT) calculations revealed that the catalytic activity of Fe or N-doped CNT sponge for ORR was affected by nature of N dopant (pyridinic N (NP) or graphitic N (NG)) and Fe doping states (adatom or substituent). The N components in the carbon network of CNT changed the catalytic properties of metal-free CNT to that of a semi-metal-like CNT by reducing the band-gap energy. Further incorporation of Fe reduced the band-gap energy to zero, producing metal-like conductor properties. When Fe is coordinated with N components on the CNT structure, coordination of Fe as an adatom than as a substituent causes the coordination sites to exhibit higher spin density distribution and stronger hybridization, enhancing the ORR catalytic properties of CNT. This catalytic activity is more substantial when the Fe adatom is combined with a pyridinic N (NP) site by lowering the adsorption energies of ORR intermediates rather than that of O2. Based on these results, we investigated the structural and electronic properties of “Fe-CNT-PA” and found that a “Fe-CNT-PA” structure with Fe adatom adjacent to an NP site had zero band-gap energy, higher binding energy, lower working function, and magnetic moment, and thus exhibited more improved ORR activity than “Fe-CNT-Py”.

Ruderman–Kittel–Kasuya–Yosida Mechanism for Magnetic Ordering of Sparse Fe Adatoms on Graphene

First-principles calculations are carried out to elucidate the mechanism of the exchange interaction among magnetic adatoms on graphene, using Fe–graphene as an example. By fitting the calculated s...

Half-metallic properties of transition metals adsorbed on WS2 monolayer: A first-principles study

Abstract Given the various applications of half-metals in the spintronics devises, we studied the structural, electronic and magnetic properties of transition-metal (TM) (Cr, Mn, Fe, Co, Ni, and Cu) adatoms on WS2 monolayer by performing first-principles calculations. Based on the adsorption energy, TM atoms prefer to occupy the TW site, above the W atoms, for all the cases. The results indicate that systems of Cr, Mn, Fe, and Co adsorbed on WS2 monolayer are magnetic, while in Ni and Cu atoms adsorbed don’t have any magnetic properties. The TM atoms are adsorbed chemically to the WS2 layer. The obtained values of the total magnetic moment vary from 4 μB to 1 μB in case of Cr to Co adsorption, respectively, which agree with the Slater-Pauling rule. Based on the magnetic moment of 0.00 μB, non-magnetic states are realized for Ni and Cu adsorption. More importantly, the high spin polarization of 100% at the Fermi level is achieved in the Cr, Mn, Fe and Co adsorption, which implies the potential for application in spintronic devices.

Transoid-to-Cisoid Conformation Changes of Single Molecules on Surfaces Triggered by Metal Coordination.

Conformational isomers are stereoisomers that can interconvert over low potential barriers by rotation around a single bond. However, such bond rotation is hampered by geometrical constraints when molecules are adsorbed on surfaces. Here, we show that the adsorption of 4,4′-bis(4-carboxyphenyl)-6,6′-dimethyl-2,2′-bipyridine molecules on surfaces leads to the appearance of prochiral single molecules on NiO(001) and to enantiopure supramolecular domains on Au(111) surfaces containing the transoid-molecule conformation. Upon additional Fe adatom deposition, molecules undergo a controlled interconversion from a transoid-to-cisoid conformation as a result of coordination of the Fe atoms to the 2,2′-bipyridine moieties. As confirmed by atomic force microscopy images and X-ray photoelectron spectroscopy measurements, the resulting molecular structures become irreversibly achiral.

Interatomic orbital magnetism: The case of 3d adatoms deposited on the Pt(111) surface

The orbital magnetic moment is still surprisingly not well understood, in contrast to the spin part. Its description in finite systems, such as isolated atoms and molecules, is not problematic, but it was only recently that a rigorous picture was provided for extended systems. Here we focus on an intermediate class of systems: magnetic adatoms placed on a non-magnetic surface. We show that the essential quantity is the ground-state charge current density, in the presence of spin-orbit coupling, and set out its first-principles description. This is illustrated by studying the magnetism of the surface Pt electrons, induced by the presence of Cr, Mn, Fe, Co and Ni adatoms. A physically appealing partition of the charge current is introduced. This reveals that there is an important nonlocal contribution to the orbital moments of the Pt atoms, extending three times as far from each magnetic adatom as the induced spin and local orbital moments. We find that it is as sizable as the latter, and attribute its origin to a spin-orbital susceptibility of the Pt surface, different from the one responsible for the formation of the local orbital moments.