Correction: Influence of 4f filling on electronic and magnetic properties of rare earth-Au surface compounds.

Correction for 'Influence of 4f filling on electronic and magnetic properties of rare earth-Au surface compounds' by L. Fernandez et al., Nanoscale, 2020, 12, 22258-22267, DOI: 10.1039/D0NR04964F.

Absence of Magnetic Proximity Effect at the Interface of Bi_{2}Se_{3} and (Bi,Sb)_{2}Te_{3} with EuS.

We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)_{2}(Se,Te)_{3} family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of ∼10^{-3} µ_{B}/at.

Influence of 4f filling on electronic and magnetic properties of rare earth-Au surface compounds.

One-atom-thick rare-earth/noble metal (RE-NM) compounds are attractive materials to investigate two-dimensional magnetism, since they are easy to synthesize into a common RE-NM2 structure with high crystal perfection. Here we perform a comparative study of the GdAu2, HoAu2, and YbAu2 monolayer compounds grown on Au(111). We find the same atomic lattice quality and moiré superlattice periodicity in the three cases, but different electronic properties and magnetism. The YbAu2 monolayer reveals the characteristic electronic signatures of a mixed-valence configuration in the Yb atom. In contrast, GdAu2 and HoAu2 show the trivalent character of the rare-earth and ferromagnetic transitions below 22 K. Yet, the GdAu2 monolayer has an in-plane magnetic easy-axis, versus the out-of-plane one in HoAu2. The electronic bands of the two trivalent compounds are very similar, while the divalent YbAu2 monolayer exhibits different band features. In the latter, a strong 4f-5d hybridization is manifested in neatly resolved avoided crossings near the Fermi level. First principles theory points to a residual presence of empty 4f states, explaining the fluctuating valence of Yb in the YbAu2 monolayer.

High Temperature Ferromagnetism in a GdAg2 Monolayer.

Materials that exhibit ferromagnetism, interfacial stability, and tunability are highly desired for the realization of emerging magnetoelectronic phenomena in heterostructures. Here we present the GdAg2 monolayer alloy, which possesses all such qualities. By combining X-ray absorption, Kerr effect, and angle-resolved photoemission with ab initio calculations, we have investigated the ferromagnetic nature of this class of Gd-based alloys. The Curie temperature can increase from 19 K in GdAu2 to a remarkably high 85 K in GdAg2. We find that the exchange coupling between Gd atoms is barely affected by their full coordination with noble metal atoms, and instead, magnetic coupling is effectively mediated by noble metal-Gd hybrid s,p-d bands. The direct comparison between isostructural GdAu2 and GdAg2 monolayers explains how the higher degree of surface confinement and electron occupation of such hybrid s,p-d bands promote the high Curie temperature in the latter. Finally, the chemical composition and structural robustness of the GdAg2 alloy has been demonstrated by interfacing them with organic semiconductors or magnetic nanodots. These results encourage systematic investigations of rare-earth/noble metal surface alloys and interfaces, in order to exploit them in magnetoelectronic applications.

Spin-Flip and Element-Sensitive Electron Scattering in the BiAg2 Surface Alloy.

Heavy metal surface alloys represent model systems to study the correlation between electron scattering, spin-orbit interaction, and atomic structure. Here, we investigate the electron scattering from the atomic steps of monolayer BiAg_{2} on Ag(111) using quasiparticle interference measurements and density functional theory. We find that intraband transitions between states of opposite spin projection can occur via a spin-flip backward scattering mechanism driven by the spin-orbit interaction. The spin-flip scattering amplitude depends on the chemical composition of the steps, leading to total confinement for pure Bi step edges, and considerable leakage for mixed Bi-Ag step edges. Additionally, the different localization of the occupied and unoccupied surface bands at Ag and Bi sites leads to a spatial shift of the scattering potential barrier at pure Bi step edges.

Spin-dependent electron scattering at graphene edges on Ni(111).

We investigate the scattering of surface electrons by the edges of graphene islands grown on Ni(111). By combining local tunneling spectroscopy and ab initio electronic structure calculations we find that the hybridization between graphene and Ni states results in strongly reflecting graphene edges. Quantum interference patterns formed around the islands reveal a spin-dependent scattering of the Shockley bands of Ni, which we attribute to their distinct coupling to bulk states. Moreover, we find a strong dependence of the scattering amplitude on the atomic structure of the edges, depending on the orbital character and energy of the surface states.

Exchange biasing single molecule magnets: coupling of TbPc2 to antiferromagnetic layers.

We investigate the possibility to induce exchange bias between single molecule magnets (SMM) and metallic or oxide antiferromagnetic substrates. Element-resolved X-ray magnetic circular dichroism measurements reveal, respectively, the presence and absence of unidirectional exchange anisotropy for TbPc(2) SMM deposited on antiferromagnetic Mn and CoO layers. TbPc(2) deposited on Mn thin films present magnetic hysteresis and a negative horizontal shift of the Tb magnetization loop after field cooling, consistent with the observation of pinned spins in the Mn layer coupled parallel to the Tb magnetic moment. Conversely, molecules deposited on CoO substrates present paramagnetic magnetization loops with no indication of exchange bias. These experiments demonstrate the ability of SMM to polarize the pinned uncompensated spins of an antiferromagnet during field-cooling and realize metal-organic exchange-biased heterostructures using antiferromagnetic pinning layers.

Coupling single molecule magnets to ferromagnetic substrates.

We investigate the interaction of TbPc(2) single molecule magnets (SMMs) with ferromagnetic Ni substrates. Using element-resolved x-ray magnetic circular dichroism, we show that TbPc(2) couples antiferromagnetically to Ni films through ligand-mediated superexchange. This coupling is strongly anisotropic and can be manipulated by doping the interface with electron acceptor or donor atoms. We observe that the relative orientation of the substrate and molecule anisotropy axes critically affects the SMM magnetic behavior. TbPc(2) complexes deposited on perpendicularly magnetized Ni films exhibit enhanced magnetic remanence compared to SMMs in the bulk. Contrary to paramagnetic molecules pinned to a ferromagnetic support layer, we find that TbPc(2) can be magnetized parallel or antiparallel to the substrate, opening the possibility to exploit SMMs in spin valve devices.

Orbital specific chirality and homochiral self-assembly of achiral molecules induced by charge transfer and spontaneous symmetry breaking.

We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers.

Correlated electrons step by step: itinerant-to-localized transition of fe impurities in free-electron metal hosts.

High-resolution photoemission spectroscopy and ab initio calculations have been employed to analyze the onset and progression of d-sp hybridization in Fe impurities deposited on alkali metal films. The interplay between delocalization, mediated by the free-electron environment, and Coulomb interaction among d electrons gives rise to complex electronic configurations. The multiplet structure of a single Fe atom evolves and gradually dissolves into a quasiparticle peak near the Fermi level with increasing host electron density. The effective multiorbital impurity problem within the exact diagonalization scheme describes the whole range of hybridizations.

Water growth on metals and oxides: binding, dissociation and role of hydroxyl groups.

We discuss the role of the presence of dangling H-bonds from water or from surface hydroxyl species on the wetting behavior of surfaces. Using scanning tunneling and atomic force microscopies and photoelectron spectroscopy, we have examined a variety of surfaces, including mica, oxides and pure metals. We find that in all cases, the availability of free, dangling H-bonds at the surface is crucial for the subsequent growth of wetting water films. In the case of mica, electrostatic forces and H-bonding to surface O atoms determine the water orientation in the first layer and also in subsequent layers with a strong influence in its wetting characteristics. In the case of oxides like TiO2, Cu2O, SiO2 and Al2O3, surface hydroxyls form readily on defects upon exposure to water vapor and help nucleate the subsequent growth of molecular water films. On pure metals, such as Pt, Pd and Ru, the structure of the first water layer and whether or not it exhibits dangling H-bonds is again crucial. Dangling H-bonds are provided by molecules with their plane oriented vertically, or by OH groups formed by the partial dissociation of water. By tying the two H atoms of the water molecules into strong H-bonds with pre-adsorbed O on Ru can also quench the wettability of the surface.

Electron confinement in surface states on a stepped gold surface revealed by angle-resolved photoemission.

STM images show that vicinal Au(788) surfaces are made up of a uniform array of (111)-oriented terraces of similar width ( approximately 3.8 nm). This uniformity makes it possible to study the electronic structure of the resulting step superlattice by angle-resolved photoemission. We show that for this terrace array the surface state appears to be broken up into one-dimensional quantum-well levels, indicating total electron confinement within the terraces. The angular resolution allows the probability density of the terrace quantum well state to be mapped in reciprocal space, complementing nicely the wave function measured in real space by STM.

Electron wave function at a vicinal surface: switch from terrace to step modulation.

The Cu(111) surface state has been mapped for vicinal surfaces with variable step densities by angle-resolved photoemission. Using tunable synchrotron radiation to vary the k dependence perpendicular to the surface, as well as the (k) dependence, we find a switch between two qualitatively different regimes at a miscut of 7 degrees (17 A terrace width). For larger miscut angles the step modulation of the wave function dominates, and for smaller miscut angles the terrace modulation dominates. These observations resolve an apparent inconsistency between prior photoemission and STM results.