Strain-induced effects in the electronic and spin properties of a monolayer of ferromagnetic GdAg2

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.

Two-Dimensional Rare Earth-Gold Intermetallic Compounds on Au(111) by Surface Alloying

Surface alloying is a straightforward route to control and modify the structure and electronic properties of surfaces. Here, we present a systematic study on the structural and electronic properties of three novel rare earth-based intermetallic compounds, namely, ReAu₂ (Re = Tb, Ho, and Er), on Au(111) via directly depositing rare earth metals onto the hot Au(111) surface. Scanning tunneling microscopy/spectroscopy measurements reveal very similar atomic structures and electronic properties, e.g., electronic states and surface work functions, for all these intermetallic compound systems because of the physical and chemical similarities between these rare earth elements. Further, these electronic properties are periodically modulated by the moiré structures caused by the lattice mismatches between ReAu₂ and Au(111). These periodically modulated surfaces could serve as templates for the self-assembly of nanostructures. In addition, these two-dimensional rare earth-based intermetallic compounds provide platforms to investigate rare earth-related catalysis, magnetisms, etc. in the lower dimensions.

A two-dimensional ErCu2 intermetallic compound on Cu(111) with moiré-pattern-modulated electronic structures

A rare-earth compound on a metal may form a two-dimensional (2D) intermetallic compound whose properties can be further modulated by the underlying substrate periodicity and coupling. Here, we present a combinational and systematic investigation using scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations on erbium (Er) on Cu(111). Experimentally, an intriguing growth mode transition from a branched island to a fractal-like island has been observed depending on whether the deposition process of Er is interrupted for a certain duration: post-deposition effects, such as nucleation and island growth controlled by diffusion, play an essential role in altering the Er island edge and its activity. Upon annealing, the branched Er islands become strands of amorphous surface alloy; in contrast, the fractal-like islands (with additional Er atoms on top) give rise to a monolayer thick 2D ErCu₂ intermetallic compound and display a moiré pattern. Theoretically, using DFT calculations, we found that the characteristic energy states, particularly the state in the unoccupied region around 582-663 meV, of the 2D ErCu₂ intermetallic compound are position-dependent, consistent with STS measurements. The moiré pattern originating from the mismatch of the periodicities of the ErCu₂ layer and the Cu(111) surface was identified to be responsible for the observed periodic modulation on the coupling interaction that affects the electronic structures. Our further DFT calculations on a free-standing ErCu₂ monolayer found it to be a 2D ferromagnet with topological band structures. Our work should stimulate further studies on such 2D rare-earth-based nanostructures and exploration of the use of the tunable electronic structures in such atomically-thin layers.

Discovery of Weyl Nodal Lines in a Single-Layer Ferromagnet

Two-dimensional (2D) materials have attracted great attention and spurred rapid development in both fundamental research and device applications. The search for exotic physical properties, such as magnetic and topological order, in 2D materials could enable the realization of novel quantum devices and is therefore at the forefront of materials science. Here, we report the discovery of twofold degenerate Weyl nodal lines in a 2D ferromagnetic material, a single-layer gadolinium-silver compound, based on combined angle-resolved photoemission spectroscopy measurements and theoretical calculations. These Weyl nodal lines are symmetry protected and thus robust against external perturbations. The coexistence of magnetic and topological order in a 2D material is likely to inform ongoing efforts study the rich physics in 2D topological ferromagnets.

Self-texturizing electronic properties of a 2-dimensional GdAu2 layer on Au(111): the role of out-of-plane atomic displacement

Here, we show that the electronic properties of a surface-supported 2-dimensional (2D) layer structure can self-texturize at nanoscale. The local electronic properties are determined by structural relaxation processes through variable adsorption stacking configurations. We demonstrate that the spatially modulated layer-buckling, which arises from the lattice mismatch and the layer/substrate coupling at the GdAu₂/Au(111) interface, is sufficient to locally open an energy gap of ∼0.5 eV at the Fermi level in an otherwise metallic layer. Additionally, this out-of-plane displacement of the Gd atoms patterns the character of the hybridized Gd-d states and shifts the center of mass of the Gd 4f multiplet proportionally to the lattice distortion. These findings demonstrate the close correlation between the electronic properties of the 2D-layer and its planarity. We demonstrate that the resulting template shows different chemical reactivities which may find important applications.

TEM nano-Moiré evaluation for an invisible lattice structure near the grain interface

Moiré technique is a powerful, important and effective tool for scientific research, from the nano-scale to the macro-scale, which is essentially the interference between two or more periodic structures with a similar frequency. In this study, an inverse transmission electron microscopy (TEM) nano-Moiré method has been proposed, for the first time, to reconstruct an invisible lattice structure near the grain interface, where only one kind of lattice structure and Moiré fringe were visible in a high resolution TEM (HRTEM) image simultaneously. The inversion process was performed in detail. Three rules were put forward to ensure the uniqueness of the inversion result. The HRTEM image of a top-coat/thermally grown oxide interface in a thermal barrier coating (TBC) structure was observed with coexisting visible lattice and Moiré fringes. Using the inverse TEM nano-Moiré method, the invisible lower layer lattice was inversed and a 3-dimensional structure near the interface was also reconstructed to some degree. The real strain field of oriented invisible and visible lattices and the relative strain field of the Moiré fringe in the grain and near the grain boundary were obtained simultaneously through the subset geometric phase analysis method. The possible failure mechanism and position of the TBC spallation from the nano-scale to the micro-scale were discussed.