Delving into the anisotropic interlayer exchange in bilayer CrI3

Bilayer CrI3 attracted much attention due to stacking-induced switching between the layered ferromagnetic and antiferromagnetic order. This discovery brought under the spotlight the interlayer Cr–Cr exchange interaction, which despite being much weaker than the intralayer exchange, plays an important role in shaping the magnetic properties of bilayer CrI3. In this work we delve into the anisotropic part of the interlayer exchange with the aim to separate the contributions from the Dzyaloshinskii–Moriya (DMI) and the Kitaev interactions (KI). We leverage the density functional theory calculations with spin Hamiltonian modeling and develop an energy mapping procedure to assess these anisotropic interactions with μeV accuracy. After inspecting the rhombohedral and monoclinic stacking sequences of bilayer CrI3, we reveal a considerable DMI and a weak interlayer KI between the sublattices of a monoclinic bilayer. We explain the dependence of DMI and KI on the interlayer distance, stacking sequence, and the spin–orbit coupling strength, and we suggest the dominant superexchange processes at play. In addition, we demonstrate that the single-ion anisotropy in bilayer CrI3 is highly stacking-dependent, increasing by 50% from monoclinic to rhombohedral bilayer. Remarkably, our findings prove that iodines are highly efficient in mediating the DMI across the van der Waals gap, much owing to spatially extended 5p orbitals which feature strong spin–orbit coupling. Our study gives promise that the interlayer chiral control of spin textures, demonstrated in thin metallic films where the DMI is with a much longer range, can be achieved with similar efficiency in semiconducting two-dimensional van der Waals magnets.ArXiv version: [https://arxiv.org/abs/2305.16142

The Anisotropic Interlayer Exchange in Van Der Waals 2D Magnets

Combining the density functional theory calculations with Hamiltonian modelling and symmetry analysis, we study the anisotropic interlayer exchange in bilayer CrI3. To calculate the anisotropic interlayer exchange that is usually an order of magnitude smaller than the isotropic Heisenberg exchange, we develop an accurate computational procedure that can be applied to any two-dimensional (2D) magnetic heterostructure. We find a considerable interlayer Dzyaloshinskii-Moriya (DM) and an order of magnitude smaller Kitaev interaction between the layers' sublattices. Our finding demonstrates the ability of iodine ligands to efficiently mediate the interlayer DM interaction owing to their delocalized 5p orbitals that feature strong spin-orbit coupling. In addition, we show that the single-ion anisotropy, that is usually perceived as the magnetic property inherent of monolayer, largely depends on stacking and increases by 50% from monoclinic to rhombohedral bilayers. Our study gives promise that semiconducting magnetic van der Waals heterostructures can be employed for the chiral control of spin textures, similar to what is experimentally realized only with metallic ferromagnetic thin films.SCMP : the 21st symposium on condensed matter physics : book of abstracts; 26-30 June 2023, Belgrad

Exceptionally Stable Cobalt Nanoclusters on Functionalized Graphene

To improve reactivity and achieve a higher material efficiency, catalysts are often used in the form of clusters with nanometer dimensions, down to single atoms. Since the corresponding properties are highly structure-dependent, a suitable support is thus required to ensure cluster stability during operating conditions. Herein, an efficient method to stabilize cobalt nanoclusters on graphene grown on nickel substrates, exploiting the anchoring effect of nickel atoms incorporated in the carbon network is presented. The anchored nanoclusters are studied by in situ variable temperature scanning tunneling microscopy at different temperatures and upon gas exposure. Cluster stability upon annealing up to 200 °C and upon CO exposure at least up to 1 × 10−6 mbar CO partial pressure is demonstrated. Moreover, the dimensions of the cobalt nanoclusters remain surprisingly small (<3 nm diameter) with a narrow size distribution. Density functional theory calculations demonstrate that the interplay between the low diffusion barrier on graphene on nickel and the strong anchoring effect of the nickel atoms leads to the increased stability and size selectivity of these clusters. This anchoring technique is expected to be applicable also to other cases, with clear advantages for transition metals that are usually difficult to stabilize

1D selective confinement and diffusion of metal atoms on graphene

The role of moiré graphene superstructures in favoring confined adsorption of different metal atoms is an intriguing problem not yet completely solved. Graphene (G) grown on Ni(100) forms a striped moiré pattern of valleys, where G approaches the nickel substrate and interacts with it rather strongly, and ridges, where G stays far away from the substrate and acts almost free-standing. Combining density functional theory (DFT) calculations and scanning-tunneling microscopy (STM) measurements, we show that this peculiar moiré constitutes a regular nanostructured template on a 2D support, confining in 1D trails single metal atoms and few atoms clusters. DFT calculations show that the confinement is selective and highly dependent on the atomic species, with some species preferring to adsorb on ridges and the other showing preference for valleys. Co and Au adsorbates, for instance, have opposite behavior, as predicted by DFT and observed by STM. The origin of such disparate behavior is traced back to the electrostatic interaction between the charged adsorbate and the nickel surface. Moreover, the selectivity is not restricted to the adsorption process only, but persists as adsorbate starts its diffusion, resulting in unidirectional mass transport on a continuous 2D support. These findings hold great promise for exploiting tailored nanostructured templates in a wide range of potential applications involving mass transport along element-specific routes

Avoided metallicity in a hole-doped Mott insulator on a triangular lattice

Doping of a Mott insulator gives rise to a wide variety of exotic emergent states, from high-temperature superconductivity to charge, spin, and orbital orders. The physics underpinning their evolution is, however, poorly understood. A major challenge is the chemical complexity associated with traditional routes to doping. Here, we study the Mott insulating CrO2 layer of the delafossite PdCrO2, where an intrinsic polar catastrophe provides a clean route to doping of the surface. From scanning tunnelling microscopy and angle-resolved photoemission, we find that the surface stays insulating accompanied by a short-range ordered state. From density functional theory, we demonstrate how the formation of charge disproportionation results in an insulating ground state of the surface that is disparate from the hidden Mott insulator in the bulk. We demonstrate that voltage pulses induce local modifications to this state which relax over tens of minutes, pointing to a glassy nature of the charge order

Ab initio istraživanje strukturnih i elektronskih osobina metala adsorbovanih na dvodimenzionalnim materijalima

Tokom poslednjih petnaest godina, svedoci smo izuzetnih istraživačkih napora usmerenih ka primeni neobičnih osobina dvodimenzionalnih (2D) materijala u razvoju naredne generacije nanoelektronskih komponenti...The last fifteen years witnessed unprecedented research efforts aimed at exploiting the peculiar properties of two-dimensional (2D) materials in the next generation of nanoelectronic devices..

How to create 2D magnets from non-magnetic 2D crystals

The introduction of point defects offers manifold possibilities to induce a magnetic response in intrinsically non-magnetic two-dimensional (2D) materials. In graphene, the presence of vacancies leads to notable paramagnetism, yet no long-range magnetic ordering has been experimentally achieved due to low defect concentration. Another approach to induce magnetism in 2D crystals is to adsorb magnetic transition metal atoms. However, when deposited on graphene, transition metal atoms tend to cluster due to strong metal-metal attraction [1], making it challenging to control the shape and size of obtained nanostructures and their magnetic properties. One route to suppress the unfavorable clusterization is to attach the metal adatoms to the vacancies, acting as the trapping sites. The embedded metal atoms might carry out non-zero magnetic moments, yet the random distribution of these defects across the 2D sheet makes the long-range ordering of localized magnetic moments highly unlikely. In this lecture, we show that with the use of borophene, a 2D boron crystal recently synthesized on Ag(111) substrate, these obstacles can be overcome [2]. Borophene, unlike graphene, possesses a regular pattern of hexagonal holes which can be used as a template to grow 2D magnets when filled with Fe atoms. We show that the obtained Fe nanostructures are composed of close-packed Fe wires featuring ferromagnetism within the chain and the inter-chain antiferromagnetic coupling. Using density functional theory calculations, we extract the exchange and single-ion anisotropy constants needed to describe the magnetic properties of these systems with the classical Ising and Heisenberg models. The corresponding Monte Carlo simulations revealed finite temperature magnetic ordering, with the estimates of critical temperatures of 105 K and 30 K derived from the anisotropic Heisenberg model, for the Fe-based magnets grown above and under the borophene.VIII International School and Conference on Photonics and HEMMAGINERO workshop : PHOTONICA2021 : book of abstracts; August 23-27, 2021; Belgrad

Understanding trends in lithium binding at two-dimensional materials

Layered structure and peculiar electronic properties of two-dimensional (2D) materials foster the concept of utilizing them as main components of lithium-ion batteries. Understanding basic physical mechanisms governing the interaction of Li with 2D crystals is of key importance to succeeding in a rational design of cathode and anode materials with superior functionalities. In this study density functional theory was applied to reveal the microscopic picture of Li interaction with 15 2D crystals, including several transition metal oxides and dichalcogenides, carbides of Group XIV elements, functionalized graphene, silicene, and germanene, as well as black phosphorus and Ti2C MXene. We found that the general trend in Li binding can be estimated from positions of conduction band minima of 2D materials, since the energy of the lowest empty electronic states shows a nice correlation with the strength of Li adsorption. At variance to the majority of studied surfaces where the electron transferred from Li is spread across the substrate, in monolayers of carbides of Group XIV elements the interaction with Li and the charge transfer are well localized. This gives rise to their capability to accommodate Li structures with a nearly constant binding energy of alkaline atoms over Li coverages ranging from well-separated adatoms to a full monolayer. © 2018 American Physical Society

Ab-initio and Monte Carlo study of Fe-based two-dimensional magnets at borophene supported by Ag(111) surface

Two-dimensional (2D) magnetic crystals are ideal platforms for the employment of simple physical models in the exploration of magnetism in a 2D limit. Instead of examining 2D van der Waals materials, the focus of our study is on adatoms that carry intrinsic magnetic moments and are assembled into 2D arrays at a suitable surface. We applied density functional theory (DFT) to investigate Fe nanostructures formed on a borophene sheet deposited at Ag(111) surface and identified stable Fe-based 2D magnets formed either on top of the borophene or at the interface between the borophene and Ag(111) surface. The structures are composed of close-packed Fe wires, featuring ferromagnetism within the chain and the interchain antiferromagnetic coupling. Exchange- and single-ion anisotropy constants extracted from DFT calculations are used to describe these systems with the classical Ising and Heisenberg models. The corresponding Monte Carlo simulations revealed finite temperature magnetic ordering, with the estimates of critical temperatures of 105 and 30 K derived from the anisotropic Heisenberg model, for the Fe-based magnets grown above and under borophene, respectively

First-principles study of nickel reactivity under two-dimensional cover: Ni2 C formation at rotated graphene/Ni(111) interface

Recent experiments indicate that the reactivity of metal surfaces changes profoundly when they are covered with two-dimensional (2D) materials. Nickel, the widespread catalyst choice for graphene (G) growth, exhibits complex surface restructuring even after the G sheet is fully grown. In particular, due to excess carbon segregation from bulk nickel to surface upon cooling, a nickel carbide (Ni2C) phase is detected under rotated graphene (RG) but not under epitaxial graphene (EG). Motivated by this experimental evidence, we construct different G/Ni(111) interface models accounting for the two types of G domains. Then, by applying density functional theory, we illuminate the microscopic mechanisms governing the structural changes of nickel surface induced by carbon segregation. A high concentration of subsurface carbon reduces the structural stability of Ni(111) surface and gives rise to the formation of thermodynamically advantageous Ni2C monolayer. We show the restructuring of the nickel surface under RG cover and reveal the essential role of G rotation in enabling high density of favorable C binding sites in the Ni(111) subsurface. As opposed to RG, the EG cover locks the majority of favorable C binding sites preventing the build-up of subsurface carbon density to a phase transition threshold. Therefore we confirm that the conversion of C-rich Ni surface to Ni2C takes place exclusively under RG cover, in line with the strong experimental evidence. © 2021 American Physical Society