¦ van der Waals gap Septuple layer J § J || T 1/2 J 0,1 || J 0,2 || J 0,3 || J 0,4 ||

Using density functional theory and Monte Carlo calculations, we study the thickness dependence of the magnetic and electronic properties of a van der Waals interlayer antiferromagnet in the twodimensional limit. Considering MnBi2Te4 as a model material, we find it to demonstrate a remarkable set of thickness-dependent magnetic and topological transitions. While a single septuple layer block of MnBi2Te4 is a topologically trivial ferromagnet, the thicker films made of an odd (even) number of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide bandgap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, MnBi2Te4 is the first stoichiometric material predicted to realize the zero plateau quantum anomalous Hall state intrinsically. This state has been theoretically shown to host the exotic axion insulator phase.After the isolation of graphene, the field of twodimensional (2D) van der Waals (vdW) materials has experienced an explosive growth and new families of 2D systems and block-layered bulk materials, such as tetradymite-like topological insulators (TIs) [1, 2], transition metal dichalcogenides [3], and others [4-6] have been discovered. The remarkable electronic properties, along with the possibility of their tuning via thickness control, doping, intercalation, proximity effects, etc., make the layered vdW materials attractive from both practical and fundamental points of view. The relative simplicity of fabrication with a number of techniques has greatly facilitated a comprehensive study of these systems. However, the important step towards magnetic functionalization of the inherently nonmagnetic layered vdW materials and a controllable fabrication of the resulting hybrid systems has proven challenging. Therefore, aiming at exploring magnetism of layered vdW materials in the 2D limit, new possibilities have been considered. One of them is the ultrathin laminae exfoliation from intrinsically ferromagnetic (FM) vdW crystals, such as Cr 2 Ge 2 Te 6 and CrI 3 , whose magnetic behaviour has been studied down to a few layers thickness [7,8]. An alternative fabrication strategy is epitaxial growth [9,10]. With this technique, a 2D FM septuple layer (SL) block of MnBi 2 Se 4 has been grown on top of the Bi 2 Se 3 TI surface [9]. Similar systems have been theoretically proposed as a promising platform for achieving the quantized anomalous Hall (QAH) and magnetoelectric effects at elevated temperatures [11,12]. Later, epitaxial growth of the Bi 2 Se 3 /MnBi 2 Se 4 multilayer heterostructure has been reported [10].The field of 2D vdW magnets is in its infancy and many more materials with new properties are to be explored. In particular, vdW antiferromagnets are expected to be of great interest. Indeed, recently it has been reported that the layered vdW compound MnBi 2 Te 4 is the first ever antiferromagnetic (AFM) TI [13]. This state of matter is predicted to give rise to exotic phenomena such as quantized magnetoelectric effect [14], axion electrodynamics [15], and Maj...

We study the dynamics of transient hot H atoms on Pd(100) that originated from dissociative adsorption of H 2 . The methodology developed here, denoted AIMDEF, consists of ab initio molecular dynamics simulations that include a friction force to account for the energy transfer to the electronic system. We find that the excitation of electron-hole pairs is the main channel for energy dissipation, which happens at a rate that is five times faster than energy transfer into Pd lattice motion. Our results show that electronic excitations may constitute the dominant dissipation channel in the relaxation of hot atoms on surfaces. DOI: 10.1103/PhysRevLett.112.103203 PACS numbers: 68.43.-h, 34.35.+a, 34.50.Bw, 82.20.Gk Electron-hole (e-h) pair excitations are an unquestioned efficient energy drain in the interaction of fast atoms with solids and surfaces [1][2][3][4]. In contrast, the relevance of this dissipation channel in gas-surface interactions that involve energies up to a few eV is not so clear. It depends not only on the specific system, but also on the elementary process considered, as shown by different studies on scattering [5][6][7][8][9][10][11][12] and adsorption [6,[13][14][15][16][17][18][19][20][21][22][23][24][25][26] of atoms and molecules on surfaces. Low-energy e-h pair excitations have been detected as chemicurrents on Schottky diode devices during the chemisorption of atomic and molecular species on metals [13][14][15]. The correlation found between chemicurrent intensities and adsorption energies is a strong indication that a large fraction of the energy dissipated in both the dissociative and nondissociative adsorption processes is used to excite e-h pairs. However, this strongly contrasts with examples showing that the dissociative adsorption is reasonably well described within the electronically adiabatic approach, which neglects the coupling between electronic excitations and the nuclear motion [6,[18][19][20]23,25], and that the effect of electronic energy dissipation seems negligible [21,24,26]. Therefore, a question that is raised here is at what stage of the dissociative adsorption process e-h pair excitations do become relevant.In a typical adsorption event, the incoming gas species gain additional kinetic energy when entering the attractive adsorption well. In the particular case of dissociative adsorption, this energy gain can lead to the formation of so-called "hot" atoms or fragments, with energies much larger than the corresponding thermal energies of the substrate atoms. The formed hot species will then propagate along the surface until they dissipate the excess kinetic energy and finally accommodate at a stable adsorption position. This stage of the dissociative adsorption process, where the relevance of e-h pair excitations has been traditionally neglected, is the focus of the present work.In this Letter we show that while e-h pair excitation may not be relevant on the molecule-bond-breaking time scale, it is an efficient dissipative channel in the subsequent relaxation of the...

Feasibility of many emergent phenomena that intrinsic magnetic topological insulators (TIs) may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi2Te4)(Bi2Te3)m with m = 0, ⋯, 6. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the m value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m = 0) to MnBi4Te7 (m = 1) and MnBi6Te10 (m = 2). Further increase in m leads to change of the overall magnetic behavior to ferromagnetic (FM) one for (m = 3), while the interlayer coupling almost disappears. In this way, the AFM and FM TI states are, respectively, realized in the m = 0, 1, 2 and m = 3 cases. For large m numbers a hitherto-unknown topologically nontrivial phase can be created, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the MnBi2Te4 building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi2Te4)(Bi2Te3)m allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.

The relaxation dynamics of hot H, N, and N 2 on Pd(100), Ag(111), and Fe(110), respectively, is studied by means of ab initio molecular dynamics with electronic friction. This method is adapted here to account for the electron density changes caused by lattice vibrations, thus treating on an equal footing both electron-hole (e-h) pair and phonon excitations. We find that even if the latter increasingly dominate the heavier is the hot species, the contribution of e-h pairs is by no means negligible in these cases because it gains relevance at the last stage of the relaxation process. The quantitative details of energy dissipation depend on the interplay of the potential energy surface, electronic structure, and kinetic factors. DOI: 10.1103/PhysRevB.92.201411 PACS number(s): 82.65.+r, 34.35.+a, 34.50.Bw, 68.43.−h In dynamic gas-surface environments, where gas-phase atomic and molecular species impinge on the surface at energies of the order of up to a few eV, energy dissipation occurs by the excitation of electron-hole (e-h) pairs and the excitation of lattice vibrations, i.e., phonons [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In the adsorption processes of atomic and molecular species, dissociative as well as nondissociative, the species trapped by the surface gradually lose their energy until they become thermalized on the surface. The competition between the e-h pairs and phonon channels governs the relaxation dynamics of the transient hot species, and thus it plays a decisive role in the system reactivity properties. The reason is that it rules the traveled length and relaxation time of a hot atom or molecule on the surface and, consequently, the probability to undergo a recombination reaction with another adsorbate [18][19][20][21][22][23].Recent ab initio molecular dynamics (AIMD) simulations with electronic friction (AIMDEF) have shown that e-h pair excitations are the dominant relaxation mechanism for hot H atoms on Pd(100) that originate from the dissociative adsorption of H 2 [16]. More particularly, this channel dissipates energy at a five times faster rate than the phonon channel [10]. The two main reasons behind this behavior are the long H-Pd interaction time, of hundreds of fs, and the low adsorbate-tosurface atom mass ratio, γ = m H /m Pd = 0.0094. The case of H on Pd(100) represents a limiting case. For heavier adsorbates, the relative weight of e-h pairs and phonons in the energy loss is expected to vary. The energy transfer to the substrate will be determined not only by kinetic factors, such as the value of γ and the incidence conditions, but also by the topography of the multidimensional potential energy surface (PES) and the electronic structure details of the configurations probed along the relaxation trajectory.In this Rapid Communication, we investigate the relaxation dynamics of hot species in three adsorption scenarios that are representative of different energy loss regimes. We have chosen atomic N on Ag(111) (γ = 0.13), N 2 on Fe(110) (γ = 0.5), and the aforemen...