Insight into the electronic structure of the centrosymmetric skyrmion magnet GdRu2Si2.

The discovery of a square magnetic-skyrmion lattice in GdRu2Si2, with the smallest so far found skyrmion size and without a geometrically frustrated lattice, has attracted significant attention. In this work, we present a comprehensive study of surface and bulk electronic structures of GdRu2Si2 by utilizing momentum-resolved photoemission (ARPES) measurements and first-principles calculations. We show how the electronic structure evolves during the antiferromagnetic transition when a peculiar helical order of 4f magnetic moments within the Gd layers sets in. A nice agreement of the ARPES-derived electronic structure with the calculated one has allowed us to characterize the features of the Fermi surface (FS), unveil the nested region along kz at the corner of the 3D FS, and reveal their orbital compositions. Our findings suggest that the Ruderman-Kittel-Kasuya-Yosida interaction plays a decisive role in stabilizing the spiral-like order of Gd 4f moments responsible for the skyrmion physics in GdRu2Si2. Our results provide a deeper understanding of electronic and magnetic properties of this material, which is crucial for predicting and developing novel skyrmion-based systems.

New Spin-Polarized Electron Source Based on Alkali Antimonide Photocathode.

New spin-dependent photoemission properties of alkali antimonide semiconductor cathodes are predicted based on the detected optical spin orientation effect and DFT band structure calculations. Using these results, the Na_{2}KSb/Cs_{3}Sb heterostructure is designed as a spin-polarized electron source in combination with the Al_{0.11}Ga_{0.89}As target as a spin detector with spatial resolution. In the Na_{2}KSb/Cs_{3}Sb photocathode, spin-dependent photoemission properties were established through detection of a high degree of photoluminescence polarization and high polarization of the photoemitted electrons. It was found that the multi-alkali photocathode can provide electron beams with emittance very close to the limits imposed by the electron thermal energy. The vacuum tablet-type sources of spin-polarized electrons have been proposed for accelerators, which can exclude the construction of the photocathode growth chambers for photoinjectors.

Surface dynamics on submonolayer Pb/Cu(001) surfaces.

The interplay of the atomic structure and phonon spectra in a variety of two dimensional phases forming during submonolayer Pb adsorption on a Cu(001) surface has been investigated using embedded atom method interatomic interaction potentials. Complementary calculations of the equilibrium atomic structure of these phases were performed using density functional theory. It has been shown that the dynamic stability of the Pb/Cu(001) structures increases with increasing the coverage from 0.375 ML to ultimately 0.6 ML, when a dense Pb layer is formed. The increase of the coverage also results in progressive shift of the Rayleigh mode of the copper surface to higher energy and the appearance of new mixed adsorbate-substrate vibration modes.

Insight into the Temperature Evolution of Electronic Structure and Mechanism of Exchange Interaction in EuS.

Discovered in 1962, the divalent ferromagnetic semiconductor EuS (TC = 16.5 K, Eg = 1.65 eV) has remained constantly relevant to the engineering of novel magnetically active interfaces, heterostructures, and multilayer sequences and to combination with topological materials. Because detailed information on the electronic structure of EuS and, in particular, its evolution across TC is not well-represented in the literature but is essential for the development of new functional systems, the present work aims at filling this gap. Our angle-resolved photoemission measurements complemented with first-principles calculations demonstrate how the electronic structure of EuS evolves across a paramagnetic-ferromagnetic transition. Our results emphasize the importance of the strong Eu 4f-S 3p mixing for exchange-magnetic splittings of the sulfur-derived bands as well as coupling between f and d orbitals of neighboring Eu atoms to derive the value of TC accurately. The 4f-3p mixing facilitates the coupling between 4f and 5d orbitals of neighboring Eu atoms, which mainly governs the exchange interaction in EuS.

Topological Magnetic Materials of the (MnSb2Te4)·(Sb2Te3)n van der Waals Compounds Family.

Using density functional theory, we propose the (MnSb2Te4)·(Sb2Te3)n family of stoichiometric van der Waals compounds that harbor multiple topologically nontrivial magnetic phases. In the ground state, the first three members of the family (n = 0, 1, 2) are 3D antiferromagnetic topological insulators, while for n ≥ 3 a special phase is formed, in which a nontrivial topological order coexists with a partial magnetic disorder in the system of the decoupled 2D ferromagnets, whose magnetizations point randomly along the third direction. Furthermore, due to a weak interlayer exchange coupling, these materials can be field-driven into the FM Weyl semimetal (n = 0) or FM axion insulator states (n ≥ 1). Finally, in two dimensions, we reveal these systems to show intrinsic quantum anomalous Hall and AFM axion insulator states, as well as quantum Hall state, achieved under external magnetic field. Our results demonstrate that MnSb2Te4 is not topologically trivial as was previously believed that opens possibilities of realization of a wealth of topologically nontrivial states in the (MnSb2Te4)·(Sb2Te3)n family.

Structural and electronic properties of C60 fullerene network self-assembled on metal-covered semiconductor surfaces.

Using first-principles density functional theory calculations, we made an accurate structural characterization of the C60 superstructures self-assembled on the Tl-adsorbed Si(111) and Ge(111) surfaces, which finds a good agreement with the recent scanning tunneling microscopy observations. Our band structure calculations revealed the semi-metallic character of the C60/Tl/Si(111) system, while the C60/Tl/Ge(111) system was found to show up the pronounced metallic character due to the cascade of the flat bands lying in the vicinity of the Fermi level. The latter is a fingerprint for strong correlation effects in the C60/Tl/Ge(111) system, which makes it a promising object for studying electrical transport phenomena and opens the prospects for its application in the molecular-based electronic devices. We elucidated the details of the molecule-substrate and intermolecular interactions and discussed the character of a charge transfer in both systems.

Magnetic and vibrational properties of small chromium clusters on the Cu(111) surface.

The structure and magnetic properties of small Cr clusters, Cr3 and Cr4, adsorbed on the Cu(111) surface have been investigated using density functional theory (DFT) calculations and their vibrational properties have been studied within calculations based on tight-binding second moment approximation interatomic interaction potentials (TBSMA). It has been shown that the magnetic ordering in the Cr clusters significantly affects their crystal structure and symmetry, which influences the vibrational modes of the clusters and nearest neighbor copper atoms. In turn, these modes select potentially possible structures of Cr3 and Cr4, prohibiting the lowest total energy cluster structure as dynamically unstable.

Fabrication of a novel magnetic topological heterostructure and temperature evolution of its massive Dirac cone.

Materials that possess nontrivial topology and magnetism is known to exhibit exotic quantum phenomena such as the quantum anomalous Hall effect. Here, we fabricate a novel magnetic topological heterostructure Mn4Bi2Te7/Bi2Te3 where multiple magnetic layers are inserted into the topmost quintuple layer of the original topological insulator Bi2Te3. A massive Dirac cone (DC) with a gap of 40-75 meV at 16 K is observed. By tracing the temperature evolution, this gap is shown to gradually decrease with increasing temperature and a blunt transition from a massive to a massless DC occurs around 200-250 K. Structural analysis shows that the samples also contain MnBi2Te4/Bi2Te3. Magnetic measurements show that there are two distinct Mn components in the system that corresponds to the two heterostructures; MnBi2Te4/Bi2Te3 is paramagnetic at 6 K while Mn4Bi2Te7/Bi2Te3 is ferromagnetic with a negative hysteresis (critical temperature  ~20 K). This novel heterostructure is potentially important for future device applications.

Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator [Formula: see text].

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator [Formula: see text] and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9-35 K), light polarizations and photon energies. We have distinguished both large (60-70 meV) and reduced ([Formula: see text]) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ([Formula: see text]). We propose that the gap above [Formula: see text] remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the "large gap" sample and apparently significantly reduced effective magnetic moment for the "reduced gap" sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.

Prediction and observation of an antiferromagnetic topological insulator.

Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order1. Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics1, such as the quantum anomalous Hall effect2 and chiral Majorana fermions3. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic4 and electronic5 properties of these materials, restricting the observation of important effects to very low temperatures2,3. An intrinsic magnetic topological insulator-a stoichiometric well ordered magnetic compound-could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi2Te4. The antiferromagnetic ordering  that MnBi2Te4  shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ2 topological classification; ℤ2 = 1 for MnBi2Te4, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi2Te4 exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling6-8 and axion electrodynamics9,10. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect2 and chiral Majorana fermions3.

Unique Thickness-Dependent Properties of the van der Waals Interlayer Antiferromagnet MnBi_{2}Te_{4} Films.

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 two-dimensional limit. Considering MnBi_{2}Te_{4} 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 MnBi_{2}Te_{4} is a topologically trivial ferromagnet, the thicker films made of an odd (even) number of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide band gap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, MnBi_{2}Te_{4} 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.

Phonons on Cu(001) surface covered by submonolayer alkali metals.

We present a theoretical investigation of the structural and vibrational properties of ordered 2D phases formed by the Li, Na and K atoms on the Cu[Formula: see text] surface. The lattice relaxation, phonon dispersions and polarization of vibrational modes as well as the local density of states are calculated using the embedded-atom method. The obtained structural parameters and vibrational frequencies are in close agreement with available experimental results.

Electronic properties of the two-dimensional (Tl, Rb)/Si(1 1 1)[Formula: see text] compound having a honeycomb-like structure.

Heavy metal layers having a honeycomb structure on the Si(1 1 1) surface were theoretically predicted to show prospects for possessing properties of the quantum spin Hall (QSH) insulators. The (Tl, Rb)/Si(1 1 1)[Formula: see text] atomic-layer compound synthesized in the present work is the first real system of such type, where atoms of heavy metal Tl are arranged into the honeycomb structure stabilized by Rb atoms occupying the centers of the honeycomb units. Electronic properties of the (Tl, Rb)/Si(1 1 1)[Formula: see text] compound has been fully characterized experimentally and theoretically and compared with those of the hypothetical (Tl, H)/Si(1 1 1)[Formula: see text] prototype system. It is concluded that the QSH-insulator properties of the Tl-honeycomb layers on Si(1 1 1) surface are dictated by the stable adsorption sites occupied by Tl atoms which, in turn, are controlled by the atom species centering the Tl honeycombs. As a result, the real (Tl, Rb)/Si(1 1 1)[Formula: see text] compound where Tl atoms occupy the T4 sites does not possess QSH-insulator properties in contrast to the hypothetical (Tl, H)/Si(1 1 1)[Formula: see text] system where Tl atoms reside in the T1 (on-top) sites and it shows up as a QSH material.

(Tl, Au)/Si(1 1 1)[Formula: see text] 2D compound: an ordered array of identical Au clusters embedded in Tl matrix.

Formation of the highly-ordered [Formula: see text]-periodicity 2D compound has been detected in the (Tl, Au)/Si(1 1 1) system as a result of Au deposition onto the Tl/Si(1 1 1) surface, its composition, structure and electronic properties have been characterized using scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density-functional-theory calculations. On the basis of these data, the structural model of the Tl-Au compound has been proposed, which adopts 12 Tl atoms and 10 Au atoms (in total, 22 atoms) per [Formula: see text] unit cell, i.e. ∼1.71 ML of Tl and ∼1.43 ML of Au (in total, ∼3.14 ML). Qualitatively, the model can be visualized as consisting of truncated-pyramid-like Au clusters with a Tl atom on top, while the other Tl atoms form a double layer around the Au clusters. The (Tl, Au)/Si(1 1 1)[Formula: see text] compound has been found to exhibit pronounced metallic properties at least down to temperatures as low as ∼25 K, which makes it a promising object for studying electrical transport phenomena in the 2D metallic systems.

Real-Time Observation of Phonon-Mediated σ-π Interband Scattering in MgB_{2}.

In systems having an anisotropic electronic structure, such as the layered materials graphite, graphene, and cuprates, impulsive light excitation can coherently stimulate specific bosonic modes, with exotic consequences for the emergent electronic properties. Here we show that the population of E_{2g} phonons in the multiband superconductor MgB_{2} can be selectively enhanced by femtosecond laser pulses, leading to a transient control of the number of carriers in the σ-electronic subsystem. The nonequilibrium evolution of the material optical constants is followed in the spectral region sensitive to both the a- and c-axis plasma frequencies and modeled theoretically, revealing the details of the σ-π interband scattering mechanism in MgB_{2}.

Spin-resolved band structure of heterojunction Bi-bilayer/3D topological insulator in the quantum dimension regime in annealed Bi2Te2.4Se0.6.

Two- and three-dimensional topological insulators are the key materials for the future nanoelectronic and spintronic devices and quantum computers. By means of angle- and spin-resolved photoemission spectroscopy we study the electronic and spin structure of the Bi-bilayer/3D topological insulator in quantum tunneling regime formed under the short annealing of Bi2Te2.4Se0.6. Owing to the temperature-induced restructuring of the topological insulator's surface quintuple layers, the hole-like spin-split Bi-bilayer bands and the parabolic electronic-like state are observed instead of the Dirac cone. Scanning Tunneling Microscopy and X-ray Photoemission Spectroscopy measurements reveal the appearance of the Bi2 terraces at the surface under the annealing. The experimental results are supported by density functional theory calculations, predicting the spin-polarized Bi-bilayer bands interacting with the quintuple-layers-derived states. Such an easily formed heterostructure promises exciting applications in spin transport devices and low-energy electronics.

Quantum spin Hall insulators in centrosymmetric thin films composed from topologically trivial BiTeI trilayers.

The quantum spin Hall insulators predicted ten years ago and now experimentally observed are instrumental for a break- through in nanoelectronics due to non-dissipative spin-polarized electron transport through their edges. For this transport to persist at normal conditions, the insulators should possess a sufficiently large band gap in a stable topological phase. Here, we theoretically show that quantum spin Hall insulators can be realized in ultra-thin films constructed from a trivial band insulator with strong spin-orbit coupling. The thinnest film with an inverted gap large enough for practical applications is a centrosymmetric sextuple layer built out of two inversely stacked non-centrosymmetric BiTeI trilayers. This nontrivial sextuple layer turns out to be the structure element of an artificially designed strong three-dimensional topological insulator Bi2Te2I2. We reveal general principles of how a topological insulator can be composed from the structure elements of the BiTeX family (X = I, Br, Cl), which opens new perspectives towards engineering of topological phases.

2D Tl-Pb compounds on Ge(1 1 1) surface: atomic arrangement and electronic band structure.

Structural transformations and evolution of the electron band structure in the (Tl, Pb)/Ge(1 1 1) system have been studied using low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density functional theory calculations. The two 2D Tl-Pb compounds on Ge(1 1 1), [Formula: see text]-(Tl, Pb) and [Formula: see text]-(Tl, Pb), have been found and their composition, atomic arrangement and electron properties has been characterized. The (Tl, Pb)/Ge(1 1 1)[Formula: see text] compound is almost identical to the alike (Tl, Pb)/Si(1 1 1)[Formula: see text] system from the viewpoint of its atomic structure and electronic properties. They contain 1.0 ML of Tl atoms arranged into a honeycomb network of chained trimers and 1/3 ML of Pb atoms occupying the centers of the honeycomb units. The (Tl, Pb)/Ge(1 1 1)[Formula: see text] compound contains six Tl atoms and seven Pb atoms per [Formula: see text] unit cell (i.e. ∼0.67 ML Tl and ∼0.78 ML Pb). Its atomic structure can be visualized as consisting of Pb hexagons surrounded by Tl trimers. The (Tl, Pb)/Ge(1 1 1)[Formula: see text] and (Tl, Pb)/Ge(1 1 1)[Formula: see text] compounds are metallic and their band structures contain spin-split surface-state bands. By analogy with the (Tl, Pb)/Si(1 1 1)[Formula: see text], these (Tl, Pb)/Ge(1 1 1) compounds are believed to be promising objects for prospective studies of superconductivity in one-atom-layer systems.

Temperature-driven topological quantum phase transitions in a phase-change material Ge2Sb2Te5.

The Ge2Sb2Te5 is a phase-change material widely used in optical memory devices and is a leading candidate for next generation non-volatile random access memory devices which are key elements of various electronics and portable systems. Despite the compound is under intense investigation its electronic structure is currently not fully understood. The present work sheds new light on the electronic structure of the Ge2Sb2Te5 crystalline phases. We demonstrate by predicting from first-principles calculations that stable crystal structures of Ge2Sb2Te5 possess different topological quantum phases: a topological insulator phase is realized in low-temperature structure and Weyl semimetal phase is a characteristic of the high-temperature structure. Since the structural phase transitions are caused by the temperature the switching between different topologically non-trivial phases can be driven by variation of the temperature. The obtained results reveal the rich physics of the Ge2Sb2Te5 compound and open previously unexplored possibility for spintronics applications of this material, substantially expanding its application potential.

Mirror-symmetry protected non-TRIM surface state in the weak topological insulator Bi2TeI.

Strong topological insulators (TIs) support topological surfaces states on any crystal surface. In contrast, a weak, time-reversal-symmetry-driven TI with at least one non-zero v1, v2, v3 ℤ2 index should host spin-locked topological surface states on the surfaces that are not parallel to the crystal plane with Miller indices (v1 v2 v3). On the other hand, mirror symmetry can protect an even number of topological states on the surfaces that are perpendicular to a mirror plane. Various symmetries in a bulk material with a band inversion can independently preordain distinct crystal planes for realization of topological states. Here we demonstrate the first instance of coexistence of both phenomena in the weak 3D TI Bi2TeI which (v1 v2 v3) surface hosts a gapless spin-split surface state protected by the crystal mirror-symmetry. The observed topological state has an even number of crossing points in (r-M)the directions of the 2D Brillouin zone due to a non-TRIM bulk-band inversion. Our findings shed light on hitherto uncharted features of the electronic structure of weak topological insulators and open up new vistas for applications of these materials in spintronics.