Modification of a Shockley-Type Surface State on Pt(111) upon Deposition of Gold Thin Layers

We present a first-principles fully-relativistic study of surface and interface states in the n one monolayer (ML) Au/Pt(111) heterostructures. The modification of an unoccupied s - p -type surface state existing on a Pt(111) surface at the surface Brillouin zone center upon deposition of a few atomic Au layers is investigated. In particular, we find that the transformation process of such a surface state upon variation of the Au adlayer thickness crucially depends on the nature of the relevant quantum state in the adsorbate. When the Au adlayer consists of one or two monolayers and this relevant state has energy above the Pt(111) surface state position, the latter shifts downward upon approaching the Au adlayer. As a result, in the 1 ML Au/Pt(111) and 2 ML Au/Pt(111) heterostructures at the equilibrium adlayer position, the Pt-derived surface state experiences strong hybridization with the bulk electronic states and becomes a strong occupied resonance. In contrast, when the number n of atomic layers in the Au films increases to three or more, the Pt(111) surface state shifts upward upon reduction of the distance between the Pt(111) surface and the Au adlayer. At equilibrium, the Pt-derived surface state transforms into an unoccupied quantum-well state of the Au adlayer. This change is explained by the fact that the relevant electronic state in free-standing Au films with n ≥ 3 has lower energy in comparison to the Pt(111) surface state.3.9003.2017/$\Pi$220 or 3.9003.2017/9.10 Ministry of Education and Science of the Russian Federation Project No. 15.61.202.2015 Saint Petersburg State University IT-756-13 Euskal Herriko Unibertsitatea FIS2016-76617-P and FIS2016-75862-P Spanish Ministry of Economy, Industry and Competitiveness MINEIC

Acoustic Plasmons in Nickel and Its Modification upon Hydrogen Uptake

In this work, we study, in the framework of the ab initio linear-response time-dependent density functional theory, the low-energy collective electronic excitations with characteristic sound-like dispersion, called acoustic plasmons, in bulk ferromagnetic nickel. Since the respective spatial oscillations in slow and fast charge systems involve states with different spins, excitation of such plasmons in nickel should result in the spatial variations in the spin structure as well. We extend our study to NiHx with different hydrogen concentrations x. We vary the hydrogen concentration and trace variations in the acoustic plasmons properties. Finally, at x=1 the acoustic modes disappear in paramagnetic NiH. The explanation of such evolution is based on the changes in the population of different energy bands with hydrogen content variation.Y.M.K. acknowledges support from the Government research assignment for ISPMS SB RAS, project FWRW-2022-0001 (in the part of band structure calculations). I.V.S. acknowledges support from the Ministry of Education and Science of the Russian Federation within State Task No. FSWM-2020-0033 (in the part of electronic structure and dielectric function calculations). E.V.C. acknowledges support from Saint Petersburg State University (Project ID No. 90383050). V.M.S. acknowledges financial support by Grant No. PID2019-105488GB-I00 funded by MCIN/AEI/10.13039/501100011033/

Formation of Surface and Quantum-Well States in Ultra Thin Pt Films on the Au(111) Surface

The electronic structure of the Pt/Au(111) heterostructures with a number of Pt monolayers n ranging from one to three is studied in the density-functional-theory framework. The calculations demonstrate that the deposition of the Pt atomic thin films on gold substrate results in strong modifications of the electronic structure at the surface. In particular, the Au(111) s-p-type Shockley surface state becomes completely unoccupied at deposition of any number of Pt monolayers. The Pt adlayer generates numerous quantum-well states in various energy gaps of Au(111) with strong spatial confinement at the surface. As a result, strong enhancement in the local density of state at the surface Pt atomic layer in comparison with clean Pt surface is obtained. The excess in the density of states has maximal magnitude in the case of one monolayer Pt adlayer and gradually reduces with increasing number of Pt atomic layers. The spin-orbit coupling produces strong modification of the energy dispersion of the electronic states generated by the Pt adlayer and gives rise to certain quantum states with a characteristic Dirac-cone shape.We acknowledge the Tomsk State University competitiveness programme (Project No. 8.1.01.2017) and partial support by the Saint Petersburg State University (Project No. 15.61.202.2015). I.V.S. acknowledges financial support from the Ministry of Education and Science of the Russian Federation within governmental program Megagrants (State Task No. 3.9003.2017/Pi 220 or 3.9003.2017/9.10). Y.M.K. acknowledges the Russian Foundation for Basic Research (Project No. 15-02-02717-a). E.V.C. acknowledges the Spanish Ministry of Science and Innovation (Grant No. FIS2016-75862-P). Calculations were performed at the SKIFCyberia supercomputer of Tomsk State University and at the Research park of St. Petersburg State University Computing Center (Russian Federation)

Superlattices of Gadolinium and Bismuth Based Thallium Dichalcogenides as Potential Magnetic Topological Insulators

Using relativistic spin-polarized density functional theory calculations we investigate magnetism, electronic structure and topology of the ternary thallium gadolinium dichalcogenides TlGdZ2 (Z= Se and Te) as well as superlattices on their basis. We find TlGdZ2 to have an antiferromagnetic exchange coupling both within and between the Gd layers, which leads to frustration and a complex magnetic structure. The electronic structure calculations reveal both TlGdSe2 and TlGdTe2 to be topologically trivial semiconductors. However, as we show further, a three-dimensional (3D) magnetic topological insulator (TI) state can potentially be achieved by constructing superlattices of the TlGdZ2/(TlBiZ2)n type, in which structural units of TlGdZ2 are alternated with those of the isomorphic TlBiZ2 compounds, known to be non-magnetic 3D TIs. Our results suggest a new approach for achieving 3D magnetic TI phases in such superlattices which is applicable to a large family of thallium rare-earth dichalcogenides and is expected to yield a fertile and tunable playground for exotic topological physics.M.M.O. and M.B. acknowledge the support by Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-103910GB-I00) and the University of the Basque Country (Grant no. IT1527-22). A.Yu.V. and E.K.P. acknowledge support from the Ministry of Education and Science of the Russian Federation within State Task No. FSWM-2020-0033 (in the part of bulk and surface electronic structure calculations). E.V.C. acknowledges support from Saint Petersburg State University (Grant ID No. 90383050). Yu.M.K. acknowledges support from the Government research assignment for ISPMS SB RAS, project FWRW-2022-0001 (in the part of the topological classification of bulk band structure)

Spin-orbit splitting of quantum well states in n -monolayer Ir/Au(111) heterostructures

The effect of spin-orbit coupling on quantum well states (QWSs) in atomically thin Ir adlayers deposited on the Au(111) substrate is studied in the framework of the density functional theory. Varying the Ir film thickness from 1 to 3 atomic layers, we find numerous Ir-derived QWSs, which are mainly of d character. The resulting band dispersion of QWSs appearing around the surface Brillouin zone center in a wide Au(111) energy gap is analyzed in the framework of the Rashba model. In all such QWSs, the fitted values of the Rashba parameter exceed 2eVÅ. The maximal value of 6.4eVÅ was obtained for the 1-monolayer-Ir/Au(111) system. We explain such large spin splitting by hybridization between different QWSs. Strong enhancement is observed in the density of electronic states at the surface in the energy region around the Fermi level caused by these QWSs.I.V.S. acknowledges the Ministry of Science and Higher Education of the Russian Federation for funding in framework of State Task (No. 0721-2020-0033). Y.M.K. acknowledges the Fundamental Research Program of the State Academies of Sciences, line of research III.23.2.9 and the Tomsk State University competitiveness improvement program (Project No. 8.1.01.2018). V.M.S. acknowledges funding from the Project of the Basque Government (Q-NANOFOT IT1164-19). E.V.C. acknowledges the Saint-Petersburg State University (Project No. 51126254). V.M.S. and E.V.C. acknowledge the Spanish Ministry of Economy, Industry and Competitiveness MINEICO (Projects Nos. FIS2016-76617-P and FIS2016-75862-P).Peer reviewe

New topological surface state in layered topological insulators: Unoccupied dirac cone

arXiv:1303.2472The unoccupied states in topological insulators Bi2Se3, PbSb2Te4, and Pb2Bi2Te2S3 are studied by the density functional theory methods. It is shown that a surface state with linear dispersion emerges in the inverted conduction band energy gap at the center of the surface Brillouin zone on the (0001) surface of these insulators. The alternative expression of ℤ2 invariant allowed us to show that a necessary condition for the existence of the second Γ Dirac cone is the presence of local gaps at the time reversal invariant momentum points of the bulk spectrum and change of parity in one of these points. © 2012 Pleiades Publishing, Ltd.This work was supported by grant of Government of the Russian Federation rergulation 220, contract # 11.G34.31.0028 (November 25, 2010).Peer Reviewe

Surface and quantum-well electronic states in ultra-thin Ir and Pt films on the Au(111) surface

Resumen del póster presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 25 de febrero al 10 de marzo de 2018.The heterostructures consisting of the noble metal substrates and the late d-metal adsorbates and other heavy atoms with the degree of coverage from one atom to several monolayers (MLs), have been actively studied for the last two decades. The interest to these systems can be explained by the possibility of exploiting them in chemical industry, namely, in heterogeneous catalysis. It is known that the properties of two-dimensional systems can drastically differ from the properties of their bulk counterparts. Moreover, frequently the degree of coverage plays a crucial role in these effects. An example of such situation is the Pt/Au(111) heterostructure with the Pt adsorbate thickness varying from one to several atomic layers. In general, favorable catalytic activity, resulting in strong enhancement in rates of certain oxidation reactions, of thin Pt films deposited on Au has been found. All of this is making this and similar systems very attractive for experimental and theoretical research. On the other hand, heterostructures containing as a substrate a heavy metal and several atoms/monolayers of metallic adsorbate demonstrate unique properties caused by spin-orbit interaction that may be attractive for spintronics, a research field experiencing nowadays explosive development. A well-known example is the Bychkov-Rashba splitting effect consisting in lifting of a spin degeneration in two-dimensional systems due to spin-orbit interaction. Large Bychkov-Rashba splitting is characteristic for surface states of both noble and late d metals such as Ir and Pt metals. Moreover the heterostructures based on light noble metals (Cu and Ag) and heavy metal adsorbate such as Bi and Pb have a giant spin-orbit splitting of surface states noticeably larger than in pristine materials. The main reason for this giant splitting is the occurrence of a local potential gradient at the surface of such heterostructures that does not exist inside the bulk materials with inversion symmetry. In this work we investigate the formation of the surface and quantum-well states in thin Ir and Pt films deposited on the Au(111) substrate. From comparison of the electronic structure of Ir(111), Pt(111), and Au(111) surfaces one can observe a strong mismatch between positions of the band gaps in the projected bulk electronic structures of these materials. As a result, the electrons related to iridium or platinum atoms are reflected from the gold substrate by its energy gaps. Together with scattering produced by the potential barrier from the vacuum side this introduces the necessary conditions for realization of the quantum-well states. Since the electronic states in Ir and Pt in the Fermi level vicinity are mainly of the d character they present strong localization at the surface. In turn, this produces strong modification in the density of states in the surface region. Studying Ir- and Pt-derived surface and quantum-well states at different regions of the surface Brillouin zone (SBZ) we find distinct formation character. We also investigate in these heterostructures the effect of spin-orbit interaction on the electronic states localized at the surface since despite many experimental and theoretical studies devoted to these systems with adsorbate thickness of several monolayers, the effect of the spin-orbit interaction on the electronic structure of these systems was not addressed.Peer reviewe

Acoustic Plasmons in Nickel and Its Modification upon Hydrogen Uptake

In this work, we study, in the framework of the ab initio linear-response time-dependent density functional theory, the low-energy collective electronic excitations with characteristic sound-like dispersion, called acoustic plasmons, in bulk ferromagnetic nickel. Since the respective spatial oscillations in slow and fast charge systems involve states with different spins, excitation of such plasmons in nickel should result in the spatial variations in the spin structure as well. We extend our study to NiHx with different hydrogen concentrations x. We vary the hydrogen concentration and trace variations in the acoustic plasmons properties. Finally, at x=1 the acoustic modes disappear in paramagnetic NiH. The explanation of such evolution is based on the changes in the population of different energy bands with hydrogen content variation

Cr-Containing Ferromagnetic Film–Topological Insulator Heterostructures as Promising Materials for the Quantum Anomalous Hall Effect

Two heterostructures consisting of a Bi2Se3 topological insulator substrate and a CrI3 or CrBi2Se4 ferromagnetic insulator thin film have been theoretically studied. The calculated band structure indicates that both heterostructures exhibit the quantum anomalous Hall effect with the splitting of the topological surface state of 19 and 92 meV for CrI3/Bi2Se3 and CrBi2Se4/Bi2Se3, respectively. It has been shown that the band gap is determined primarily by the degree of localization of the wavefunction of the topological state in the ferromagnetic film. This degree of localization depends on the height and width of the interface barrier between the substrate and ferromagnetic material. Both the height and width of the interface barrier in the CrBi2Se4/Bi2Se3 system are similar to the respective characteristics of the van der Waals barrier in bulk Bi2Se3

Quantum Plasmonics in Sub-Atom-Thick Optical Slots.

Publication status: PublishedWe show using time-dependent density functional theory (TDDFT) that light can be confined into slot waveguide modes residing between individual atomic layers of coinage metals, such as gold. As the top atomic monolayer lifts a few Å off the underlying bulk Au (111), ab initio electronic structure calculations show that for gaps >1.5 Å, visible light squeezes inside the empty slot underneath, giving optical field distributions 2 Å thick, less than the atomic diameter. Paradoxically classical electromagnetic models are also able to reproduce the resulting dispersion for these subatomic slot modes, where light reaches in-plane wavevectors ∼2 nm-1 and slows to <10-2c. We explain the success of these classical dispersion models for gaps ≥1.5 Å due to a quantum-well state forming in the lifted monolayer in the vicinity of the Fermi level. This extreme trapping of light may explain transient "flare" emission from plasmonic cavities where Raman scattering of metal electrons is greatly enhanced when subatomic slot confinement occurs. Such atomic restructuring of Au under illumination is relevant to many fields, from photocatalysis and molecular electronics to plasmonics and quantum optics