Angle-resolved photoemission spectroscopy study of the (101) surface of the Kondo insulator CeNiSn

The electronic structure of CeNiSn, which is considered a possible topological Kondo insulator, has been investigated by employing synchrotron radiation excited angle-resolved photoemission spectroscopy (ARPES). We have found that the easy cleavage plane in CeNiSn is (101), for which we have investigated the Fermi surface (FS) and band structures. The measured FS and ARPES for the (101) plane are described well by the calculated FS and band structures, obtained from the DFT calculations. The measured ARPES bands and photon energy map show that the metallic states crossing the Fermi level have the 3D nature, casting a negative suspicion for the existence of the topological surface states of the 2D character in CeNiSn. The Ce 4f Kondo resonance peak is observed in Ce 4d → 4f resonant photoemission spectroscopy, suggesting the importance of the Ce 4f electrons in determining the temperature-dependent topological electronic structure of CeNiSn.     

Determination of the surface states from the ultrafast electronic states in a thermoelectric material

We reveal the electronic structure in Yb Cd₂Sb₂,a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center,consistent with the density functional theory(DFT)calculation.It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right-and left-handed circularly polarized probe.In addition,a hole band of surface states,which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation,is identified.We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states.Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.     

First principles studies of the dependence of magnetism on the crystal phase in 4d and 5d late transition metals

We investigate the possibility of inducing ferromagnetic order in 4d and 5d late transition metals through crystal symmetry change. First principles, self-consistent density functional theory calculations, with spin-orbit coupling included, performed at 0 K show that ferromagnetism occurs in the bulk of Rh and Pd at the optimum lattice constant if Rh is in the bcc and Pd in the hcp/dhcp phase. The ferromagnetic order originates in the d-band occupancy of Rh or Pd which locates the Fermi energy at the top of the highest peak of the respective (paramagnetic) density of states induced by the bcc or hcp/dhcp structure. This peak in the density of states is caused by flat bands which lie at the surface of the respective Brillouin zone. For a bcc crystal these flat bands have the eg character and are positioned at the surface of the bcc Brillouin zone along the N-P line. The origin of the flatness of the bands was found to be the translation symmetry of the cubic lattice which causes the bands with the eg character to be narrow along the k-lines whose k-vector directions are furthest off the directions to which the orbitals of the eg symmetry point. Due to the d-band occupancy of Rh these flat bands lie in the paramagnetic state at the Fermi energy, whereas in the ferromagnetic state they exhibit the largest energetic split. This indicates that a smaller degree of orbital overlap narrows electronic bands enhancing the tendency of the system for ferromagnetic band split. For the hcp/dhcp structure the states contributing to the high density of para-magnetic states at the Fermi level of Pd lie in the vicinity of the M-L line of the hcp Brillouin zone boundary, which possesses a high number of symmetry (M and L) points. Moreover, the M-L line is aligned with the stacking sequence direction ([0001]) which is furthest off the densest-packed atomic chain direction of an hcp-crystal and, consequently, the weakest-bond direction in the crystal. This makes the narrow bands along the M-L line flat. The instability of the bcc and the meta-stability of the hcp crystal phase modifications for metals with native close-packed crystal structures is subsequently analysed in order to find whether they can be grown as films on suitable substrates.     

Electronic Structure of FeSe Monolayer Superconductors: Shallow Bands and Correlations

Electronic spectra of typical single FeSe layer superconductor—FeSe monolayer film on SrTiO₃ substrate (FeSe/STO) obtained from ARPES data reveal several puzzles: what is the origin of shallow and the so called “replica” bands near the M-point and why the hole-like Fermi surfaces near the Γ-point are absent. Our extensive LDA+DMFT calculations show that correlation effects on Fe-3d states can almost quantitatively reproduce rather complicated band structure, which is observed in ARPES, in close vicinity of the Fermi level for FeSe/STO. Rather unusual shallow electron-like bands around the M-point in the Brillouin zone are well reproduced. Detailed analysis of the theoretical and experimental quasiparticle bands with respect to their origin and orbital composition is performed. It is shown that for FeSe/STO system the LDA calculated Fe-3d _xy band, renormalized by electronic correlations within DMFT gives the quasiparticle band almost exactly in the energy region of the experimentally observed “replica” quasiparticle band at the Mpoint. However, correlation effects alone are apparently insufficient to eliminate the hole-like Fermi surfaces around the Γ-point, which are not observed in most ARPES experiments. The Fermi surfaces remain here even if Coulomb and/or Hund interaction strengths are increased while overall agreement with ARPES worsens. Increase of number of electrons also does not lead to vanishing of this Fermi surface and makes agreement of LDA+DMFT results with ARPES data much worse. We also present some simple estimates of “forward scattering” electron-optical phonon interaction at FeSe/STO interface, showing that it is apparently irrelevant for the formation of “replica” band in this system and significant increase of superconducting T _c .     

Surface-induced orbital-selective band reconstruction in kagome superconductor CsV3Sb5

The two-dimensional (2D) kagome superconductor CsV₃Sb₅ has attracted much recent attention due to the coexistence of superconductivity, charge orders, topology and kagome physics, which manifest themselves as distinct electronic structures in both bulk and surface states of the material. An interesting next step is to manipulate the electronic states in this system. Here, we report angle-resolved photoemission spectroscopy (ARPES) evidence for a surface-induced orbital-selective band reconstruction in CsV₃Sb₅. A significant energy shift of the electron-like band around Γ and a moderate energy shift of the hole-like band around M are observed as a function of time. This evolution is reproduced in a much shorter time scale by in-situ annealing of the CsV₃Sb₅ sample. Orbital-resolved density functional theory (DFT) calculations reveal that the momentum-dependent band reconstruction is associated with different orbitals for the bands around Γ and M, and the time-dependent evolution points to the change of sample surface that is likely caused by the formation of Cs vacancies on the surface. Our results indicate the possibility of orbital-selective control of the band structure via surface modification, which may open a new avenue for manipulating exotic phenomena in this material system, including superconductivity.     

角分辨光电子能谱技术及其应用进展

角分辨光电子能谱（ARPES）是研究晶体表面电子结构,如能带,费米面,以及多体相互作用的重要工具。本文概述了光电子激发的一般过程和单粒子近似下的理论模型。详细讨论了角分辨光电子能谱的能带勾画（Energy Band Mapping）和费米面成像（Fermi Surface Mapping）技术,以及高分辨下的角分辨光电子能谱在强相关体系研究中的应用。文章最后简单介绍了当前角分辨光电子能谱研究的新进展,如研究宽禁带半导体材料的表面电子结构,有机功能材料与金属的界面,金属超薄膜中的量子阱态,以及高温超导机理研究等。     

AB INITIO STUDY ON THE ELECTRONIC STRUCTURE AND MAGNETISM OF MnAs, MnSb, AND MnBi

We report the results of first-principles calculations on the electronic structure in ferromagnetic and non-magnetic hexagonal MnV (V=As, Sb, Bi). The calculations are based on the local-spin-density approximation (LSDA) of the density-functional theory (DFT) as well as the atomic sphere approximation (ASA) in the linear muffin-tin orbitals (LMTO) method. For the non-spin-polarized case, the calculated bands in these compounds exhibit p-d mixing in the vicnity of Fermi energy and the Mn 3d bands dominate the antibonding parts of p-d hybride. The spin-polarization in ferromagnetic states are mainly due to the splitting of anti-bonding bands from p-d mixing. The calculated spin moments in these compounds agree fairly well with experimental values and refine previous band calculations. In the spin-polarized band structure, the Mn 3d electrons are found to exhibit week dispersions.     

Intrinsic electron accumulation at clean InN surfaces

The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Gamma point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5 (+/-0.2)x10(13) cm(-2) and a surface Fermi level of 1.64+/-0.10 eV above the valence band maximum.     

Existence, character, and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe(2-x)Co(x)As2

Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe(2-x)Co(x)As2 (x = 0.1,0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe2-As block. As a result, surface-related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe(2-x)Co(x)As2 system for accurate determination of self-energies using ARPES.     

Quantum size effects on the (0001) surface of double hexagonal close packed americium

Electronic structures of double hexagonal close-packed americium and the (0001) surface have been studied via full-potential all-electron density-functional calculations with a mixed APW+lo/LAPW basis. The electronic and geometric properties of bulk dhcp Am as well as quantum size effects in the surface energies and the work functions of the dhcp Am (0001) ultra thin films up to seven layers have been examined at nonmagnetic, ferromagnetic, and antiferromagnetic configurations with and without spin orbit coupling. The anti-ferromagnetic state including spin-orbit coupling is found to be the ground state of dhcp Am with the 5f electrons primarily localized. Our results show that both magnetic configurations and spin-orbit coupling play important roles in determining the equilibrium lattice constant, the bulk modulus as well as the localized feature of 5f electrons for dhcp Am. Our calculated equilibrium lattice constant and bulk modulus at the ground state are in good agreement with the experimental values respectively. The work function of dhcp Am (0001) 7-layer surface at the ground state is predicted to be 2.90 eV. The surface energy for dhcp Am (0001) semi-infinite surface energy at the ground state is predicted to be 0.84 J/m². Quantum size effects are found to be more pronounced in work functions than in surface energies.     

Ab initio study of platinum induced reconstructions on Ge(001)-(1×2) surface with dimerization

We present first principle total energy calculation of Pt induced reconstructions on Ge(001)-(1×2) surface with dimerization. Study was undertaken using localized orbitals basis set DFT using SIESTA to compare pure Ge dimerized Ge(001)-(1×2) surface with 0.5 and 1.0 Pt covered dimerized Ge(001)-(1×2) surface with the possibility of homo (Ge-Ge and Pt-Pt) and hetro (Pt-Ge) dimers. From total energy calculation results we calculated dimer bond lengths, buckling angles and formation energy of dimers on Ge(001)-(1×2) surface. By calculating the formation energy of different configurations we find that Ge-Ge buckled dimerized surface has least (−1.23 eV/dimer) and Pt-Pt symmetric dimerized surface has largest (+0.09 eV/dimer) formation energy with respect to unreconstructed surface. We further calculated the electronic DOS and band structure of Ge dimerized as well as Pt dimerized surface to see the change in semiconducting behavior on dimerization. By comparing the DOS and electronic band structure of homo Ge dimerized surface, we found metallicity of Ge(001)-(1×2) surface results from dimer formation. Also by comparing the electronic band structure of homo Ge dimerized surface with unreconstructed surface we find that less number of bands crossing the Fermi level which is perhaps due to the saturation of one dangling bond per Ge surface atom. By introducing Pt at 0.5 and 1.0 coverage in place of Ge, except for homo Pt buckled dimerized surface having 1.0 coverage of Pt, we find in all other cases increase in number of bands are crossing the Fermi level, indicating strong metallic behavior of Ge(001)-(1×2) surface.     

Insight into the f-derived Fermi surface of the heavy-fermion compound YbRh2Si2

Angle-resolved photoelectron spectroscopy (ARPES) was used to study the Fermi surface of the heavy-fermion system YbRh(2)Si(2) at a temperature of about 10 K, i.e., a factor of 2 below the Kondo energy scale. We observed sharp structures with a well-defined topology, which were analyzed by comparing with results of band-structure calculations based on the local-density approximation (LDA). The observed bulk Fermi surface presents strong similarities with that expected for a trivalent Yb state, but is slightly larger, has a strong Yb-4f character, and deviates from the LDA results by a larger region without states around the Γ point. These properties are qualitatively explained in the framework of a simple f-d hybridization model. Our analysis highlights the importance of taking into account surface states and doing an appropriate projection along k(z) when comparing ARPES data with results from theoretical calculations.     

Quantized electron accumulation states in indium nitride studied by angle-resolved photoemission spectroscopy

Electron accumulation states in InN have been measured using high resolution angle-resolved photoemission spectroscopy (ARPES). The electrons in the accumulation layer have been discovered to reside in quantum well states. ARPES was also used to measure the Fermi surface of these quantum well states, as well as their constant binding energy contours below the Fermi level E(F). The energy of the Fermi level and the size of the Fermi surface for these quantum well states could be controlled by varying the method of surface preparation. This is the first unambiguous observation that electrons in the InN accumulation layer are quantized and the first time the Fermi surface associated with such states has been measured.     

On the origin of the shallow and “replica” bands in FeSe monolayer superconductors

We compare the electronic structures of single FeSe layer films on SrTiO₃ substrate (FeSe/STO) and K _x Fe_2-y Se₂ superconductors obtained from extensive LDA and LDA + DMFT calculations with the results of ARPES experiments. It is demonstrated that correlation effects on Fe-3d states are sufficient in principle to explain the formation of the shallow electron-like bands at the M(X)-point. However, in FeSe/STO these effects alone are apparently insufficient for the simultaneous elimination of the hole-like Fermi surface around the Γ-point which is not observed in ARPES experiments. Detailed comparison of ARPES detected and calculated quasiparticle bands shows reasonable agreement between theory and experiment. Analysis of the bands with respect to their origin and orbital composition shows, that for FeSe/STO system the experimentally observed “replica” quasiparticle band at the M-point (usually attributed to forward scattering interactions with optical phonons in SrTiO₃ substrate) can be reasonably understood just as the LDA calculated Fe-3d _xy band, renormalized by electronic correlations. The only manifestation of the substrate reduces to lifting the degeneracy between Fe-3d _xz and Fe-3d _yz bands near M-point. For the case of K _x Fe_2-y Se₂ most bands observed in ARPES can also be understood as correlation renormalized Fe-3d LDA calculated bands, with overall semi-quantitative agreement with LDA + DMFT calculations.     

Pd对O吸附在ZnO（0001）面上的影响的第一性原理研究

本文用第一性原理方法计算了Pd 在ZnO(0001)面上的吸附、Pd对O吸附的影响及Pd替代表面Zn原子能量的变化.结果表明：(1) Pd的吸附位置不随覆盖度变化,Pd稳定吸附位为H3位;(2)Pd在1/4单层吸附时比1个单层吸附时稳定;(3)Pd的存在增强了氧在ZnO(0001)面上的吸附,O原子可以扩散到Pd吸附层的下,Pd处于最上面, 具有催化作用.     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

First-principles study of high-pressure phase transformations in LaBi

An investigation into the structural stability and the electronic properties of LaBi under high pressure was conducted using first-principles calculations based on density functional theory (DFT), in the presence and absence of spin–orbit coupling (SOC). Our results demonstrate that there exists a structural phase transition from the NaCl-type (B1) structure to a primitive tetragonal (PT) structure at the transition pressure of 11.2 GPa (without SOC) and 12.9 GPa (with SOC). The chemical bond between La and Bi is mainly ionic. The band structure shows that B1-LaBi is metallic. A pseudogap appears around the Fermi level of the total density of states (DOS) of the B1 phase of LaBi, which may contribute to its stability.     

Band edge singularities and the surface density of states in Nb (110)

Photoemission measurements of the Nb(110) surface show surface sensitive structure near the Fermi energy which can be attributed to bulk band structure effects persisting right out to the surface layer, in agreement with recent layer density of states calculations for Mo(110).     

硼烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理的方法计算H修饰边缘不同宽度硼稀纳米带的电荷密度、电子能带结构、总态密度和分波态密度。结果表明,硼烯纳米带的宽度大小影响着材料的导电性能,宽度5的硼烯纳米带是间接带隙简并半导体,带隙值为0.674 eV,而宽度7的硼烯纳米带却具有金属材料的性质。分波态密度表明,宽度5的硼烯纳米带的费米能级附近主要是由B-2s、2p电子态贡献,H-1s主要贡献于下价带且具有局域性,消除了材料边缘的不稳定性。宽度7的B-2p和H-1s电子态贡献的导带和价带处于主导地位,费米能级附近B-2p和H-1s电子态的杂化效应影响材料的整体发光性能。     

Scanning tunneling microscopy chemical signature of point defects on the MoS2(0001) surface

Using ab initio calculations, we have studied the modification of the electronic structure of the MoS2(0001) surface by several point defects: a surface S vacancy and different transition metal atoms substituting a S atom (Pd, Au, Fe, and V). With a S vacancy, a gap state appears with weight mostly on the Mo and S atoms surrounding the vacancy. The substitutional atoms of complete d band (Pd and Au) do not present magnetic polarization and slightly modify the DOS near the Fermi energy. On the other hand, the incomplete d band atoms (Fe and V) present spin polarization and modify significantly the states near the band edges. From calculated STM images and STS curves, we show that this chemical signature can be measured and used to characterize the surface defects of the substrate which are suitable nucleation centers for nanocluster growth.