Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ～1 and ～10 nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)?Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.     

Angle-resolved photoemission study of the cobalt oxide superconductor Na(x)CoO(2) x yH(2)O: observation of the Fermi surface

The cobalt oxide superconductor Na(x)CoO(2) x yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E(F)) in the normal state of Na(x)CoO(2) x yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to Na(x)CoO(2). The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within approximately 30 meV below E(F), closer to E(F) than in Na(x)CoO(2). We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.     

Raising Bi-O bands above the Fermi energy level of hole-doped Bi2Sr2CaCu2O8+delta and other cuprate superconductors

The Fermi surface (FS) of Bi2Sr2CaCu2O8+delta (Bi2212) predicted by band theory displays Bi-related pockets around the (pi, 0) point, which have never been observed experimentally. We show that when the effects of hole doping either by substituting Pb for Bi or by adding excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi energy (E(F)) and the resulting first-principles FS is in remarkable accord with measurements. With decreasing hole doping the Bi-O bands drop below and the system self-dopes below a critical hole concentration. Computations on other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the cation-derived band with hole doping is a general property of the electronic structures of the cuprates.     

Valence surface electronic states on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.     

Quasiparticle interference of C2-symmetric surface states in a LaOFeAs parent compound

We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. High resolution spectroscopic imaging reveals strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. Fourier analysis shows that the distribution of scattering wave vectors exhibits pronounced twofold (C(2)) symmetry, strongly reminiscent of the nematic electronic state found in CaFe(1.94)Co(0.06)As(2). The implications of these results to the electronic structure of the pnictide parent states will be discussed.     

Coherent and Incoherent Excitations of Electron-Doped SrTiO3

Resonant photoemission at the Ti 2p and O 1s edges on a Nb-doped SrTiO(3) thin film revealed that the coherent state (CS) at the Fermi level (E(F)) had a mainly Ti 3d character whereas the incoherent in-gap state (IGS) positioned approximately 1.5 eV below E(F) had a mixed character of Ti 3d and O 2p states. This indicates that the IGS is formed by a spectral-weight transfer from the CS and subsequent spectral-weight redistribution through d-p hybridization. We discuss the evolution of the excitation spectrum with 3d band filling and rationalize the IGS through a mechanism similar to that proposed by Haldane and Anderson.     

Direct observation of the mass renormalization in SrVO3 by angle resolved photoemission spectroscopy

Band dispersions and Fermi surfaces of the three-dimensional Mott-Hubbard system SrVO3 are directly observed by angle-resolved photoemission spectroscopy. An observed spectral weight distribution near the Fermi level (E(F)) shows cylindrical Fermi surfaces as predicted by band-structure calculations. By comparing the experimental results with calculated surface electronic structures, we conclude that the obtained band dispersion reflects the bulk electronic structure. The enhanced effective electron mass obtained from the energy band near E(F) is consistent with the bulk thermodynamic properties and hence with the normal Fermi-liquid behavior of SrVO3.     

Strain and high temperature superconductivity: unexpected results from direct electronic structure measurements in thin films

Angle-resolved photoemission spectroscopy reveals very surprising strain-induced effects on the electronic band dispersion of epitaxial La(2-x)Sr(x)CuO(4-delta) thin films. In strained films we measure a band that crosses the Fermi level (E(F)) well before the Brillouin zone boundary. This is in contrast to the flat band reported in unstrained single crystals and in our unstrained films, as well as in contrast to the band flattening predicted by band structure calculations for in-plane compressive strain. In spite of the density of states reduction near E(F), the critical temperature increases in strained films with respect to unstrained samples. These results require a radical departure from commonly accepted notions about strain effects on high temperature superconductors, with possible general repercussions on superconductivity theory.     

Spin-polarized electron tunneling in bcc FeCo/MgO/FeCo(001) magnetic tunnel junctions

In combining spin- and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe(1-x)Co(x)(001)/MgO/Fe(1-x)Co(x)(001) magnetic tunnel junctions. Contrary to previous theoretical predictions, we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic% as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with Δ(1) symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe(1-x)Co(x)(001) free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system.     

Effect of atomic impurities on the helical surface states of the topological insulator Bi2Te3

The effect of atomic impurities including N, O, Na, Ti and Co on the surface states of the topological insulator (TI) Bi(2)Te(3) is studied using pseudopotential first principles methods. The robustness of the TI surface states is particularly investigated against magnetic and non-magnetic atomic adsorption by calculating the electronic band structure, charge transfer, and magnetic moments. Interestingly, it is found that a non-magnetic nitrogen atom has produced a residual magnetic moment and opens a gap in the surface states whereas Na and O atoms preserve the Dirac-like dispersion. The charge transfer from the adatoms produces an electric dipole field that causes Rashba splitting in the surface bands. For atomic impurities with 3d orbitals (Ti and Co), the TI surface states are destroyed and two spin-resolved resonance peaks are developed near the Fermi level in the DOS.     

Spatial chemical inhomogeneity and local electronic structure of Mn-doped Ge ferromagnetic semiconductors

We have investigated the chemical distributions and the local electronic structure of potential diluted magnetic semiconductor Ge0.94Mn0.06 single crystals using scanning photoelectron microscopy (SPEM), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The SPEM image shows the stripe-shaped microstructures, which arise from the chemical phase separation between the Mn-rich and Mn-depleted phases. The Mn 2p XAS shows that the Mn ions in the Mn-rich region are in the divalent high-spin Mn2+ states but that they do not form metallic Mn clusters. The Mn 3d PES spectrum exhibits a peak centered at approximately 4 eV below E(F) and the negligible spectral weight near E(F). This study suggests that the observed ferromagnetism in Ge1-xMnx arises from the phase-separated Mn-rich phase.     

Surface-state depopulation on small Ag(111) terraces

The dependence of the local density of states near the Fermi energy E(F) on the width of terraces T is investigated by tunneling scanning spectroscopy on Ag(111) at 7 K. With decreasing T, the electronic density in the occupied surface state shifts monotonically towards E(F), leading to a depopulation at T=3.2 nm in quantitative agreement with a Fabry-Pérot model. Depopulation coincides with a switch from confinement by terrace modulation to step modulation.     

Topological surface states of Bi2Te2Se are robust against surface chemical modification

The robustness of the Dirac‐like electronic states on the surfaces of topological insulators (TIs) during materials process‐ing is a prerequisite for their eventual device application. Here, the (001) cleavage surfaces of crystals of the topological insulator Bi₂Te₂Se (BTS) were subjected to several surface chemical modification procedures that are common for electronic materials. Through measurement of Shubnikov–de Hass (SdH) oscillations, which are the most sensitive measure of their quality, the surface states of the treated surfaces were compared to those of pristine BTS that had been exposed to ambient conditions. In each case – surface oxidation, deposition of thin layers of Ti or Zr oxides, or chemical modification of the surface oxides – the robustness of the topological surface electronic states was demonstrated by noting only very small changes in the frequency and amplitude of the SdH oscillations. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Density functional study of LaFeAsO(1-x)F(x): a low carrier density superconductor near itinerant magnetism

Density functional studies of 26 K superconducting LaFeAs(O,F) are reported. We find a low carrier density, high density of states, N(E(F)), and modest phonon frequencies relative to T(c). The high N(E(F)) leads to proximity to itinerant magnetism, with competing ferromagnetic and antiferromagnetic fluctuations and the balance between these controlled by the doping level. Thus LaFeAs(O,F) is in a unique class of high T(c) superconductors: high N(E(F)) ionic metals near magnetism.     

Topological phases in graphitic cones

The electronic structure of graphitic cones exhibits distinctive topological features associated with the apical disclinations. Ahranov-Bohm magnetoconductance oscillations (period Phi(0)) are completely absent in rings fabricated from cones with a single pentagonal disclination. Close to the apex, the local density of states changes qualitatively, either developing a cusp which drops to zero at the Fermi energy, or forming a region of nonzero density across E(F), a local metallization of graphite.     

Quantum interference of Rashba-type spin-split surface state electrons

We studied the quantum interference of electrons in the Bi (p(x), p(y)) orbital-derived j = 1/2 spin-split surface states at Bi/Ag(111)√3 × √3 surfaces of 10 monolayer thick Ag(111) films on Si(111) substrates. Surface electron standing waves were observed clearly at the energy (E) below the intersection of the two spin-split downward dispersing parabola bands (E(x)). The E dependence of the standing wave pattern reveals the dispersion as the average of the two spin-split surface bands due to the interference between |(k + Δ), ↑> and |-(k - Δ), ↑> [or (|(k - Δ), ↓>) and |-(k + Δ), ↓>] states. In contrast, it was impossible to deduce the dispersion from the standing wave pattern at E ≥ E(x) because the surface electron cannot find its backscattered state with the same spin polarization.     

超精细能级中85Rb激发态(｜52P1/2F>、｜52P3/2F>)原子与理想金属表面间范德瓦尔斯作用系数C3的估算

利用碱金属原子与理想金属表面间范德瓦尔斯(vdW)作用势和不可约张量方法,首次计算了85Rb激发态52P1/2 (F=2,3)及52P3/2 (F=1,2,3,4) 原子超精细结构的C3系数.数值分别对应为:4.0542 、4.0553 及4.3012 、4.3312 、4.3369 、4.3104 .为了说明其结果的准确性,还与其他作者的理论数据和相关实验数据进行了比较,结果表明本文所得到的85Rb激发态52P1/2 (F=2,3)及52P3/2(F=1,2,3,4)原子的C3数值是可靠的     

超精细能级中85Rb激发态(｜52P1/2F>、｜52P3/2F>)原子与理想金属表面间范德瓦尔斯作用系数C3的估算

利用碱金属原子与理想金属表面间范德瓦尔斯(vdW)作用势和不可约张量方法,首次计算了85Rb激发态52P1/2 (F=2,3)及52P3/2 (F=1,2,3,4) 原子超精细结构的C3系数.数值分别对应为:4.0542 、4.0553 及4.3012 、4.3312 、4.3369 、4.3104 .为了说明其结果的准确性,还与其他作者的理论数据和相关实验数据进行了比较,结果表明本文所得到的85Rb激发态52P1/2 (F=2,3)及52P3/2(F=1,2,3,4)原子的C3数值是可靠的     

超精细能级中133CS激发态(｜62D3/2F>)原子与理想金属表面间范德瓦尔斯作用系数C3的估算

利用碱金属原子与理想金属表面间范德瓦尔斯(vdW)作用势和不可约张量方法,首次计算了133CS激发态62D3/2 (F=2,3,4,5)原子超精细结构的C3系数.数值分别对应为:17.9956 、18.0796 、18.0857 及18.1756 .为了说明其结果的准确性,还与其他作者的理论数据和相关实验数据进行了比较,结果表明本文所得到的133CS激发态62D3/2 (F=2,3,4,5)原子的C3数值是可靠的.     

超精细能级中~(133)C_S激发态(|6~2D_(3/2)F>)原子与理想金属表面间范德瓦尔斯作用系数C_3的估算

利用碱金属原子与理想金属表面间范德瓦尔斯(vd W)作用势和不可约张量方法,导出超精细能级中碱金属原子特定量子态C3的解析表达式,完成133CS原子62D3/2(F=2,3,4,5)的C3数值计算,计算结果表明随着的增大,C3增大,z(原子与金属表面之间的距离)不变时,铯原子与理想金属表面间vd W作用增强.同时对于由电四极矩作用产生的C31效果因贡献不显著,C3数值结果并未见到有异常波动现象.