K-shell double core-hole spectroscopy in molecules

A great deal of attention has been devoted in the last few years to photoionization processes in isolated molecules leading to the formation of double core-hole (DCH) states. There are two main experimental avenues to induce such processes, namely single-photon absorption followed by the simultaneous ejection of two core electrons, and x-ray-induced multiphoton processes leading to the production of DCH states via the sequential absorption of two soft x- ray photons on a time scale on the order of the molecular Auger lifetime (4–8 femtoseconds for light elements). The formation of molecular two-site (ts) DCH states, in particular, shows great potential as a powerful tool for chemical analysis. A compelling motivation for the study of ts-DCH states is their ability to probe the local chemical environment more sensitively than either single core-hole (SCH) or single-site (ss) DCH states. The enhanced sensitivity originates from the fact that the double ionization potential (DIP) of ts-DCH states is directly coupled to induced changes in the valence charge distribution at the two different atomic sites. Here a review of the recent literature is presented on both types of experiments, and on the related theoretical work.     

Evidence for suppressed screening on the surface of high temperature La(2-x)SrxCuO4 and Nd2(2-x)CexCuO4 superconductors

Hard x-ray photoemission spectroscopy (PES) of Cu core electronic states, with a probing depth of approximately 60 A, is used to show that the Zhang-Rice singlet feature is present in La2CuO4 but is absent in Nd2CuO4. Hole and electron doping in La(2-x)SrxCuO4 (LSCO) and Nd(2-x)CexCuO4 (NCCO) result in new well-screened features which are missing in soft x-ray PES. Impurity Anderson model calculations establish screening from doped states as its origin, which is strongly suppressed within 15 A of the surface. Complemented with x-ray absorption spectroscopy, the small chemical-potential shift in core levels (approximately 0.2 eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap (approximately 1 eV) upon hole and electron doping in LSCO and NCCO.     

Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering

We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T(S)=365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ～1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.     

Calculation of the one- and two-loop lamb shift for arbitrary excited hydrogenic states

General expressions for quantum electrodynamic corrections to the one-loop self-energy [of order alpha(Zalpha)6] and for the two-loop Lamb shift [of order alpha2(Zalpha)6] are derived. The latter includes all diagrams with closed fermion loops. The general results are valid for arbitrary excited non-S hydrogenic states and for the normalized Lamb shift difference of states, defined as Delta N = n3deltaE(nS) - delta E(1S). We present numerical results for one-loop and two-loop corrections for excited S, P, and D states. In particular, the normalized Lamb shift difference of states is calculated with an uncertainty of order 0.1 kHz.     

锂原子里德伯态能级的精细结构分裂

对锂原子的四体问题的哈密顿算符进行幂级数展开,通过计算原子实的 极化(包括偶极矩,四极矩至八级电极矩)对能级的贡献,同时考虑相对论效应和LS耦合以 及有效势的高阶修正,求得了锂原子里德伯态的能级位移和能级公式。用此公式计算N=5～1 2,L=4～9,J=L±1/2的相关的精细结构位移以及能级间隔,并与有关实验结果进行了比较 。     

Migration of weakly bonded oxygen atoms in a-IGZO thin films and the positive shift of threshold voltage in TFTs

Amorphous indium-gallium-zinc oxide (a-IGZO) thin films are prepared by pulsed laser deposition and fabricated into thin-film transistor (TFT) devices. In-situ x-ray photoelectron spectroscopy (XPS) illustrates that weakly bonded oxygen (O) atoms exist in a-IGZO thin films deposited at high O₂ pressures, but these can be eliminated by vacuum annealing. The threshold voltage (V_th) of the a-IGZO TFTs is shifted under positive gate bias, and the V_th shift is positively related to the deposition pressure. A temperature variation experiment in the range of 20 K-300 K demonstrates that an activation energy of 144 meV is required for the V_th shift, which is close to the activation energy required for the migration of weakly bonded O atoms in a-IGZO thin films. Accordingly, the V_th shift is attributed to the acceptor-like states induced by the accumulation of weakly bonded O atoms at the a-IGZO/SiO₂ interface under positive gate bias. These results provide an insight into the mechanism responsible for the V_th shift of the a-IGZO TFTs and help in the production of reliable designs.     

Influence of Bipartite Qubit Coupling on Geometric Phase

The geometric phase of the bipartite Heisenberg spin-1/2 system with one spin driven by rotating magnetic field is investigated. It is found that in the one-site drive case, the intersubsystem coupling can be equivalent to a static quasi-magnetic field in the parameter space. This perspective has satisfactorily explained the irregular asymptote effect of geometric phase. We discuss the property of the two-site magnetic drive spin system and discover that a stationary state with no geometric phase shift is generated.     

The fine structure splitting of the level of lithium in Rydberg states

The Hamiltonian of the four-body problem for a lithium atom is expanded in series. The level shift and level formula of a lithium atom in Rydberg states are achieved by means of the calculation of polarization of the atomic core (including the contribution of dipole, quadrupole and octupole components). We also consider the effect of relativity theory, the orbital angular momentum L and the spin angular momentum S coupling scheme (LS coupling) and high-order correction of the effective potential to the level shift. The fine structure splitting (N=5-12, L=4-9, J=L±1/2) and level intervals in Rydberg states have been calculated by the above-mentioned formula and compared with recent experimental data.     

Influence of Bipartite Qubit Coupling on Geometric Phase

The geometric phase of the bipartite Heisenberg spin-1/2 system with one spin driven by rotating magnetic field is investigated. It is found that in the one-site drive case, the intersubsystem coupling can be equivalent to a static quasi-magnetic field in the parameter space. This perspective has satisfactorily explained the irregular asymptote effect of geometric phase. We discuss the property of the two-site magnetic drive spin system and discover that a stationary state with no geometric phase shift is generated.     

Effect of nitrogen doping on optical properties and electronic structures of SrZrO3 films

The effect of nitrogen doping on optical properties and electronic structures of SrZrO₃ (SZO) films was investigated both experimentally and theoretically. A shift of the absorption edge to the longer wavelength, i.e., the narrowing of the band gap caused by nitrogen doping was observed. The first-principles calculations revealed that the band gap reduced in nitrogen-doped SZO because of the localized N 2p states above O 2p states and the bottom of conduction band of Zr 4d shifting to a lower energy.     

X-Ray photoemission study of CuIr2S4: Ir3+-Ir4+ charge ordering and the effect of light illumination

We have studied the electronic structure of the spinel-type compound CuIr2S4 using x-ray photoemission spectroscopy (XPS). CuIr2S4 undergoes a metal-insulator transition (MIT) at approximately 226 K. In going from the metallic to insulating states, the valence-band photoemission spectrum shows a gap opening at the Fermi level and a rigid-band shift of approximately 0.15 eV. In addition, the Ir 4f core-level spectrum is dramatically changed by the MIT. The Ir 4f line shape of the insulating state can be decomposed into two contributions, consistent with the charge disproportionation of Ir3+:Ir4+=1:1. XPS measurements under laser irradiation indicate that the charge disproportionation of CuIr2S4 is very robust against photo-excitation in contrast to Cs2Au2Br6 which shows photo-induced valence transition.     

Irradiation-induced magnetism in carbon nanostructures

Nitrogen (15N) and carbon (12C) ion implantations with implant energy of 100 keV for different doses were performed on nanosized diamond (ND) particles. Magnetic measurements on the doped ND show ferromagnetic hysteresis behavior at room temperature. The saturation magnetization (M(s)) in the case of 15N implanted samples was found to be higher compared to the 12C implanted samples for dose sizes greater than 10(14) cm(-2). The role of structural modification or defects along with the carbon-nitrogen (C-N) bonding states for the observed enhanced ferromagnetic ordering in 15N doped samples is explained on the basis of x-ray photoelectron spectroscopy measurements.     

Surface phase transition in H/W(110) induced by tuning the fermi surface nesting vector by hydrogen loading

At a hydrogen coverage of one monolayer, W(110) is known to exhibit a Fermi nesting in its electronic surface states with a nesting vector q{N} of 0.9 A{-1} along [001]. Here we show that additional H adsorption allows a controlled tuning of q{N}. As q{N} approaches the commensurate value of 1.0 A{-1}, its signature in inelastic He-atom scattering becomes more pronounced, finally disappearing as a surface charge density wave (CDW) develops and the surface symmetry changes from c(2 x 2) to a p(8 x 2) superstructure. The gradual change in q{N} is attributed to an energetic shift of the spin-polarized electronic surface states that eventually form the surface CDW.     

Bosonic amplification of noise-induced suppression of phase diffusion

We study the effect of noise-induced dephasing on collisional phase diffusion in the two-site Bose-Hubbard model. Dephasing of the quasimomentum modes may slow down phase diffusion in the quantum Zeno limit. Remarkably, the degree of suppression is enhanced by a bosonic factor of order N/logN as the particle number N increases.     

Total-reflection inelastic X-ray scattering from a 10-nm thick La0.6Sr0.4CoO3 thin film

To study equilibrium changes in composition, valence, and electronic structure near the surface and into the bulk, we demonstrate the use of a new approach, total-reflection inelastic x-ray scattering, as a sub-keV spectroscopy capable of depth profiling chemical changes in thin films with nanometer resolution. By comparing data acquired under total x-ray reflection and penetrating conditions, we are able to separate the O K-edge spectra from a 10 nm La0.6Sr0.4CoO3 thin film from that of the underlying SrTiO3 substrate. With a smaller wavelength probe than comparable soft x-ray absorption measurements, we also describe the ability to easily access dipole-forbidden final states, using the dramatic evolution of the La N4,5 edge with momentum transfer as an example.     

Regional versus global entanglement in resonating-valence-bond states

We investigate the entanglement properties of resonating-valence-bond states on two and higher dimensional lattices, which play a significant role in our understanding of various many-body systems. We show that these states are genuinely multipartite entangled, while there is only a negligible amount of two-site entanglement. We comment on possible physical implications of our findings.     

Relationships between geometrical and electronic structures and optical properties of 1,8-naphthosultam substituents and derivatives: TDDFT study

Geometrical and electronic structure and optical properties of several substituents and derivatives of 1,8-naphthosultam are studied by quantum-chemical DFT and TDDFT. It is found that the substituents –NO₂, –CF₃, and –N(CH₃)₂ affect the geometrical and electronic structure the most. It is shown that the Stokes shift is greatest (≈260 nm) for compounds with the strong donor substituent –N(CH₃)₂, while strong acceptor substituents provide a quite small Stokes shift. The dependence of the Stokes shift on the difference in energies of the frontier orbitals of the ground and excited states of molecules is found. Of the 1,8-naphthosultam substituents considered, compounds with –N(CH₃)₂ substituent, which emit in the biological window region, can be advised for use in optical bioimaging. The results can be used as a basis for the development and creation of new functional materials and biologically active compounds.     

双势阱中冷原子的关联隧穿

研究了在不同的相互作用和隧穿耦合强度比值情况下双势阱中冷原子的隧穿现象.两格点Bose-Hubbard模型不能完全解释实验.为此,对此模型做了高阶修正.取双势阱的基态和第一激发态作基矢,写出哈密顿量的表示,解释了用两格点Bose-Hubbard模型分析实验时参数不能定量符合的原因.另外,通过计算两阱中原子纠缠度的变化,结合三态上原子布居数分布,直观描述了整个隧穿过程.     

双势阱中冷原子的关联隧穿

研究了在不同的相互作用和隧穿耦合强度比值情况下双势阱中冷原子的隧穿现象.两格点Bose-Hubbard模型不能完全解释实验.为此,对此模型做了高阶修正.取双势阱的基态和第一激发态作基矢,写出哈密顿量的表示,解释了用两格点Bose-Hubbard模型分析实验时参数不能定量符合的原因.另外,通过计算两阱中原子纠缠度的变化,结合三态上原子布居数分布,直观描述了整个隧穿过程.