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.     

典型磁性材料价电子结构研究面临的机遇与挑战

目前在磁性材料磁有序现象研究中广泛使用的交换作用、超交换作用和双交换作用模型形成于1950年代及其以前,这些模型都涉及材料中的价电子状态,但那时还没有充分的价电子状态实验依据.1970年代以来,有关价电子结构实验结果的报道越来越多,这些实验结果表明传统的磁有序模型需要改进.首先,大量电子谱实验表明,在氧化物中除存在负二价氧离子之外,还存在负一价氧离子,并且负一价氧离子的含量可达30%或更多.这说明以所有氧离子都是负二价离子为基本假设的超交换和双交换作用模型需要改进.其次,一些实验证明,铁、钴、镍自由原子的一部分4s电子在形成铁磁性金属的过程中变成了3d电子,这为探讨金属磁性与电输运性质的关系提供了依据.此外,即使在现代的密度泛函计算中,仍不能给出磁性交换作用能的函数表达式,只能采取各种不同模型进行模拟计算,从而使磁性材料的模拟计算遇到严重困难.寻求一个磁有序能的函数表达式可能是解决这个困难的途径.这些研究表明磁性材料价电子结构研究面临着重大的机遇与挑战.本文首先介绍一些典型的实验例证,然后介绍了基于这些实验结果的一套典型磁性材料的磁有序新模型,随后介绍了基于新模型的磁性材料价电子结构与旧模型的主要区别,最后指出了未来研究工作面临的挑战.     

Observation of multiple charge density wave phases in epitaxial monolayer 1T-VSe2 film

As a special order of electronic correlation induced by spatial modulation, the charge density wave (CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning—tunneling microscopy in various temperatures, we discover a hidden incommensurate stripe-like CDW order besides the ($sqrt{7}$ × $sqrt{3}$) CDW phase at low-temperature of 4 K in the epitaxial monolayer 1T-VSe₂} film. Combining the variable-temperature angle-resolved photoemission spectroscopic (ARPES) measurements, we discover a two-step transition of an anisotropic CDW gap structure that consists of two parts Δ₁ and Δ₂. The gap part Δ₁ that closes around ～ 150 K is accompanied with the vanish of the ($sqrt{7}$ × $sqrt{3}$) CDW phase. While another momentum-dependent gap part Δ₂ can survive up to ～ 340 K, and is suggested to the result of the incommensurate CDW phase. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure Δ₁ + Δ₂, which suggests different forming mechanisms between the ($sqrt{7}$ × $sqrt{3}$) and the incommensurate CDW phases. Our findings provide significant information and deep understandings on the CDW phases in monolayer 1T-VSe₂} film as a two-dimensional (2D) material.     

The role of 3d electrons in the appearance of ferromagnetism in the antiferromagnetic Ru2MnGe Heusler compound: a magnetic Compton scattering study

The antiferromagnetism in Ru(2)MnGe can be suppressed by the substitution of V by Mn and ferromagnetism appears. Synchrotron-based magnetic Compton scattering experiments are used in order to investigates the role of 3d electrons in the indirect/direct exchange interactions for the appearance of ferromagnetism. A small spin moment for the itinerant electron part on the magnetic Compton profile indicates that the metallic ferromagnet Ru(2)Mn(0.5)V(0.5)Ge has a weak indirect exchange interaction between the d-like and sp-like (itinerant) electrons. This suggests that the appearance of ferromagnetism is caused by the enhancement of the direct exchange interactions between d-d electrons in the Ru(2)MnGe Heusler compound. These findings indicate that the indirect exchange interaction between itinerant electrons and localized electrons is a significant key point for the appearance of ferromagnetism in this system.     

Kinks and waterfalls in the dispersion of ARPES features

In this paper we use a generic form for the Green function G(k, ω) in a correlated metal, already proven successful in describing ARPES line shapes [1]. The associated many body self-energy function has only a single pole. We now investigate, whether this generic model can be used all the way to the limit of strong correlations and, when applied to ARPES intensities, whether it is able to explain some of the ubiquitous dispersive crossover phenomena that have been attributed to dynamical, i.e.: ω-dependent effects. We argue that a quantitative interpretation of experimental data requires to calculate extrema not only in the momentum distribution curve but also in the energy distribution curve. In passing, we give a formula for the extrema in the latter distribution that is valid for the general G(k, ω) in a many body system. To our knowledge, this is a new formula, not found in the literature. The investigation of the generic model proceeds on two levels: on the one hand, we explore the rich variety of crossovers that can be predicted and linked to well defined features in the complex ω-plain. On the other hand, we show that the generic one-pole self-energy can be viewed as a projection on the low energy sector of a microscopic solution, belonging to a lattice model of interacting fermions. To obtain approximate microscopic solutions, we use our continued fraction method [2] and [3]. As an explicit example, we study the projection for the case of a hole doped Hubbard model in infinite dimension. A discussion section gives examples, how the generic model is able to cope with the ubiquity of the crossover phenomena, also in finite dimension and beyond the Hubbard model.     

Raman scattering by electron and phonon excitations in FeSi

The temperature evolution of Raman scattering by electron and phonon excitations in FeSi is studied within the range of 10–500 K. At low temperatures, the frequency dependence for the spectra of light scattered by electrons exhibits vanishing intensity in the range up to 500–600 cm^–1, which suggests the existence of an energy gap of about 70 meV. The calculations of the electronic excitation spectra based on the band structure determined using the LDA+DMFT technique (local electron density + dynamic mean field approximation) are in good agreement with the low-temperature experimental data and confirm that FeSi is a material with intermediate electron correlations. The changes in the shape of the electronic excitation spectrum and in the self-energy of optical phonons indicate a transition to the metallic state above 100 K. The analysis of experimental data demonstrates an appreciable decrease in the electron lifetime with the growth of temperature determining the (insulator–poor metal) transition.     

Strong and complex electron-lattice correlation in optimally doped Bi2Sr2CaCu2O8+delta

We discuss the nature of electron-lattice interaction in optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} samples, using the isotope effect (IE) in angle resolved photoemission spectroscopy (ARPES) data. The IE in the ARPES linewidth and the IE in the ARPES dispersion are both quite large, implying a strong electron-lattice correlation. The strength of the electron-lattice interaction is "intermediate," i.e., stronger than the Migdal-Eliashberg regime but weaker than the small polaron regime, requiring a more general picture of the ARPES kink than the commonly used Migdal-Eliashberg picture. The two IEs also imply a complex interaction, due to their strong momentum dependence and their differing sign behaviors. In sum, we propose an intermediate-strength coupling of electrons to localized lattice vibrations via charge density fluctuations.     

On the theory of cyclotron resonance linewidth

Using self-consistent perturbation theory a formula for the a.c. magneto-conductivity is derived. This formula takes account of both self-energy and vertex correction effects and can be used to calculate the cyclotron resonance linewidth. For realistic values of the strength and the range of the impurity potential good agreement with results of experiments on InSb is obtained.     

Angle-resolved photoemission spectroscopy study of Fe(1 1 0) single crystal: Many-body interactions between quasi-particles at the Fermi level

A high-resolution angle-resolved photoemission spectroscopy (ARPES) study of Fe(1 1 0) single crystal was conducted to elucidate many-body interactions between quasi-particles at the Fermi level at low-temperature. Two kink structures were observed in the energy-band dispersion at the binding energies of ∼40 meV and ∼270 meV for the bulk-derived band on the majority-spin Fermi surface around the Γ point. Based on analyses of the experimentally obtained real parts of the self-energy, these kink structures are derived from electron-phonon and electron-magnon interactions.     

Velocity renormalization and carrier lifetime in graphene from the electron-phonon interaction

We present a first-principles investigation of the phonon-induced electron self-energy in graphene. The energy dependence of the self-energy reflects the peculiar linear band structure of graphene and deviates substantially from the usual metallic behavior. The effective band velocity of the Dirac fermions is found to be reduced by 4%-8%, depending on doping, by the interaction with lattice vibrations. Our results are consistent with the observed linear dependence of the electronic linewidth on the binding energy in photoemission spectra.     

Dynamics of the self-energy of the Gd(0001) surface state probed by femtosecond photoemission spectroscopy

Transient changes of the complex self-energy of the 5d(z2) surface state on Gd(0001) after intense optical excitation are investigated by femtosecond time-resolved photoemission. We observe an ultrafast (<100 fs) broadening of the linewidth due to e-e scattering followed by a decrease of the binding energy due to thermal expansion of the lattice. In addition, we resolve a periodic breathing of the band structure which originates from a coherent phonon. An amplitude of 1 pm is derived from the binding energy shift upon lattice displacement calculated by density functional theory.     

Destruction of the Fermi surface due to pseudogap fluctuations in strongly correlated systems

We generalize the dynamical-mean field theory (DMFT) by including into the DMFT equations dependence on the correlation length of the pseudogap fluctuations via the additional (momentum dependent) self-energy Σ_k. This self-energy describes nonlocal dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures, these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT + Σ_k approach is used for the numerical solution of the weakly doped one-band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbor hopping. The effective single impurity problem is solved by using a numerical renormalization group (NRG). Both types of strongly correlated metals, namely, (i) doped Mott insulator and (ii) the case of the bandwidth W ? U (U-value of local Coulomb interaction) are considered. By calculating profiles of the spectral densities for different parameters of the model, we demonstrate the qualitative picture of Fermi surface destruction and formation of Fermi arcs due to pseudogap fluctuations in qualitative agreement with the ARPES experiments. Blurring of the Fermi surface is enhanced with the growth of the Coulomb interaction.     

Theoretical investigation of the superconducting gaps in the Fe-based superconductors

Based on an antiferromagnetic (AFM) spin fluctuation approximation, we study the superconducting gaps in Fe-based compound using two-band model. We find that our results are consistent with the previous work that concludes sign-reversal extended s-wave pairings between different Fermi surface sheets. The different superconducting gap magnitude around different Fermi surface sheets is probably due to the different density of states on them. This calculation can give insight to the recent angle-resolved photoemission (ARPES) experiments on these materials. To detect the phase variation of the superconducting gap over the Fermi surfaces, we propose a new method for measuring the particular wave vector phonon linewidth. In the case of the sign-reversal superconducting pairing, the linewidth shows continuities compared to the case of no phase variation.     

铜氧化物超导体Bi2Sr2CaCu2O8+δ 多体相互作用的超高能量分辨和时间分辨 角分辨光电子能谱研究

自发现30 多年来,铜氧化物的高温超导机理仍未得到解释。传统超导电性起源于电 子–声子相互作用形成的电子配对,研究传统超导体中的多体相互作用为BCS 理论提供了有 力的证据。目前已证实铜氧化物高温超导体中存在着电子配对,但是引起配对的机制仍不清 楚。因此,对铜氧化物高温超导体中的多体相互作用研究是揭示高温超导机理的关键。角分辨 光电子能谱是研究固体电子结构最直接的技术手段,随着其分辨率的不断提升,在研究高温超 导体的多体相互作用中日益发挥重要的作用。近年来兴起的时间分辨角分辨光电子能谱在常规 角分辨光电子能谱的基础上增加了独特的时间维度,从而成为研究多体相互作用动力学的有力 手段。本文详细地介绍了我们利用超高能量分辨和时间分辨角分辨光电子能谱在铜氧化物超导 体Bi2Sr2CaCu2O8+δ 中多体相互作用的研究,包括在节点区域、反节点区域扭折的研究,多体 相互作用的动量依赖关系,配对电子自能的提取以及库珀对在激光泵浦下的受激辐射现象。     

Element-specific magnetization reversal in Fe/Ce multilayers: : a study by X-ray magnetic circular dichroism and the magneto-optic Kerr effect

Fe/Ce multilayers are magnetically soft with coercive fields of a few Oersteds. In this artificial system, the itinerant 5d electrons of Ce are magnetically polarized by hybridization with the spin–split 3d states of Fe. To obtain an insight into the magnetization reversal process, the element selectivity of X-ray magnetic circular dichroism was used to measure the magnetization of the Ce-5d electrons as a function of an applied magnetic field. Comparison with the magnetization curves studied by the magneto-optic Kerr effect, which averages over the whole system, revealed that the coercivity in the hysteresis of the ordered Ce-5d moments is reduced by 50%. We propose that this is an effect of the magnetically disturbed interface or of the complex non-collinear magnetic structure of the Ce layers detected by recent experiments of X-ray resonant magnetic scattering. The results are compared to the X-ray dichroic and Kerr hysteresis loops of the multilayers Fe/La/Ce/La and Fe/CeH_2−δ. These systems are magnetically harder and their coercivities are identical.     

Mössbauer-Effect Study of Metastable c-FeSi Synthesized by Molecular Beam Epitaxy (MBE) and by Ion Implantation

Conversion electron Mössbauer spectroscopy (CEMS) has been applied to the study of the metastable c-FeSi phase (i.e. an iron silicide with CsCl lattice structure) that was synthesized by implantation of Si + ions of 50 keV in energy into f -Fe (95% 57 Fe) near room temperature with a nominal dose of 5 2 10 17 cm m 2 , and by molecular beam epitaxy (MBE). Iron silicide layers with different stoichiometry (FeSi 0.85 , FeSi, Fe 0.85 Si) were grown by codeposition of 57 Fe and Si onto an Fe buffer layer on MgO(100). For all FeSi layers the defective CsCl structure was observed after annealing at different temperatures. X-ray diffraction measurements were performed to determine the structure and epitaxial relationship of the c-FeSi films. The lattice parameter perpendicular to the film plane was found to be 2.77(5) Å. CEMS measurements revealed a lower than cubic site symmetry of the iron atoms for both the c-FeSi layers synthesized by ion implantation and by MBE. The formation of nearly undistorted c-FeSi after annealing is favored by excess Fe atoms in the deposited film.     

High energy pseudogap and its evolution with doping in Fe-based superconductors as revealed by optical spectroscopy

We report optical spectroscopic measurements on electron- and hole-doped BaFe2As2. We show that the compounds in the normal state are not simple metals. The optical conductivity spectra contain, in addition to the free carrier response at low frequency, a temperature-dependent gap-like suppression at fairly high energy scale near 0.6 eV. This suppression evolves with the As–Fe–As bond angle induced by electron or hole doping. Furthermore, the feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that the feature is mainly caused by the strong Hund's rule coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. The coupling strength changes with the environment of the Fe atom. Our experiments demonstrate the coexistence of itinerant and localized electrons in iron-based compounds, which would then lead to a more comprehensive picture of the metallic magnetism in the materials.     

Orbital polarization in itinerant magnets

We propose a parameter-free scheme of calculation of the orbital polarization (OP) in metals, which starts with the strong-coupling limit for the screened Coulomb interactions in the random-phase approximation (RPA). For itinerant magnets, RPA can be further improved by restoring the spin polarization of the local-spin-density approximation through the local-field corrections. The OP is then computed as the self-energy correction in the static GW method, which systematically improves the orbital magnetization and the magnetic anisotropy energies in transition-metal and actinide compounds.     

Nuclear orientation and nuclear structure

The purpose of this paper is to demonstrate how recent experimental results of¹⁵¹Eu and⁵⁷Fe high pressure Mössbauer studies in 4f and 3d metallic magnetic systems can contribute to a deeper understanding of the nature of local moment (4f) and itinerant (3d) magnetism in these systems. Special emphasis is given on the comparison of the experimental results with related theoretical models.     

Size effects and magnetoelastic couplings: a link between Hall–Petch behaviour and coercive field in soft ferromagnetic metals

Size effects regarding Hall–Petch (HP) relation are studied in this work for cobalt, nickel and Fe–3wt.%Si (FeSi), from polycrystalline to multicrystalline states. The materials show a breakdown in HP plot for thickness (t) to grain size (d) ratio less than a critical value. This appears in the beginning of plasticity for cobalt and FeSi whereas a plastic strain threshold must be overcome for nickel. Measurements of the coercive field on strained samples are able to depict such modification for low t/d ratio. Values of the coercive field in the polycrystalline domain allow an estimation of the magnetocrystalline anisotropy energy, related to the grain volume fraction concerned by reversal mechanisms for magnetic domains. Multicrystalline samples of cobalt and FeSi becomes magnetically softer at the yield stress. This is linked to a delay of the maximum intergranular stress towards higher strains for FeSi. For cobalt, non-linear elasticity and strong basal texture modify the magnetoelastic effects in coarse grain samples. For nickel, size effect on the coercive field appears after a few per cent of plastic strain as for HP relationship. A mean internal stress can be captured by magnetic measurements on polycrystals, related to the intragranular part of the kinematic stress. The softening of the magnetic properties for strained nickel multicrystals is due to a competition between the apparition of dislocation cells, which increases the coercive field by mechanisms of magnetic domain wall pinning, and surface softening of multicrystals, which tends to decrease the value of H_c.