Dispersion of neutron spin resonance mode in Ba0.67K0.33Fe2As2

We report an inelastic neutron scattering investigation on the spin resonance mode in the optimally hole-doped iron-based superconductor Ba_0.67K_0.33Fe₂As₂ with T_c=38.2 K. Although the resonance is nearly two-dimensional with peak energy E_R≈14 meV, it splits into two incommensurate peaks along the longitudinal direction ([H, 0, 0]) and shows an upward dispersion persisting to 26 meV. Such dispersion breaks through the limit of total superconducting gaps ∆_tot=|∆_k|+|∆_k+Q|(about 11-17 meV) on nested Fermi surfaces measured by high resolution angle resolved photoemission spectroscopy (ARPES). These results cannot be fully understood by the magnetic exciton scenario under s^±-pairing symmetry of superconductivity, and suggest that the spin resonance may not be restricted by the superconducting gaps in the multi-band systems.     

Structural and electronic properties of thin Ni layers on Cu(1 1 1) as investigated by ARPES, STM and GIXD

The growth and the crystalline and electronic structure of Ni deposited on single crystalline Cu(1 1 1) were studied by scanning tunnelling microscopy (STM), grazing incidence X-ray diffraction (GIXD) and angle-resolved photoemission spectroscopy (ARPES). In the early stages of growth monoatomic-high flat Ni islands, partially covered by Cu migrating from the surface, are observed. Starting from a pseudomorphic epitaxial relationship the in-plane lattice parameter progressively relaxes with increasing coverage. For a 20 monolayer (ML) thick deposit the in-plane lattice parameter is still found halfway between the bulk Ni and Cu lattice parameters. ARPES data also rule out the layer-by-layer growth and provide the values of the Ni exchange splitting.     

A relation between the resonance neutron peak and ARPES data in cuprates

We argue that the resonant peak observed in neutron scattering experiments on superconducting cuprates and the peak/dip/hump features observed in ARPES measurements are byproducts of the same physical phenomenon: both are due to feedback effects on the damping of spin fluctuations in a d-wave superconductor. We argue that in the superconducting phase, the dynamical spin susceptibility possesses the resonance peak at Ω_res∝ξ⁻¹ where ξ is the magnetic correlation length. The scattering of the resonant magnetic excitations by electrons gives rise to a peak/dip/hump behavior of the electronic spectral function, the peak-dip separation is exactly Ω_res.     

Enhanced core polarization in (70)Ni and (74)Zn

The reduced transition probabilities B(E2;0(+) --> 2(+)(1)) of the neutron-rich (74)Zn and (70)Ni nuclei have been measured by Coulomb excitation in a (208)Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d'Ions Lourds via interactions of a 60A MeV (76)Ge beam with a Be target. The B(E2) value for (70)Ni(42) is unexpectedly large, which indicates that neutrons added above N=40 strongly polarize the Z=28 proton core. In the Zn isotopic chain, the steep rise of B(E2) values beyond N=40 continues up to (74)Zn(44). The enhanced proton core polarization in (70)Ni is attributed to the monopole interaction between the neutron in the g(9/2) and protons in the f(7/2) and f(5/2) spin-orbit partner orbitals. This interaction could result in a weakening of magicity in (78)Ni(50).     

Analysis of single-particle energies of the neutron states in the 64,66,68,70Zn isotopes within a mean field model with dispersive optical potential

The dynamics of the shell parameters of ^{64, 66, 68, 70}Zn nuclei close to the Fermi energy, obtained by the joint evaluation of data from the stripping and pick-up reaction of a nucleon on one and the same nucleus, demonstrates how the difference between the neutron single-particle spectra of the levels of ⁷⁰Zn and ⁶⁸Ni nuclei with the number N = 40 arises. The experimental data are analyzed within a mean field model with dispersive optical potential. The calculation results describe very well the dimunution of the energy gap between the 1g _9/2 and 2p _1/2 levels in Zn isotopes, in comparison with Ni isotopes.     

Angle-resolved and resonant photoemission study of the ZrO-like film on ZrC(1 0 0)

Angle-resolved photoemission spectroscopy (ARPES) and resonant photoemission spectroscopy (RPES) have been used to study the electronic structure of the ZrO-like film formed on a ZrC(1 0 0) surface. It is found that, in addition to the O 2p band observed at 6-8 eV, states exist at 0.15 and 0.75 eV around the point. ARPES measurements show that the states at 0.15 and 0.75 eV disperse towards the lower binding energy side and cross the Fermi level along the direction. RPES measurements show that the former state shows a resonant behavior characteristic of the intra-atomic Zr 4d resonance, suggesting that the state includes substantial contribution of Zr 4d orbitals. On the other hand, the latter state shows a resonant behavior similar to that of the O 2p state in ZrO. The resonance is thought to be caused through the inter-atomic deexcitation mechanism involving the emissions of O 2p electrons, and thus the latter state is ascribed to that mostly composed of the O 2p component.     

Quantum impurities and the neutron resonance peak in YBa2Cu3O7: Ni versus Zn

The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin dynamics of an optimally doped high temperature superconductor is compared in YBa2(Cu0.97Ni0.03)3O7 (Tc=80 K) and YBa2(Cu0.99Zn0.01)3O7 (Tc=78 K). In the Ni-substituted system, the magnetic resonance peak (which is observed at Er approximately 40 meV in the pure system) shifts to lower energy with a preserved Er/Tc ratio while the shift is much smaller upon Zn substitution. By contrast Zn, but not Ni, restores significant spin fluctuations around 40 meV in the normal state. These observations are discussed in the light of models proposed for the magnetic resonance peak.     

单能中子源中低能成份对活化截面测量的影响

对用单能中子源作活化截面测量时低能中子干扰的影响作了分析研究,并从测量的两种常用中子源T(d,n)⁴He和D(d,n)³He的中子源能谱中验证了这种定性分析.最后给出了两个比较典型的反应⁵⁸Ni(n,P)⁵⁸Co和⁶⁴Zn(,P)⁶⁴Cu反应的截面测量结果.     

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9-27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron-electron interaction in the SS.     

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.     

ZnSiF66H2O∶Fe2+晶体光谱结构的杨特勒效应和电子顺磁共振g因子

研究了过渡金属络合物ZnSiF66H2O∶Fe2+晶体光谱结构的杨特勒效应和电子顺磁共振g因子。由单晶的中子衍射方法得到ZnSiF66H2O∶Fe2+的晶体结构,这种结构可以用SiF2-6和Zn(H2O)++∶Fe2+两个离子来描述。而局域三角对称的Zn(H2O)++∶Fe2+离子反映了这种晶体的主要光谱性质。利用不可约张量的理论构成了晶体场和自旋轨道相互作用哈密顿矩阵和电子顺磁共振理论公式,求出了晶体ZnSiF66H2O∶Fe2+中Fe2+离子的电子顺磁共振零场分裂参量(D,F-a)及g因子,并研究了低自旋3L态对电子顺磁共振零场分裂参量的贡献是不能忽略的,而对g因子的贡献是非常小的,并理论计算了它的晶体结构,证实了杨特勒效应的存在,理论计算的结果与实验值是相符的。     

Stripes in doped antiferromagnets: single-particle spectral weight

Recent angle-resolved photoemission spectroscopy (ARPES) experiments on cuprate superconductors provide important guidelines for a theory of electronic excitations in the stripe phase. Using a cluster perturbation theory, where short-distance effects are accounted for by exact cluster diagonalization and long-distance effects by perturbation (in the hopping), we calculate the single-particle Green's function for a striped t-J model. The data obtained quantitatively reproduce salient (ARPES) features and may serve to rule out "bond-centered" in favor of "site-centered" stripes.     

Bulk vs. surface effects in ARPES experiment from La2/3Sr1/3MnO3 thin films

Experiments directly probing the electronic states using angle-resolved photoemission (ARPES) were carried out on La_2/3Sr_1/3MnO₃ in order to elucidate its electronic properties. ARPES is a surface sensitive technique where bulk and surface states are usually both present. We present high-resolution ARPES studies in the (1 0 0) and (1 1 0) mirror planes and compare them with simulated ARPES based on GGA + U band structure calculations. In the (1 1 0) mirror plane we identify surface umklapps accounted by surface reconstruction which couple to bulk electronic states. As predicted by the simulated spectra there is additional spectral intensity at the Fermi level detected in ARPES data due to k_⊥-broadening effects in the photoemission final states. We demonstrate that this additional spectral intensity is a convenient spectral marker for determination of the k_F Fermi momenta.     

Impurity states and interlayer tunneling in high temperature superconductors

We argue that the scanning tunneling microscope (STM) images of resonant states generated by doping Zn or Ni impurities into Cu-O planes of BSCCO are the result of quantum interference of the impurity signal coming from several distinct paths. The impurity image seen on the surface is greatly affected by interlayer tunneling matrix elements. We find that the optimal tunneling path between the STM tip and the metal (Cu, Zn, or Ni) d(x(2)-y(2)) orbitals in the Cu-O plane involves intermediate excited states. This tunneling path leads to the fourfold nonlocal filter of the impurity state in Cu-O plane that explains the experimental impurity spectra. Applications of the tunneling filter to the Cu vacancy defects and "direct" tunneling into Cu-O planes are also discussed.     

Study of the energy levels of 62Ni using the (γ, γ′) reaction

Elastic and inelastic scattering of monochromatic photons were used for studying nuclear energy levels in ⁶²Ni; the photons were produced by thermal neutron capture in iron. The energy of the resonance level in ⁶²Ni was 7646 keV. The angular distributions of the elastic and two inelastic lines were measured and the corresponding level spins determined. The parity of the resonance level was found to be odd using polarization measurements.     

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.     

Stagnancy of the pygmy dipole resonance

The pygmy dipole resonance(PDR) of nickel isotopes is studied using the deformed random phase approximation method. The isoscalar character of the pygmy resonance is confirmed, and the correlation between the pygmy resonance and neutron skin thickness is discussed. Our investigation shows a linear correlation between PDR integral cross section and neutron skin thickness when the excess neutrons lie in pf orbits, with a correlation rate of about 0.27 fm~(-1). However, in more neutron-rich nickel isotopes, the growth of the pygmy dipole resonance is stagnant. Although the neutron skin thickness increases, the whole skin is not active. There is an inertial part in the nuclei~(70-78)Ni which does not participate in the pygmy resonance actively and as a result, contributes little to the photo-absorption cross section.     

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 .     

重费米子化合物 CemMnIn3m+2n(m = 1, 2, 3; n = 0, 1, 2; M = Co,Rh,Ir,Pt 及Pd) 的角分辨光电子能谱研究

重费米子材料以其新奇多变的宏观性质,复杂而难以理解的微观物理过程而受到广泛的关注,长期是凝聚态物理研究的重点。角分辨光电子能谱作为一种能够直接探测材料电子结构的实验手段,随着近年来实验技术的高度发展,能量和动量分辨率得到了极大的提高,能够有力地探测到强关联材料中更加精细的电子结构。使用角分辨光电子能谱探测重费米子材料的电子结构,为理解其各种物理过程提供强有力的实验证据,进而推进重费米子理论的发展。本文回顾 Ce 基重费米子材料 CemMnIn3m+2n (m = 1; 2; 3;n = 0; 1; 2;M =Co, Rh, Ir, Pt, Pd) 系列化合物的角分辨光电子能谱研究,包括电子结构的维度、近藤共振、f 电子与导带电子的杂化和电子结构随温度的变化等。     

Evolution of the <Emphasis Type="Italic">N</Emphasis> = 40 neutron subshell in 20 ≤ <Emphasis Type="Italic">Z</Emphasis> ≤ 30 nuclei within the dispersive optical model

The evolution of single-particle neutron spectra in the N = 40 isotones ⁶⁰Ca, ⁶²Ti, ⁶⁴Cr, ⁶⁶Fe, ⁶⁸Ni, and ⁷⁰Zn is calculated on the basis of the mean-field model featuring a dispersive optical potential. The results of these calculations agree with the idea that the degree of collectivity becomes higher in the ⁶⁴Сr nucleus and that the coupling of single-particle motion to this collectivity becomes stronger, as well as with available experimental data, which are indicative of the closure of the N = 40 subshell in ⁶⁸Ni and of the trend toward this closure in ⁶⁰Ca.