M. Lindroos

Matrix element effects in angle-resolved photoemission from Bi2Sr2CaCu2O8: Energy and polarization dependencies, final state spectrum, spectral signatures of specific transitions, and related issues

We have carried out extensive simulations of the angle-resolved photoemission (ARPES) intensity in Bi2212 within the one-step- and three-step-type models using a first-principles band theory framework. The focus is on understanding the behavior of emissions from the antibonding and bonding hands arising from the CuO 2 bilayers around the M(π,0) symmetry point. The specific issues addressed include: Dependencies of the photointensity on the energy and polarization of the incident light; character of the initial and final states involved as well as the spectrum of the relevant final states; and changes in the spectral intensity as a function of the perpendicular component k 1 of the momentum of the photoelectron. Considerable insight into the nature of individual transitions is adduced by examining the momentum matrix element for hulk transitions within the solid and by further decomposing this matrix element into contributions from various atomic sites and angular momentum channels. These results indicate that via remarkable interference effects, the ARPES matrix element can in particular cases help zoom in on the properties of electrons excited from specific sites and/or angular momentum channels even in a complex material.

Observation of top-site adsorption for Xe on Cu(111)

Abstract A low-energy electron diffraction study of Cu(111)–(√3×√3)R30°–Xe at 50 K indicates that Xe atoms occupy the top sites. The equilibrium Xe–Cu interlayer spacing is 3.60±0.08 A and the spacings of the three top Cu layers are essentially bulk-like. The Xe–Cu spacing agrees well with estimates based on hard-sphere packing. Top-site adsorption for Xe on Cu(111) was unexpected on the basis of previous experimental and theoretical results for noble gas adsorption on metal surfaces. This result is discussed in the light of earlier studies of physisorbed atoms, anticorrugated potentials in He-atom scattering, and possible links to alkali metal adsorption.

Existence, character and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe2-xCoxAs2

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.

A LEED structural analysis including multilayer relaxation: An alkali-metal-induced (1 × 2) reconstruction of Pd(110)

Abstract The geometric structure of an alkali-metal-induced Pd(110)-(1×2) reconstructed surface has been investigated by LEED using quantitative R -factor analyses to test the level of theory-experiment agreement. The missing-row, “saw-tooth”, paired-row and rumpled-surface models have been tested. For the most promising structure (the missing row) we have extended our analysis to include the possibility of multilayer reconstruction. We incorporate shifts of second- and third-layer Pd atoms away from bulk equilibrium sites in the form of row pairing and rumpling. With the framework of the multilayer models tested, a missing-row model with a small second-layer pairing (0.1 A) and third-layer rumpling (0.1 A) is favoured ( R P = 0.28). First- and second-layer contractions of 9% and 1% respectively have been determined. These values compare with first- and second-interlayer spacing changes of 6% and 1% (expansion) respectively for the clean Pd{110}-(1×1) surface. This analysis suggests that the clean {110} surfaces of Au and Ir and the alkali-induced reconstruction of Pd{110} share a similar geometric structure.

Evolution of midgap states and residual three dimensionality in La2-xSrxCuO4.

We carry out extensive first-principles doping-dependent computations of angle-resolved photoemission (ARPES) intensities in La2-xSrxCuO4 over a wide range of binding energies. Intercell hopping and the associated three dimensionality, which is usually neglected in discussing cuprate physics, is shown to play a key role in shaping the ARPES spectra. Despite the obvious importance of strong coupling effects (e.g., the presence of a lower Hubbard band coexisting with midgap states in the doped insulator), a number of salient features of the experimental ARPES spectra are captured to a surprising extent when kz dispersion is properly included in the analysis.

The adsorption sites of rare gases on metallic surfaces: a review

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

The surface geometry of a caesium induced (1 × 2) multilayer reconstruction of Ag(110)

A structural analysis of a caesium induced (1 × 2) reconstruction of Ag(110) (θCs ≅ 0.10 ML) by low energy electron diffraction is presented. Evidence in favour of a topmost layer missing-row structure has been found in agreement with a recent ion scattering study utilising channeling and blocking. The saw-tooth (Bonzel-Ferrer) model can be clearly excluded. Inclusion of the possibility of multilayer reconstruction particularly in the form of third layer buckling leads to a small yet significant improvement in the R-factor. A second layer row pairing of 0.10 ± 0.10 A (towards the toplayer “missing rows”) and a third layer bucking amplitude of 0.10 ± 0.05 A have been determined. The surface relaxations of the outermost three atomic layer spacings are modified to −11, −2 and −9% in comparison to the clean surface damped oscillatory relaxation of −7, + 1 and −2% respectively. Thus, the missing row model with multilayer relaxation/reconstruction favoured for the clean (110) surfaces of Au, Ir and Pt has conclusively been shown to extend to the alkali-induced reconstructions. However, the magnitude of the first layer contraction and third layer buckling are definitely smaller in the case of the alkali-induced (1 × 2) structure, yet still larger than theoretical predictions based on the embedded atom method.

The Deformation, Strain Hardening, and Wear Behavior of Chromium-Alloyed Hadfield Steel in Abrasive and Impact Conditions

Abstract The alloying of Hadfield steels aims at enhanced mechanical properties and improvements in the wear resistance. In this work, the impact and abrasive properties of a chromium-alloyed high-manganese Hadfield steel were experimentally studied using a wide variety of testing techniques and characterization methods. In addition, an in-service sample was characterized to identify the wear and hardening mechanisms in a real application (jaw crusher). The dynamic mechanical behavior of the steel was determined using the Hopkinson split bar technique. The abrasion properties were studied with three-body abrasion tests using several different natural abrasives. The effects of existing plastic strain and normal loading on the surface hardening and wear rate were further investigated with scratch testing. High-velocity impact testing was performed to evaluate the effect of pre-strain on the impact wear behavior of the material. It was shown that the dynamic loading affects both the yield behavior and the strain hardening rate of the studied steel. The connection between pre-strain, hardness, and wear rate in abrasion was established. In impact conditions, plastic straining of the surface layer first has a positive effect on the wear resistance, but when strain hardening reached the observed ductility limit, it showed an adverse effect on the material’s performance. The addition of chromium and an increase in the manganese content from the nominal ASTM Hadfield composition provided some improvements in the strength, ductility, and surface hardening of the studied steel.

LEED determination of the structures of Ni(111) and the p(2*2) overlayer of potassium on Ni(111)

The authors have used dynamical low-energy electron diffraction (LEED) to determine the structure of both the clean Ni(111) surface and the p(2*2) structure of potassium adsorbed on Ni(111). The result of the clean surface study indicates that the Ni(111) is essentially a truncation of the bulk crystal. The p(2*2) structure of potassium on this surface consists of the potassium atoms adsorbed on top of the Ni atoms with a slight reconstruction of the top-layer Ni atoms combined with vertical relaxations of the first and second layers of Ni. The potassium-nickel bond length is 2.82+or-0.04 AA corresponding to an effective potassium radius of about 1.57 AA. This result fits well in the bond length versus coordination number trend observed for other alkali metal overlayers.

Origin of the high-energy kink in the photoemission spectrum of the high-temperature superconductor Bi2Sr2CaCu2O8

Susmita Basak, Tanmoy Das, Hsin Lin, J. Nieminen, M. Lindroos, R.S. Markiewicz, and A. Bansil 1 Physics Department, Northeastern University, Boston MA 02115, USA 2 Institute of Physics, Tampere University of Technology, P.O. Box 692, 33101 Tampere, Finland SMC-INFM-CNR, Dipartimento di Fisica, Università di Roma “La Sapienza”, P. Aldo Moro 2, 00185 Roma, Italy. ISC-CNR, Via dei Taurini 19, 00185 Roma