The spontaneous magnetostriction of invar and its interpretation on the itinerant electron model of very weak ferromagnetism

The spontaneous magnetostriction of invar is calculated from the forced magnetostriction and susceptibility to be about 50% larger than recent estimation from thermal expansion measurements, and 5 times as large as predicted by the itinerant electron model of very weak ferromagnetism.     

Orientation of adsorbed benzene from angle-resolved photoemission measurements

Angle-resolved photoemission using polarised radiation has been used to confirm the orientation of benzene adsorbed in a (2 × 2) R45° structure on Pd(100). The molecule is found to lie flat on the surface with apparent C_6v local symmetry.     

Strong lateral electron coupling of pb nanowires on stepped si(111): angle-resolved photoemission studies

We employ angle-resolved photoemission to characterize the electronic band structure of the Pb "nanowire" array self-assembled on a stepped Si(111) surface. Despite the highly oriented nanowires observed in scanning tunneling microscopy images, we find essentially two-dimensional Fermi contours modulated one dimensionally perpendicular to the wires. This strong two-dimensional and quasi-one-dimensional nature of the band structure explains the stability and anisotropy of the metallic phase down to 4 K as reported recently. A simple tight-binding model with each Si nanoterrace covered by a densely packed Pb overlayer successfully reproduces this modulated band structure and quantifies the electron coupling within the "nanostripes" and the step-edge potential.     

Electronic structure of MgB2 from angle-resolved photoemission spectroscopy

The first angle-resolved photoemission spectroscopy results from MgB2 single crystals are reported. Along the GammaK and GammaM directions, we observed three distinct dispersive features approaching the Fermi energy. These can be assigned to the theoretically predicted sigma (B 2p(x,y)) and pi (B 2p(z)) bands. In addition, a small parabolic-like band is detected around the Gamma point, which can be attributed to a surface-derived state. The overall agreement between our results and the band calculations suggests that the electronic structure of MgB2 is of a conventional nature, thus implying that electron correlations are weak and may be of little importance to superconductivity in this system.     

Spin- and angle-resolved photoemission from ferromagnetic Fe(001)

For near-normal photoemission from ferromagnetic Fe(001) excited by linearly polarized synchrotron radiation, energy-resolved spin polarization and intensity distribution have been measured at 60 eV photon energy. Calculations using a one-step theory of photoemission consistently reproduce. the present spin-resolved data as well as earlier spin-averaged measurements. The quasi-particle exchange splitting deduced from the data is 2 eV. The agreement with band structure calculations is suggested to be coincidental due to a compensation of real and imaginary self-energy corrections.     

Evidence of electron fractionalization from photoemission spectra in the high temperature superconductors

In the normal state of the high temperature superconductors Bi(2)Sr(2)CaCu(2)O(8+delta) and La2(-x)Sr(x)CuO4, and in the related "stripe ordered" material, La(1.25)Nd(0.6)Sr(0.15)CuO4, there is sharp structure in the measured single hole spectral function, A<(k-->,omega), considered as a function of k--> at fixed small binding energy omega. At the same time, as a function of omega at fixed k--> on much of the putative Fermi surface, any structure in A<(k-->,omega), other than the Fermi cutoff, is very broad. This is characteristic of the situation in which there are no stable excitations with the quantum numbers of the electron, as is the case in the one-dimensional electron gas.     

Electronic structure of the YH3 phase from angle-resolved photoemission spectroscopy

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on angle-resolved photoemission experiments of the Y trihydride phase. Most importantly, we find the absence of metal d bands at the Fermi level and a set of flat, H-induced bands located at much higher binding energy than predicted, indicating an increased electron affinity at H sites.     

A simple model for magnetism in itinerant electron systems

A new lattice model of interacting electrons is presented. It can be viewed as a classical Hubbard model in which the energy associated to electron itinerance is proportional to the total number of possible electron jumps. Symmetry properties of the Hubbard model are preserved. In the half-filled band with strong interaction the model becomes the Ising model. The main features of the magnetic behavior of the model in the one-dimensional and mean-field cases are studied.     

Experimental aspects of angle-resolved photoemission from clean and adsorbate-covered metal surfaces

Instrumental techniques for the observation of angle-resolved photoelectron spectra from surfaces are reviewed. Results from the work of various groups on clean metals, particularly tungsten, copper, nickel and palladium, are described. Spectra induced by adsorption of hydrogen, oxygen, carbon monoxide and benzene on some of these metals are discussed.     

Theory of angle-resolved photoemission from localised orbitals at solid surfaces

The theory of angle resolved photoemission from localised orbitals is reviewed and is cast in a form requiring the calculation of the purely outgoing wave emanating from an emitting atom, that describes the final state of the photoelectron, rather than using the more usual approach based on time reversed scattering states. An explicit expression is written down for the superposition of partial waves that results from emission from an atomic orbital, and it is pointed out that emission from more complex initial states such as localised bonds or Bloch and surface states, can be described by coherently combining such sets of partial waves. The effect of the crystal surface environment in damping and scattering the waves is described briefly. Model calculations are performed to investigate the major influences on the angular distribution of the photoelectrons. The profound effect of varying the polarisation direction of the incident light relative to the surface is discussed, with examples from the literature, showing how it can be used to determine the type of initial orbital. Emission from directed orbitals is studied and it is shown that scattering by the emitter potential can be an important effect, so that the radial wave function of the outgoing electron, which determines the amplitudes and phases of the outgoing waves, must be calculated with care. Different choices of these quantities lead in the model calculations to very different angular profiles, that sometimes bear little relation to the shape of the initial orbital. The consideration of emission from localised bonds shows that provided that bonds are not too strongly polarised, interference between waves from different centres is always significant at higher energies, and can also be important at energies of a few eV relative to the vacuum. Scattering by the ion cores of the surface region can strongly distort the angular distribution, or may have little effect. But it is generally difficult to decide a priori which influences are dominant for a particular case, so that the interpretation of angular profiles must be based on careful calculations including all these effects. The optimum energy range for the interpretation of experimental data is that from about 30 to 100 eV.     

Electronic band structure and Fermi surface of CaB6 studied by angle-resolved photoemission spectroscopy

We report high-resolution angle-resolved photoemission spectroscopy (ARPES) on CaB6. The band structure determined by ARPES shows a 1 eV energy gap at the X point between the valence and the conduction bands. We found a small electron pocket at the X point, whose carrier number is estimated to be (4-5) x 10(19) cm(-3), in good agreement with the Hall resistivity measurement with the same crystal. The experimental results are discussed in comparison with band structure calculations and theoretical models for the high-temperature ferromagnetism.     

Spin-orbit splitting in the valence bands of PbSe from angle-resolved uv photoemission

Angle-resolved photoelectron spectra of single-crystal PbSe have been measured employing 16.85 eV and 21.22 eV photon energies. Characteristic double peaks in the valence band spectra are attributed to the effect of spin-orbit interaction. The dependence of peak intensities on the photon incidence angle furnishes an additional justification for this interpretation. Experimentally obtained values of the spin-orbit splitting agree well with the prediction of band structure calculations.     

Observation of p- and d-like surface states on CuCl(100) by angle-resolved photoemission

In the course of a systematic ultraviolet photoemission study of the electronic band structure of CuCl, we have identified two occupied surface states on CuCl(100), situated at 0.25 and 3.0 eV below valence band maximum in normal emission spectra. They essentially show pure p- and d-like orbital symmetry, respectively. We interpret them as a chlorine p_x,y-like occupied antibonding resonance and a copper Γ₁₂-derived state split off from the bulk orbitals by the surface potential. We also present critical point energies along Γ-X and Γ-L.     

Fermi surface and quasiparticle dynamics of Na0.7CoO2 investigated by angle-resolved photoemission spectroscopy

We present the first angle-resolved photoemission study of Na0.7CoO2, the host material of the superconducting NaxCoO2.nH(2)O series. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band, a small Fermi velocity, and a large Hubbard U. The quasiparticle band crosses the Fermi level from M toward Gamma suggesting a negative sign of effective single-particle hopping t(eff) (about 10 meV) which is on the order of magnetic exchange coupling J in this system. Quasiparticles are well defined only in the T-linear resistivity (non-Fermi-liquid) regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics may have implications for understanding the phase of matter realized in this new class of a strongly interacting quantum system.