Photoemission spectroscopy in metals:: band structure-Fermi surface–spectral function

Angular resolved photoelectron spectroscopy plays a key role in the study of the electronic structure of solids. We discuss recent methodical developments in its application to metallic systems. These include a new procedure for absolute E(k) band structure determination, which allows complete control of the three-dimensional wave-vector k, as well as a method for Fermi surface mapping based on measurements of the angular photoelectron intensity distribution. Going beyond a simple one-electron picture, we examine under which conditions the photoemission signal can be interpreted in terms of the electron removal spectrum of an interacting electron system and discuss an experimental test on a suitable Fermi liquid metal, which supports this many-body interpretation.     

Absolute determination of band structureE(k) by combined very-low-energy electron diffraction and photoemission

Very-low-energy electron diffraction (VLEED) is a direct probe for the unoccupied electronic band structureE(k) corresponding to the upper bands of photoemission (PE). These bands can contain significant non-free-electron and self-energy effects. Use of the experimental VLEED bands in PE and inverse PE allows to determine the valence and conduction bandE(k) absolutely, i.e. with complete and approximation-free control of the 3D wavevector k. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999.     

Symmetry-breaking phase transition without a Peierls instability in conducting monoatomic chains

The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains of single atoms on a surface, is scrutinized for lattice instabilities predicted in the Peierls paradigm. By scanning tunneling microscopy and electron diffraction we reveal a second-order phase transition at 585 K. It leads to charge ordering with transversal and vertical displacements and complex interchain correlations. However, the structural phase transition is not accompanied by the electronic signatures of a charge density wave, thus precluding a Peierls instability as origin. Instead, this symmetry-breaking transition exhibits three-dimensional critical behavior. This reflects a dichotomy between the decoupled 1D electron system and the structural elements that interact via the substrate. Such substrate-mediated coupling between the wires thus appears to have been underestimated also in related chain systems.     

Photoemission of a doped Mott insulator: spectral weight transfer and a qualitative Mott-Hubbard description

The spectral weight evolution of the low-dimensional Mott insulator TiOCl upon alkali-metal dosing has been studied by photoelectron spectroscopy. We observe a spectral weight transfer between the lower Hubbard band and an additional peak upon electron doping, in line with quantitative expectations in the atomic limit for changing the number of singly and doubly occupied sites. This observation is an unconditional hallmark of correlated bands and has not been reported before. In contrast, the absence of a metallic quasiparticle peak can be traced back to a simple one-particle effect.     

Unoccupied electronic structure of TiOCl studied using x-ray absorption near-edge spectroscopy

We study the unoccupied electronic structure of the spin-1/2 quantum magnet TiOCl using x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. We acquire data both in total electron and fluorescence yield modes (TEY and FY, respectively). While only the latter allows us to access the unconventional low-temperature spin-Peierls (SP) phase of TiOCl, the signal is found to suffer from significant self-absorption in this case. Nevertheless, we conclude from FY data that effects of the SP distortion on the electronic structure are absent in the incommensurate intermediate phase within experimental accuracy. The similarity of room-temperature FY and TEY data, the latter not being obscured by self-absorption, allows us to use TEY spectra for comparison with simulations. These are performed by means of cluster calculations in D(4h) and D(2h) symmetries using two different codes. We extract values of the crystal-field splitting and parameterize our results using the commonly seen notation of Slater, Racah and Butler. In all cases, good agreement with published values from other studies is found.     

Synthetic magnetic opals

We present studies of novel nanocomposites of BiNi impregnated into the structure of opals as well as inverse opals. Atomic force microscopy and high resolution elemental analyses show a highly ordered structure and uniform distribution of the BiNi filler in the matrix. These BiNi-based nanocomposites are found to exhibit distinct ferromagnetic-like ordering with transition temperature of about 675 K. As far as we know there exists no report in literature on any BiNi compound which is magnetic.     

Possible metallization of the Mott insulators TiOCl and TiOBr: Effects of doping and external pressure

We report the results of doping- and pressure-dependent experimental investigations on the low-dimensional Mott-Hubbard insulators TiOCl and TiOBr. As observed by photoelectron spectroscopy, doping with Na and K shifts the valence band to higher binding energies associated with a jump of the chemical potential. With increasing doping, a broad hump develops close to the Fermi energy and the energy gap “softens”. The application of pressure induces the appearance of spectral weight close to the Fermi energy, as probed by infrared transmittance and reflectance measurements. The pressure-induced changes in the electronic properties coincide with a structural phase transition according to powder X-ray diffraction studies under pressure. The experimental findings are compared to theoretical predictions.     

Inelastic light scattering in hole-accumulation layers on silicon

Hole-accumulation layers on silicon (100) are studied using resonant inelastic light scattering techniques. The observed intersubband transitions are in good agreement with Hartree calculations of Bangert and with recent infrared absorption experiments. The width of the spectra can be explained qualitatively using the nonparabolic k_| dispersion of the individual subbands.