Inverse photoemission from Cu(001)

Bremsstrahlung ultraviolet spectra for Cu(001) are calculated for a photon energy ηω=9.7 eV within the framework of an inverse one-step model of photoemission for several angles of incidence of the electron beam. The comparison with recently reported experimental data shows that both the calculated energy dispersion of the peak in the spectra and the variation of the peak intensity are in reasonable agreement with the experiment.     

Evidence for an anisotropy-induced non-collinear spin state in exchange-coupled Co/Cu(001)

The magnetization behaviour of a Co/Cu/Co(001) sandwich has been studied by magneto-optical Kerr effect measurements. The sample was grown by molecular beam epitaxy onto a sapphire (1 .2) substrate with a Cu/Cr/Nb(001) buffer system. The copper layer had the form of a wedge with the thickness range chosen to be around the second region of antiferromagnetic exchange coupling. The hysteresis loops in the regime of weak antiferromagnetic coupling show characteristic steps, which can be explained by an anisotropy-induced non-collinear spin state. Indication for a similar behaviour is also found in the regime of strong antiferromagnetic coupling. This behaviour is explained by taking into account the competition between anisotropy, interlayer exchange coupling and external field energy. The nature of this metastable non-collinear magnetization state is in marked contrast to the biquadratic (90°) exchange coupling which was discovered in Fe/Cr(001).     

Scattering of electrons on the Cu(001) surface by Co adatoms

Excited electrons at surfaces can be scattered by adsorbate atoms or defects, which changes the energy or momentum. Such scattering processes can be studied by energy, time and angle-resolved two-photon photoelectron spectroscopy. In this article the influence of statistically distributed Co adatoms on a Cu(001) surface on the dynamics of electrons in image-potential states is investigated. Different scattering mechanisms, such as interband, intraband, and bulk scattering are identified and analyzed quantitatively. Cobalt adatoms cause mainly quasielastic scattering of electrons in image-potential states. Inelastic processes are due to interactions with electrons in the substrate and are not significantly increased by Co adatoms. The results are compared to previous experimental and theoretical work on Cu adatoms. PACS 73.20.At; 68.49.Jk; 79.60.Ht     

Mechanisms of high-order perturbative photoemission from Cu(001)

We observed high-order 2- to 4-photon photoemission and above threshold photoemission (ATP) processes with 3.07 eV light from the Cu(001) surface. The intensity of 3-photon photoemission via excitation through the n = 1 image potential state significantly exceeded that of the 2-photon process. The ATP occurs either via single photon transitions from the image potential resonances above the vacuum level or by multiphoton transitions from image potential states below the vacuum level. The experimental ratio of the m- to (m + 1)-photon process yields is sensitive to the electronic band structure of the solid.     

Kerr-effect magnetometry of the single monolayer Co/Cu(001)

Experimental evidence is given for a four-fold in-plane magnetic anisotropy of the p(1×1) Co monolayer epitaxially grown on a Cu(001) substrate. Temperature dependent hysteresis curves show the magnetization remaining almost constant up to 400 K and the coercive field drastically increasing as the temperature is decreased.     

Photon polarization effects in inverse photoemission from Cu(001)

A study of the polarization of Bremsstrahlung emitted from a Cu(001) single crystal surface helps to identify surface states and allows to determine the parity of bulk final states involved in a particular radiative transition if the angle of electron incidence is varied in a mirror plane of the crystal. In case of overlapping peaks with different polarization, a polarization dependent light detection increases in addition the apparent experimental resolution considerably.     

Emission direction dependence of the magnetic linear dichroism in valence band and core level photoemission from Co/Cu(001)

We have investigated the angular dependence of the magnetic linear dichroism occurring in angle-resolved photoemission (MLDAD) from epitaxial Co films on Cu(001) using unpolarised HeI- and MgK_α-radiation. Spectra were taken while rotating the sample about the magnetisation direction for fixed angle between the directions of light incidence and photoelectron collection. We observe a strong dependence of the MLDAD asymmetry on the electron emission direction with respect to the surface normal showing the importance of photoelectron diffraction in MLDAD.     

Uniaxial magnetic anisotropy in nanostructured Co/Cu(001): from surface ripples to nanowires

We have investigated the correlation between morphology and magnetic anisotropy in nanostructured Co films on Cu(001). The formation of nanoscale ripples by ion erosion is found to deeply affect the magnetic properties of the Co film. A surface-type uniaxial magnetic anisotropy with easy axis parallel to the ripples is observed. The origin of the magnetic anisotropy has been identified with the modification of thermodynamic-step distribution induced by ripple formation. At higher ion doses, when Co ripples detach and crystalline nanowires form, a strong enhancement of the magnetic anisotropy due to magnetostatic contributions is observed.     

Diffusion kinetics in the Pd/Cu(001) surface alloy

We use atom-tracking scanning tunneling microscopy to study the diffusion of Pd in the Pd/Cu(001) surface alloy as a function of temperature. By following the motion of individual Pd atoms incorporated in the surface, we show that Pd diffuses by a vacancy-exchange mechanism. We measure an activation energy for the diffusion of incorporated Pd atoms of 0.88 eV, which is in good agreement with our ab initio calculated energy of 0.94 eV.     

Vibrational spectroscopy of ordered oxygen adiayers on Ni/Cu(001) and Co/Cu(001) thin film systems

We report surface vibrations in c(2 × 2) oxygen adlayers on Ni and Co thin films on a Cu(001) substrate measured at gG by high resolution EELS. For the Ni thin film surface, one phonon peak is measured for varying film thicknesses from 1.3 ML (monolayer) to 6 ML with a constant energy of 221 cm⁻¹. For the Co thin film surface, three loss peaks are found, whose relative intensities change as the film thicknesses are varied. One loss peak at ˜520 cm⁻¹ is tentatively assigned to the Fuchs-Kliewer mode of cobalt oxide (CoO). The other two peaks at 317 and 376 cm⁻¹ are likely related to different bonding sites. Surface phonons on the p(2 × 2) Co thin film (389 cm⁻¹) and a bulk resonance mode (115 cm⁻¹) are also reported.     

Angle-resolved photoemission studies of a surface state for Ag overlayers on Cu(111)

We have performed an angle-resolved photoemission study of a surface state on Cu(111) covered by various thicknesses of Ag. The growth mode of Ag on Cu(111) was determined to be layer-by-layer despite the large mismatch between the two lattices. The Cu(111) surface state was observed to evolve monotonically for increasing Ag overlayer thickness to eventually become the Ag(111) surface state. The measured rate of shift of the surface state binding energy can be explained qualitatively in terms of the degree of localization of the surface-state wave functions.     

Angle-resolved photoemission study of the Li-induced structures of the Si(001) surface

We have investigated dispersion relations of surface bands induced by Li adsorption on the Si(001) surface at room temperature by Angle-Resolved PhotoE-mission Spectroscopy (ARPES). The characteristic spectral changes in the Energy Distribution Curves (EDCs) with increasing Li coverage implicate that Li adatoms occupy on-top sites of the dimer-forming silicon atoms resulting in 0.5 and 1.0 monolayers for the completion of the first two Li-induced ordered (2×2): Li and (2×1): Li phase, respectively. The unique upward dispersions of the surface bands for these Li-induced structures are ascribed to the size effect of a Li adatom. We further discuss a mechanism which might account for the metallie peak found at the early stage of Li adsorption.     

Three-photon photoemission from Cu(100): observation of an unoccupied surface state near gG

The photon energy dependence has been measured for three-photon photoemission from Cu(100). A narrow resonance (FWHM = 0.02 eV) was observed centered at 2.02 eV photon energy. The resonance can be explained by the presence of an unoccupied surface state near the center of the surface Brillouin zone at gG and 2.0 eV above E_F.     

Joys and pitfalls of fermi surface mapping in Bi(2)Sr(2)CaCu(2)O(8+delta) using angle resolved photoemission

On the basis of angle-scanned photoemission data recorded using unpolarized radiation, with high (E,k) resolution, and an extremely dense sampling of k space, we resolve the current controversy regarding the normal state Fermi surface (FS) in Bi(2)Sr(2)CaCu(2)O(8+delta). The true picture is simple, self-consistent, and robust: the FS is holelike, with the form of rounded tubes centered on the corners of the Brillouin zone. Two further types of features are also clearly observed: shadow FSs, which are most likely to be due to short range antiferromagnetic spin correlations, and diffraction replicas of the main FS caused by passage of the photoelectrons through the modulated Bi-O planes.     

Diffusion mechanism of Cu adatoms on a Cu(001) surface

Ab initio calculations on surface diffusion of Cu adatoms on Cu(001) are presented. The hopping mechanism with a calculated energy barrier of 0.69 eV is found to be favorable over the exchange mechanism with 0.97 eV. We find from the geometry relaxations that adatoms are significantly attracted to the surface and push away nearest-neighbor atoms in the surface. Lateral relaxations of atoms in the surface are larger than vertical ones.