Mössbauer study of very dilute Pd(Fe) alloys at very low temperatures

The magnetic behaviour of very dilute (7 ppm and 0.6 ppm) Pd(Fe) alloys has been studied by Mössbauer spectroscopy in external fields up to 6 T in the temperature range between 0.018 and 300 K. No magnetic ordering was observed even at the lowest temperature. Above 0.067 K the magnetization exhibits free-spin behaviour. The saturation moment was found to be independent of temperature. Magnetic hyperfine patterns observed below 0.067 K were interpreted in terms of paramagnetic hyperfine interactions in the regime of slow electronic relaxation.     

Chemical effects in the N7 VV Auger spectra from W(100) and W(110) surfaces

Fine structure in the N₇ VV Auger spectra from clean W(100) and W(110) surfaces has been interpreted by Lander's band model for the doubly ionized final state. It is shown that the energies of the prominent emissions in the spectra are similar for the two surfaces and furthermore are consistent with the self convolution of a bulk density of states for tungsten. An additional feature in the spectrum from the W(100) surface has been attributed to emission from an intrinsic surface state at ?0.4 eV. The localization of this state at the surface was confirmed by its sensitivity to adsorbates (H₂, CO, O₂ and I₂). During the interaction of these gases with the surface the Auger spectra always retained the features attributed to the bulk density of states which were modified only by a shift in the background intensity profile. New emission features in this part of the spectra were not seen except for the example of hydrogen adsorption when a single new emission could be seen on each of the two tungsten surfaces. However, each adsorbate produce either one (H₂) or two (CO, O₂ and I₂) new emissions at lower energies which were attributed to emissions from new adsorbate derived levels which reside at energies below the prominent features of the tungsten valence band. The location of these new adsorbate levels is compared and contrasted with the equivalent ultraviolet photoelectron spectroscopic determinations.     

Optical intersubband transitions and femtosecond dynamics in Ag/Fe(100) quantum wells

The optical intersubband transitions and femtosecond dynamics of electrons in quantum well states in Ag/Fe(100) are investigated by interferometric time-resolved two-photon photoemission. The quantum well wave functions and transition probabilities are evaluated from the two-photon photoemission resonance energies and intensities using an extended phase accumulation model. Direct femtosecond pump-probe correlation measurements elucidate the importance of interfaces in confined structures.     

High-resolution phonon study of the Ag(100) surface

Using high-resolution electron energy loss spectroscopy the phonon dispersion of Ag(100) has been studied at two different sample temperatures of 86 and 300 K. The dominant feature in the spectra corresponds to the Rayleigh wave. Its full dispersion is determined along the ΓX high symmetry direction in the first and second Brillouin zones. The Rayleigh phonon maximum at the X point shows a redshift with increasing temperature. This is explained based on a surface anharmonicity with an anharmonicity constant of 0.014, comparable to the value reported for Cu(100). In the vicinity of the Γ point two additional phonon features have been discovered at about 110 and 160 cm(-1), which are tentatively assigned to high density of states features from the bulk phonon bands. However, the observed steep dispersion is in contrast to theoretical calculations. Along ΓX two surface resonance branches have been observed with maximum frequencies in the range of 90-110 cm(-1) near to the zone boundary. These branches agree with helium atom scattering data where available, but are not predicted by theory.     

Structural phase transitions of the Si(100) surface

A Monte Carlo study of the structural phase transitions on the Si(100) surface for both the “(2×1)” and “c(2×2)” reconstruction families is carried out using the asymmetric dimer model. A second order phase transition is observed at about 250 K from a p(2×2) (layered antiferromagnetic) to a disordered (paramagnetic) state in the “2×1” family. A similar transition is observed about 800 K from a c(2×2) (ferromagnetic) to a disordered state in the “c(2×2)” family. These results are in agreement with real-space renormalization group results. The variation of the specific heat, susceptability and the absolute value of the order parameter as a function of temperature is obtained in the range 200 to 300 K for the “2×1” family and 750 to 850 K for the “c(2×2)” family. Also, the order parameter correlation function is computed for ten different temperatures in the above ranges.     

Absorption and emission Mössbauer line shapes in the presence of electronic transitions

We compare Mössbauer absorption and emission line shapes in the presence of electronic transitions. We show that under certain conditions, which are actually fulfilled in most cases, the emission line shape can be obtained from the absorption line shape in a straightforward manner. We emphasize that ?interference terms” are present in both types of line shapes.     

Determination of the magnetic transition temperature of dilute ferromagnetic alloys - the Pd(Gd) case

The determination of T_c in dilute alloys is examined. As an example, magnetization vs field data in a Pd_0.95Gd_0.05 alloy are presented. For this alloy, it was not possible to determine a value of T_c using Arrott-Belov-Kouvel (ABK) plots; the conditions for the applicability of ABK plots were not satisfied. Results of very-low-field ac and dc magnetization vs T, which depend mainly on the demagnetization factor, yield a more precise measure of T_c. We conclude that the latter procedures are more useful than ABK plots for determination of T_c in dilute alloys.     

Dependence of the intrinsic line width of surface states on the wave vector: The Cu(111) and Ag(111) surfaces

The dependence of the intrinsic line width Γ of electron and hole states due to inelastic scattering on the wave vector k _∥ in the occupied surface state and the first image potential state on the Cu(111) and Ag(111) surfaces has been calculated using the GW approximation, which simulates the self-energy of the quasiparticles by the product of the Greens’s function and the dynamically screened Coulomb potential. Different contributions to the relaxation of electron and hole excitations have been analyzed. It has been demonstrated that, for both surfaces, the main channel of relaxation of holes in the occupied surface states is intraband scattering and that, for electrons in the image potential states, the interband transitions play a decisive role. A sharp decrease in the intrinsic line width of the hole state with an increase in k _∥ is caused by a decrease in the number of final states, whereas an increase in Γ of the image potential state is predominantly determined by an increase of its overlap with bulk states.     

Potassium Auger emission by K ion bombardment of the Ni(100) surface

Angle-resolved electron energy spectra induced by bombardment of a Ni(100) monocrystal surface with 0.5–4.5 keV energy K⁺ ions have been measured. In the case when the target is previously implanted by K⁺ ions a peak (discrete structure) in the energy range 15 < E < 17 eV is observed. This phenomenon has been ascribed to deactivation of M₂₃, vacancies of the K atom formed in the symmetrical collisions of projectiles with implanted or adsorbed surface K atoms. The phenomenon of the peak splitting at the higher projectile energies has been discussed within the frame of the Doppler effect in the specific case of symmetrical K-K collisions, where both collision participants can appear as Auger electron emitters.     

First-principles study on the electronic transport properties of M/SiC Schottky junctions (M=Ag,Au and Pd)

In this work, we investigate the electronic transport properties of M/SiC Schottky junctions (M=Ag, Au and Pd). The results show that the band structures of hydrogenated zigzag SiC nanoribbons (ZSiCNRs) and hydrogenated armchair SiC nanoribbons (ASiCNRs) are almost unaffected by their width changes. When the hydrogenated 7-ASiCNR is directly connected to the Ag, Au and Pd electrode, the transmission spectra of three metal-semiconductor junctions show that the Fermi level of metal is pinned to a fixed position in the semiconductor band gap of hydrogenated 7-ASiCNR. The nearly same rectifying current-voltage characteristics are found in three metal-semiconductor junctions. The average rectification ratios of three M/SiC Schottky junctions are all in the neighborhood of 10⁶. In other word, the M/SiC Schottky junction has remarkable application prospect as the candidate for Schottky Diode.     

Transition density of states for Si(100) from L1L23V and L23VV Auger spectra

The L₁L₂₃V and L₂₃VV Auger spectra of sputtered and annealed Si(100) have been measured and the transition density of states extracted. The line shapes for the two transitions differ, indicating the importance of matrix element effects. Whereas the L₁L₂₃V line shape closely resembles results of other measurements of the Si density of states, the L₂₃VV line shows a strong emphasis on p-like states.