Novel photoemission studies of the PtCu alloys

The photoemission spectra of PtCu alloys of a number of different compositions has been measured. A new method of sample preparation by an interdiffusion technique allowed rapid measurement as a function of composition. The results are in general agreement with the coherent potential approximation to the electronic structures of alloys.     

Interpretation of photoemission from Na (1 1 0)

A self-consistent calculation of the electronic structure of the Na (1 1 0) surface, using an embedding method to treat the semi-infinite system, gives a prominent surface resonance peak at 0.75 eV above the Fermi energy. The tail of this resonance extends below E_F at the surface, and it is suggested that surface photoemission from this tail is responsible for the peak in the photocurrent at E_F observed experimentally, A photoemission calculation, with a non-self-consistent surface potential, reproduces the enhancement of the peak as a direct transition moves through it.     

The photon angle dependence of photoemission from a Cu(100) crystal

The symmetry properties of direct interband transitions are shown to have important consequences for angle-resolved u.v. photoemission. Predictions are made regarding the photon incidence angle dependence of Ar I (11.7 eV) excited photoemission from a Cu(100) single crystal surface, and these are shown to be in close agreement with the experimental results.     

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.     

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.     

Final state band gap effects in UV photoemission from Cu(111)

The crystal momentum dependence of the final states involved in bulk photoemission from the sp-bands through a (111) surface of copper was investigated with a photon energy hν = 11.7 eV. Plane wave hybridization was observed and is described in terms of a 2-OPW model Hamiltonian with parameters determined to be V₀=7.1±0.1 eV for the inner potential and E_g = 1.6 ± 0.2 eV for the energy gap. The model is also shown to account for a rapid intensity variation with crystal momentum of the form |a_G(k)|², where a_G(k) is a plane wave amplitude of the final state wave function.     

Observation of light-phase-sensitive photoemission from a metal

We demonstrate that multiphoton-induced photoelectron emission from a gold surface caused by low-energy (unamplified) 4-fs, 750-nm laser pulses is sensitive to the timing of electric field oscillations with respect to the pulse peak. This observation confirms recent theoretical predictions and opens the door to measuring the absolute value of the carrier-envelope phase difference of few-cycle light pulses with a solid-state detector.     

Spin fluctuations and the peak-dip-hump feature in the photoemission spectrum of actinides

We present first-principles multiband spin susceptibility calculations within the random-phase approximation for four isostructural superconducting PuCoIn{5}, PuCoGa{5}, PuRhGa{5}, and nonsuperconducting UCoGa{5} actinides. The results show that a strong peak in the spin-fluctuation dressed self-energy is present around 0.5 eV in all materials, which is mostly created by 5f electrons. These fluctuations couple to the single-particle spectrum and give rise to a peak-dip-hump feature, characteristic of the coexistence of itinerant and localized electronic states. Results are in quantitative agreement with photoemission spectra. Finally, we show that the studied actinides can be understood within the rigid-band filling approach, in which the spin-fluctuation coupling constant follows the same materials dependence as the superconducting transition temperature T{c}.     

Origin and doping dependence of the photoemission pseudogap in Cu oxides

A recent report on ARPES on insulating , compared to previous data from and Dy-doped , sheds new light on the origin of the anisotropic pseudogap observed in the normal state of underdoped cuprate oxides. The energy dispersion of the insulator is attributed to strong AF correlations enhanced by the diagonal hopping between magnetic sites, which is progressively deformed by the possibility of nearest neighbour hopping, that increases with hole doping. Received 9 April 1999     

Bulk,surface and thermal effects in inverse photoemission spectra from Cu(100), Cu(110) and Cu(111)

We have performed a study of empty electronic bulk and surface states on the three low indexed copper surfaces employing momentum resolved inverse photoemission. The bulk electronic features may be well understood in the frame work of the bulk direct transition model using state of the art band structure calculations. Surface states of both, the crystal derived and the image potential induced type have been identified and were found to agree with previous work. Several radiative transitions into unoccupied bands were also investigated at elevated temperatures. Characteristic temperatures of an exponential attenuation law are distinctly different between surface and bulk transitions. However, no systematic behaviour of bulk transitions at different points of the Brillouin zone could be established.     

Interpretation of UPS from Cu(111) using a time reversed LEED formalism

Angle resolved photoelectron spectra from Cu(111) have been studied experimentally and theoretically. A one-step model has been found able to reproduce all observed features. In particular a recently observed resonance at hν = 70 eV is interpreted in terms of joint density of states modulations.     

Some polarisation effects in photoemission from a cubic crystal

The energy distribution spectra of photoelectrons emitted normal to the (100) face of tungsten were investigated with respect to their polarisation dependence. The results are compatible with the assumption that surface photoemission vanishes if the exciting light is polarised parallel to the emitting surface. This is demonstrated for the examples of emission from a high density of states at the surface, a surface resonance, and an adsorbate induced level. The measurements also show that emission following direct optical excitation in the bulk is polarisation sensitive, supporting the idea of an anisotropic excitation along a star of k-vectors in the bulk. Some indication is also given of emission in ‘secondary cones’.     

Plasma resonance absorption in interfacial photoemission from very thin silver films on Cu(111)

Very thin silver films deposited on a Cu(111) substrate were studied with the photoemission-into-electrolyte technique. The optical resonance absorption at the silver bulk plasma frequency was observed for films thicker than two monolayers. The correlation between optical absorption and photoemission intensities depends in a complicated fashion on the photon energy as well as the film thickness.     

Interpretation of hyperfine fields in concentrated ferromagnetic alloys

A simple physical interpretation is given for hyperfine fields systematics at the sites of nonmagnetic ions in concentrated ferromagnetic alloys. Application is made to Heusler alloys and numerical results are given.     

Microspectroscopy and spectromicroscopy with photoemission electron microscopy using a new kind of imaging energy filter

The use of an imaging retarding field analyser attached to the FOCUS IS-PEEM is described. This kind of energy filter is a simple, powerful tool to obtain microspectra from areas of down to about 1 μm using (V)UV and X-ray excitation sources. First results of microspectroscopy measured by excitation with a laboratory as well as a synchrotron X-ray source are presented.     

Development of a noise sources classification system based on new method for feature selection

Feature selection of noise sources is important for noise sources detection and classification. In this paper, a new rough set based feature selection method has been given. Based on the method, a noise sources automatic classification system (NSACS) has been designed and validated. The key idea of the method is that most effective features can distinguish the most number of samples belonging to different classes of noise sources, if they are used for classification. This new approach has been applied into the system NSACS to select relevant features for artificial datasets and real-world datasets and the results have shown that this approach can correctly select all the relevant features of artificial datasets and at the same time it can drastically reduce the number of features. From the experiments, it can be found that to consider all the five datasets, the number of classification features after selection drops to 35% and the accurate classification rate increases about 14%. For the underwater noise sources dataset the number of features drops to 1/5 and the accurate classification rate increases about 6% after feature selection.