Narrow Results

We review some of the problems still affecting photoemission as a probe of high-temperature superconductivity, as well as important recent results concerning their solution. We show, in particular, some of the first important results on thin epitaxial films grown by laser ablation, which break the monopoly of cleaved BCSCO in this type of experiments. Such results, obtained on thin LSCO, may have general implications on the theory of high-temperature superconductivity.

In this contribution, the peak position of the spectral function in the ARPES spectrum has been calculated for several gap symmetries and electronic band structures. As a result, the d–wave, s + id and s* + id symmetries appear to be consistent with ARPES results. An additional comparison with previous calculations of the specific heat in normal and superconducting states as well as the single particle tunnelling current, reveals the existence of two characteristic temperatures, intrinsically associated with the Van Hove Singularity.

The unusual properties of a doped high-T_c oxide can be attributed to its unique electronic structure, here we put emphasis on importance of the O2p band for the origin of magnetism and superconductivity. Antiferromagnetism of a charge transfer type insulator should be described by the Kramers-Anderson theory, the superexchange between localized Cu3d spins is mediated by O2p electrons. A doped hole resided in the O2p orbital in a Cu-O-Cu bond effectively suppresses the superexchange between Cu3d spins creating a local moment or a "Kondo impurity" in the magnetically ordered CuO₂ layer. The interaction between a local moment and O2p electrons yields an effective spin coupling between O2p electrons and pairing. Accordingly, itinerant O2p spins may condense into a paired state or a spin liquid state via local exchange interactions. On the contrary, Cu3d spins merely provide a magnetic background, the existing local field breaks the time-reversal symmetry in a photoemission process. We use a semiclassical approach to estimate the difference between the intensities of photocurrents generated by the left and the right circularly polarized photons observed in Bi₂Sr₂CaCu₂O_8+δ by Kaminski et al.

Increasing experimental evidence points towards a two-fluid model for the low-temperature behavior of 5f systems. Assuming some 5f electrons to be localized atomic-like and others to be band-like itinerant can explain various unusual low-energy properties of U compounds including the formation of heavy fermion and their unconventional superconductivity. The dual model, however, provides an effective Hamiltonian which is valid only for the low-energy regime. It has been shown that intra-atomic correlations among the 5f electrons are a possible mechanism for orbital-selective (partial) localization. In this paper we theoretically derive criteria and specify properties which experimentally characterize correlation-induced partial localization in actinide compounds. Of particular interest are the 5f spectral function which can be related to photoemission data. Results for the U-based heavy fermion superconductor UPd₂Al₃ are presented emphasizing the consequences for the interpretation of experimental data.

To enhance the near-infrared response of photocathodes in the application of image detection, two kinds of multilayer complex structures of GaAs-based photocathodes are developed by changing the structure of the buffer layer underneath the emission layer, wherein one is the graded bandgap structure, and the other is the distributed Bragg reflector (DBR) structure. The theoretical quantum efficiency suitable for the reflection-mode (r-mode) GaAs-based photocathode with these two kinds of buffer structures are deduced based on one-dimensional continuity equations, among which the reflectivity changing with the wavelength of incident light is considered in particular. By comparison of spectral characteristics of photocathodes with the two different structures, and analysis of cathode structure parameters, it is found that the photoelectric performance of the photocathodes with the two structures are quite different, and the structure parameters especially the thickness of GaAs emission layer have a great impact on the spectral characteristic. The quantum efficiency of photocathode with graded bandgap structure is improved due to the introduction of the built-in electric field and the decrement of interface recombination, while for cathode with DBR structure, the quantum efficiency is improved by the multiple reflection of light between two parallel mirrors in the stack of alternating quarter-wave layers of high and low refractive. The theoretical quantum efficiency calculation and analysis would provide theoretical guidance for the better design of GaAs-based photocathodes.

A resonance-enhanced GaInAsSb nanopillar array negative electron affinity potential photocathode based on Mie resonance excitation at infrared wavelengths can significantly enhance the absorption and quantum efficiency of nanopillar arrays. Our study indicates the different periodic structures have very little effect on the QE for the nanopillar arrays. However, the smaller the period of the array, the higher the quantum efficiency is achieved, but the probability of being collected by the anode is reduced, so a compromise should be considered in the structure design. Moreover, the incident light inclination and the external electric field can significantly affect the quantum efficiency of the nanopillar arrays. The photocathodes studied here have a quantum efficiency of over 5.7% with an angle of incidence of 30^∘ and an applied electric field intensity of 1.2V/$\mu$m. This work provides an important reference and theoretical basis for the design and optimization of resonance-enhanced GaInAsSb nanopillar arrays photocathode.

In 6H- or 4H-SiC(0001) surface technology, a Si-rich 3 × 3 reconstruction is usually first prepared by heating at 800°C under Si flux, and two other most stable or reconstructions are obtained by further extensive annealing at higher temperatures ranging between 900 and 1250°C. The 3 × 3 Si excess is thus progressively depleted up to a graphitized C-rich surface. By crystallographic (LEED) and chemical surface characterizations (XPS and UPS), we show that all these reconstructions can be obtained at a unique, low formation temperature of 800°C if the Si richness is controlled before annealing. This control is achieved by exposing the 3 × 3 surface to atomic hydrogen at room temperature. This procedure allows one to etch or partially deplete the (3 × 3)-associated Si excess, and make it more comparable to the final Si coverages, required to form the less Si-rich or reconstructions. After annealing at 800°C, the latter reconstructions are no longer determined by the heating time or temperature but only by the initial Si coverage set by the H doses inducing the low temperature etching. The high temperature treatment, required to remove by sublimation a significant Si amount associated with the Si-rich 3 × 3 reconstruction, is thus avoided. Such a methodology could be applied to other binary systems in the formation of reconstructions that depends on surface richness.

The combination of traditional surface-spectroscopic methods and microscopic imaging techniques is gaining popularity along with the advances of nanotechnology. Among the available techniques, scanning photoelectron microscopy (SPEM) and photoelectron emission microscopy (PEEM) are two methods recently developed at the Synchrotron Radiation Research Center (SRRC). The SPEM station uses a Fresnel zone-plate optics to focus the soft X-ray beam and form a microprobe. Photoelectrons emitted from the illuminated spot are used to perform micro-photoelectron spectroscopy (μ-PES) measurements or to image the sample. The PEEM station, on the other hand, collects secondary electrons emitted from the sample upon photon irradiation, and uses an all-electrostatic column to magnify the field of view defined by the objective lens. By stepping the photon energy, micro-X-ray absorption spectra (μ-XAS) can be extracted from a sequence of images after proper image processing.

Increasing experimental evidence points towards a two-fluid model for the low-temperature behavior of 5f systems. Assuming some 5f electrons to be localized atomic-like and others to be band-like itinerant can explain various unusual low-energy properties of U compounds including the formation of heavy fermion and their unconventional superconductivity. The dual model, however, provides an effective Hamiltonian which is valid only for the low-energy regime. It has been shown that intra-atomic correlations among the 5f electrons are a possible mechanism for orbital-selective (partial) localization. In this paper we theoretically derive criteria and specify properties which experimentally characterize correlation-induced partial localization in actinide compounds. Of particular interest are the 5f spectral function which can be related to photoemission data. Results for the U-based heavy fermion superconductor UPd₂Al₃ are presented emphasizing the consequences for the interpretation of experimental data.

The following sections are included:

• Introduction

• Background

• Layer Resolved Spectroscopy

• CO Adsorption

• Oxidation

• Sulfidation

• SO₂ Reaction

• Etching and Passivation

• Outlook

• Acknowledgments

• References

The following sections are included:

• Introduction

• Semiconductor Technology
  • Surface Chemistry During Growth, Etching, and Passivation
  • Interfaces

• Magnetic Materials
  • Electronic States in Magnetoelectronics
  • Magnetic Multilayers

• Nanostructures, Tailored Materials
  • Quantum Wells
  • One- and Zero-Dimensional Structures

• Acknowledgments

• References