QUASIPARTICLES IN HIGH-TEMPERATURE SUPERCONDUCTORS

We study the quantum criticality effects induced by a singular charge vertex on the quasiparticle spectral function of an extended single-band Hubbard model. It is shown that the spectral intensity computed in a strong-coupling approach, reproduces the Momentum Distribution Curve (MDC) and the Energy Distribution Curve (EDC) of ARPES experiments on Bi 2 Sr 2 CaCu 2 O 8+δ.

Quantitative Auger and XPS Analysis of Thin Films

In 1925, P. Auger first observed the so-called Auger electrons in a Wilson cloud chamber. He explained this occurrence as being due to a radiationless transition in atoms excited by a primary x-ray photon source. In 1953, Lander first pointed out that Auger electrons arising from solid samples can be detected in the energy distribution curve of secondary electrons from surfaces subjected to electron bombardment. Moreover, low-energy Auger electrons (∼1 keV kinetic energy) can escape from only the first several atomic layers of a surface since they are strongly absorbed by even a monolayer of atoms. Thus Auger electron spectroscopy (AES) possesses high surface sensitivity. This is one characteristic that makes AES very useful for the study of thin films. For such applications, an important development in AES occurred when Harris showed that the sensitivity of the detection of Auger electrons can be improved by differentiating the electron energy distribution curve with respect to the energy. Furthermore, Weber and Johnson demonstrated that, provided the Auger line profile does not change, the peak-to-peak height in the differentiated energy distribution curves is proportional to the Auger current in the peak. Therefore, in addition to its surface sensitivity, AES also can be used for quantitative studies of thin films.Like AES, x-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique that uses the energy distribution of electrons ejected from a thin film for quantitative analysis. However, in many ways the information provided by AES and XPS is complementary.

The Oxidation Behavior of the √3×√3 Ag and Au/Si(111) Surfaces at Room Temperature Studied by Photoemission

ABSTRACTThe differences of the room temperature oxidation behavior of ordered Ag/Si(111) and Au/Si(111) surfaces were studied by surface sensitive soft x-ray photoemission spectroscopy obtained with synchrotron radiation. Si surfaces covered with a monolayer of Ag or Au, once annealed to display a √3×√3 LEED pattern, were believed to be passivated against oxidation according to earlier reports. This work shows that these two surfaces oxidize but in a different way. Up to 104 L O2 exposures, the observed valence band of the Au/Si surface's valence band electron energy distribution curve is almost identical to that of the surface before oxygen exposure. But the corresponding Si 2p core level spectrum shows a small chemically shifted component indicating an initial stage of the formation of Si oxide. Thi3 chemically shifted signal becomes a strong peak at -3.7 eV below the clean Si position, characteristic of SiO2, after subsequent O2 exposures up to 1010 L. The Ag/Si system behaves in a similar fashion, but oxide growth saturates at 108 L, and the final oxides formed include a distribution of suboxides in addition to SiO2. Clearly, oxide formation is not prohibited by the presence of the ordered Au or Ag metal overlayer but delayed. Although the onset of oxidation is delayed compared to that for the clean Si surface, due to the metal-silicon bonding, the oxide formation is much faster once the surface starts to oxidize.

The β -ray spectrum of radium E

The problem of the β -ray spectrum of Ra E is too familiar now to require more than brief introduction. Owing to the eminent experimental suitability of this substance, its β -ray spectrum has been investigated widely, and various workers have given summaries of the results up to date, e.g. O’Conor (1937) and Martin and Townsend (1939). Most work has been done on the upper limit or on the form of the intermediate part of the spectrum, with fairly concordant results. Only a scanty amount of work has been done on an accurate investigation of the lower regions of the spectrum, where the experimental difficulties are much greater on account of the necessity of avoiding scattering in the source and along the track of the particles in the spectrograph, and of reducing the absorption in the window of the counter. Richardson (1934) first demonstrated by cloud chamber measurements, using a satisfactorily thin source mounting, that there is a considerable intensity of slow electrons between 15 and 60 ekV, and that the ordinate of the energy distribution curve is about as high in this region as at the previously estimated maximum. However, on account of straggling and other causes, it would be extremely difficult to determine exactly the form of the energy spectrum below 100 ekV by cloud chamber methods, and most other work has been done by semicircular magnetic deflexion with Geiger counters for registration of the particles. The results of chief importance are those of Alichanian and Zavelsky (1937) and of Flammersfeld (1937, 1939). In the discussion below the conclusion is reached that in both these sets of investigations the source mountings were quite satisfactory even down to 30 or 20 ekV, but that in the former the thickness of the counter window (1 µ cellulose acetate) can scarcely be regarded as small enough for measurements below 50 ekV, whilst in the latter absorption effects in the 0.3 µ Zaponlak window must begin to enter between 30 and 20 ekV.* Moreover, in both cases the spectrographs are not sensibly free from scattering. The experiments to be described below were made in an attempt at a precise and direct observation of the undistorted spectrum, especially below 100 ekV, and it is believed that the results do represent a practical realization of this ideal, down to the region of 20 ekV at least, without the need for the application of corrections of any kind.

The β-ray spectrum of Ra E

Interest in the continuous β-ray spectrum has been revived during the past few years by the discovery of induced β-ray activity and the difficulty which has been experienced in incorporating an account of the phenomenon in the theory of the nucleus. Attention has been focused on two features of the spectrum: the high-energy limit, the accurate measurement of which yields the total change in nuclear energy associated with the β disintegration, and the form of the energy distribution curve, which is discriminative in theories of the β-ray emission process. Owing to the convenience of R aE as a source, the β-ray spectrum of this element has received considerable attention, and a comprehensive table of previous work published in a recent paper by O’Conor (1937) shows that recent values of the high-energy limit obtained with magnetic spectrometers are in fair agreement. The form of the R aE spectrum, however, is still not known with any certainty. This can be made clear with the help of Table I, which sets out the results and significant experimental details of the work carried out since 1935 with magnetic spectrometers. Some recent work with cloud expansion chambers is not included because the results are rather discordant. With the relatively low energy electrons of R aE and the high probability of nuclear collisions in the chamber, measurements of the energies of the β-particles are extremely difficult, and the results are probably not as reliable as those obtained with magnetic spectrometers.

The β-particle emission of radium D

The main object of this investigation was to obtain an energy distribution curve for the electrons emitted by radium D during its disintegration. Such a distribution curve may be expected to be made up by the electrons coming from the nucleus, which have a continuous distribution of energy, together with secondary particles forming groups of homogeneous energy, produced by the action of the single γ-ray of radium D. The nuclear electrons are of great interest because owing to their very low energy they have never been identified with any certainty. The energy distribution finally obtained is shown in fig. 4, each point on the curve representing the number of particles having energies lying within 1000 volts on either side of the point. A secondary object of the work was to attempt to estimate the absolute number of particles emitted per disintegration, which number should, of course, be unity for the nuclear particles and some fraction less than unity for the secondary groups. The latter are due to the ejection of electrons from the L and outer atomic levels only, since the energy of the γ-ray, 47,200 volts, is insufficient to ionise the K level.

The colorimetric properties of the spectrum

The paper describes an investigation carried out at the National Physical Laboratory to determine the colorimetric properties of a group of seven subjects as obtained from direct measurements of the trichromatic coefficients of the spectrum on a trichromatic colorimeter. The “spectral distribution curves of the primaries,” by means of which the colorimetric quality of a heterochromatic stimulus may be computed from its energy distribution curve, are obtained by combining the experimentally determined trichromatic coefficients with the International Standard visibility curve. This procedure is a simplification, applicable to the mean results of a normal group, of a general method by which the chromatic and luminosity functions of any subject or group of subjects can be determined from one set of observations. The general method is described in an Appendix.

A differential retarding potential method for the study of the energy distribution of slow electron emissions

The method described gives the differential energy distribution curve directly from readings of a galvanometer and a voltmeter. When adjusted, the arrangement is convenient in operation and very good “resolution” can be obtained. The existence of a maximum frequency of secondary electrons at a finite small energy has been established, and its position is shown to be independent of the energy of the primary beam over a fairly wide range. The effect of deposition of tungsten on a clean copper surface is studied with respect to the secondary emission properties of the surface. From these experiments some interesting conclusions concerning the mechanism of the process may be drawn.I am very grateful to Prof. Sir Ernest Rutherford for the encouragement he has given and the interest he has taken in this work.