Study of electron shadow patterns of the intermediate state of superconducting lead

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Abstract
This paper reports on a specimen magnetization stage and on the study of superconducting lead cylinders by means of shadow electron microscopy.The magnetization stage is made up of three Helmholtz pairs to produce the required magnetic field on the specimen. It is shown that this system will introduce only third-order aberration to the electron beam if the middle pair is adjustable with respect to the others so that only central incidence can occur.Using shadow electron microscopy, observations have been made of the superconducting intermediate state of hollow lead cylinders in transverse magnetic fields. The resultant structure gives rise to distorted cycloid patterns with secondary focusing spots. The images have been analysed and interpreted both qualitatively and quantitatively on the basis of a one-dimensional periodic structure (lamellar model) assuming the full expressions for the z and x components of the magnetic fields as calculated by the method described by Kittel (1946, 1949) for an infinite, flat specimen. In particular it is shown that distortion of the cycloid patterns is produced where the width of the normal lamellae is different from that of the superconducting lamellae and that secondary spots are formed when a fine structure modulates the overall periodicity of the magnetic field distribution at the surface of the specimen.