H Implantation Improves Superconductivity in Non-Transition Metals
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H IMPLANTATION IMPROVES SUPERCONDUCTIVITY IN NON-TRANSITION METALS F. OCHMANN and B. STRITZKER Institut fdr Festk6rperforschung, D-5170 J~lich, FRG
Kernforschungsanlage
J~lich D-5100 Julich
ABSTRACT The superconducting transition temperature, TC , of a series of non-transition metals was improved by implantation of H and D into these metals kept below 10 K. In all cases the increase of Tc was considerably larger for H and D implantation with respect of He implantation causing only radiation effects. It could be shown that the enhancement of T depends on the electron-phonon coupling constant of the host metal, suggesting that mainly phonon effects dominate the improvement of Tc . INTRODUCTION During the last decade many metal-hydrogen systems have been discovered showing interesting superconducting properties, i. e. hydrogenated Pd 5 5 Cu4 5 alloys with a T 0 17 K [i]. Although the general superconducting properties of the widely studied Pd-H system are rather well understood, a detailed understanding of its inverse isotope effect or the influence of noble metals is still missing. This is even more true for the other superconducting metal-hydrogen systems. In this paper we performed a systematic study of the influence of H on the superconducting properties of non-transition metals. We chose non-transition metals with broad s-p bands in order to avoid considerations about the influence of small changes within a complicated electronic band structure due to the H interstitials. Thus one should be able to extract a well defined H influence. EXPERIMENTAL AND RESULTS Since H and D are almost insoluble in all non-transition metals we used the method of low temperature implantation to introduce H and D. The target temperature 4-10 K hinders the diffusion of H and D. Thus the hydrogen isotopes can be accumulated also in the non-transition metals. However, this method has the disadvantage that not only H and D but also lattice defects are introduced into the target material. It is known from a large variety of experiments on vapor quenched materials [2] that lattice disorder increases the superconducting transition temperature of non-transition metals due to a "weakening" of the phonon spectrum. In order to simulate the effect of radiation damage we implanted inert He atoms with the appropriate energy. For each non-transition metal we implanted H, D and He in three different experiments using similar starting materials. +
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The implantation of H , D and He was performed with energies between 2 2 7 and 100 KeV depending on the sample type. The implantations yield Gaussian type of concentration profiles in a depth of 50 to 300 nm and a width of 100 to 400 nm.
Mat. Res.
Soc. Sym.
Proc. vol. 27 (1984) 0Elsevier Science Publishing Co.,
Inc.
572
The elements under investigation were Be, Zn, Cd, In, Sn, Ti and Pb. In the case of Zn, In and Pb we used both evaporated thin films of 100 to 200 nm and rolled foils of 10 to 20 pm thickness. We obtained the same results for films and foils of the same material. Only foils w
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