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The transition temperature of a superconductor depends on ct2F(co), the spectral function of the effective interaction due to phonon exchange. We discuss how strongly the transition temperature is influenced by different frequency parts of ~2F(co). For this purpose the functional derivative ~Tc/&~2F(co) is calculated. It is shown that all frequency regions of ~2F(O) yield a positive contribution to T c and that the most effective range covers frequencies, slightly above 2 nT c. The functional derivative is calculated numerically for several superconductors from their measured ~2F(co)-spectra. Finally, we discuss the change in transition temperature due to the softening of 0~2F(co) which has been observed in amorphous superconductors. IntroductionIn superconducting tunneling experiments the Eliashberg function ~2F(co) and the Coulomb pseudopotential #* can be determined. The transition temperature of a superconductor can be calculated in terms of these quantities using the linearized Eliashberg equations [1]. Such calculations reflect an integral property of the total spectrum ~2F(co) and do not answer the question of how much different frequency parts of ~2 F(co) contribute to the calculated transition temperature T c.To investigate this problem one can calculate A Tc, the change of the transition temperature which would result if ~2F(co) is set equal to zero in a frequency interval between coo and coo + A co. For small A co the change AT c is proportional to c~2F(coo) multiplied by a weighting factor. This weighting factor is simply the appropriate functional derivative, i.e. (1) ATe= ,~.2 F(co )The functional derivative, considered as a function of frequency, can therefore be thought of as a measure of the strength of the influence of a particular frequency on T c. In this sense a calculation of the functional derivative tells one how favourable a certain frequency range is for an increase of the transition temperature. By means of the functional

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We summarize investigations on four different nematic main-chain elastomer systems. In the first part, the synthetic approaches towards nematic elastomers with clearing temperatures suitable for mechanical investigations are described. In the second part, the coupling between the liquid crystalline order and the chain conformation is quantified by the Landau-de Gennes coefficient U. Comparing different types of elastomers shows that the coupling depends on the type of mesogens forming the polymer backbone, and on the elastic modulus of the polymer network. In the third part, we compare the orientation behavior of the different systems under strain.