The optical properties of the heavy-fermion compound UPd 2 Al 3 have been measured in a frequency range from 0.04 to 5 meV (0.3-40 cm Ϫ1 ) at temperatures 2 KϽTϽ300 K. Below the coherence temperature T* Ϸ50 K, a hybridization gap opens around 10 meV. As the temperature decreases further (Tр20 K), a wellpronounced pseudogap of approximately 0.2 meV develops in the optical response; we relate this to the antiferromagnetic ordering which occurs below T N Ϸ14 K. The frequency-dependent mass and scattering rate give evidence that the enhancement of the effective mass mainly occurs below the energy which is associated to the magnetic correlations between the itinerant and localized 5 f electrons. In addition to this correlation gap, we observe a narrow zero-frequency conductivity peak which at 2 K is less than 0.1 meV wide, and which contains only a fraction of the delocalized carriers. The analysis of the spectral weight infers a loss of kinetic energy associated with the superconducting transition.

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The short wavelength cutoff has been introduced in the calculation of ac fluctuation conductivity of superconductors. It is shown that a finite cutoff leads to a breakdown of the scaling property in frequency and temperature. Also, it increases the phase φ of the complex conductivity (tan φ = σ2/σ1) beyond π/4 at Tc. Detailed expressions containing all essential parameters are derived for 3D isotropic and anisotropic fluctuation conductivity. In the 2D case we obtain individual expressions for the fluctuation conductivity for each term in the sum over discrete wavevectors perpendicular to the film plane. A comparison of the theory to the experimental microwave fluctuation conductivity is provided.

Perturbation of a microwave cavity by a small sample with variable dielectric, magnetic, or conducting properties is considered. The complex frequency shift is derived in terms of a volume integral, or equivalently, in terms of a surface integral. These are used to obtain a general formula for thin films in the microwave electric field maximum. The complex frequency shift depends on the depolarization factor of the film and on its thickness in a nontrivial way. The previously known expressions for the complex frequency shift are shown to be good approximations of the present solution in the low and high conductivity limits. Our formula is applied to calculate the signal shapes in superconducting films of various geometric parameters and conductivities. It is shown that a diversity of signal shapes can result, and experimental support of those shapes is provided. The role of the dielectric substrate on which the thin film is grown is simply reduced to an asymmetry effect.

We have developed a new and fast method for the determination of the complex frequency shift of a microwave resonant cavity. The method is based on frequency modulation of the microwave source around the cavity resonance and detection of the 2nd and 4th harmonic of the modulation frequency. With this procedure the static measurement of the response amplitude is not necessary and all the data are obtained through a single ac channel. The optimal frequency deviation is shown to be comparable to the cavity resonance width.