The complex conductivity of a MgB2 film has been investigated in the frequency range 4 cm −1 < ν < 30 cm −1 and for temperatures 2.7 K < T < 300 K. The overall temperature dependence of both components of the complex conductivity is reminiscent of BCS-type behavior, although a detailed analysis reveals a number of discrepancies. No characteristic feature of the isotropic BCS gap temperature evolution is observed in the conductivity spectra in the superconducting state. A peak in the temperature dependence of the real part of the conductivity is detected for frequencies below 9 cm −1 . The superconducting penetration depth follows a T 2 behavior at low temperatures.

Far-infrared reflectance of a MgB2 film has been measured by Fourier-transform spectroscopy for frequencies 10 cm −1 < ν < 4000 cm −1 above and below the superconducting transition. The data provide clear experimental evidence for the onset of a superconducting gap at 24 cm −1 at T = 5 K. On increasing temperature the gap energy increases, contrary to what is expected in isotropic BCS superconductors. The small zero-temperature gap value and its unconventional increases on increasing temperature can only be explained by a highly anisotropic or multiple gap function.

No abstract

A complex study of the electron-phonon interaction in thin NbC films with electron mean free path lϭ2 -13 nm gives strong evidence that electron scattering is significantly modified due to the interference between electron-phonon and elastic electron scattering from impurities. The interference T 2 term, which is proportional to the residual resistivity, dominates over the Bloch-Grüneisen contribution to resistivity at low temperatures up to 60 K. The electron energy relaxation rate is directly measured via the relaxation of hot electrons heated by modulated electromagnetic radiation. In the temperature range 1.5-10 K the relaxation rate shows a weak dependence on the electron mean free path and strong temperature dependence ϳT n , with the exponent nϭ2.5-3. This behavior is explained well by the theory of the electron-phonon-impurity interference taking into account the electron coupling with transverse phonons determined from the resistivity data.

Detailed temperature dependence of both superconducting gaps was obtained directly by means of SnS-Andreev spectroscopy. The ( ) , T σ π Δ -curves were shown to be deviated from standard BCS-like behavior, due to k-space proximity effect between σ -and πcondensates, which could give a key to experimental determination of interband electronphonon coupling constants. For the first time, an excellent qualitative agreement with theoretical predictions of Nicol and Carbotte, and Moskalenko and Suhl was shown. dI(V)/dV-spectra of SnS-Andreev contacts based on 2 * Corresponding author. dimensional (2D) hole condensate described by 7 12 σ Δ = ÷ meV gap and the small 3D one with 2 0.5 π Δ = ÷ meV. According to the theory elaborated by Choi et al. [27] 2 MgB quasiparticle density of states is characterized by two well-legible split gap peculiarities related to the four different bands including two σ -and two π -bands (the four-band model) that can be considered as two effective ones (the two-band approach) [28]. The rather high C T value originates from σ -bands coupling on the 2 g E boron phonon modes [5]. A large boron isotope effect [29] gives support unambiguously to phonon-mediated superconductivity [30].