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In the present investigation (2002) the current-voltage characteristics (CVCs) of break junctions in polycrystalline $MgB_2$ samples have been studied in the temperature range $4.2 K < T < T_c$. An inelastic Cooper pair tunneling accompanied by the emission of Leggett plasmons has been found for the first time in $MgB_2$ Josephson contacts. A fine structure in the CVCs of $MgB_2$ Andreev point contacts caused by a resonant emission of Leggett plasmons has been observed for the first time. An energy of the Leggett plasmon has been estimated: $E_0$ ~ 4 meV. Distinct two-gap structures have been found in the CVCs of $MgB_2$ tunneling contacts and $MgB_2$ Andreev point contacts with 7.5 meV $< \Delta_L <$ 11 meV and 1.5 meV $< \Delta_S <$ 2.5 meV. An intrinsic tunneling effect (ITE) and intrinsic multiple Andreev reflections effect (IMARE) have been observed due to the layered structure of $MgB_2$. The obtained experimental results give a direct evidence for a two-gap superconductivity in $MgB_2$. ("V. Tarasov's Readings", November 2002, Russia, Moscow, conference proceedings.), Comment: 20 pages (8 pages - preface, 7 pages - electron copy of the "V. Tarasov's Readings" International conference proceedings published in December 2002, 4 pages - electron layout of references and figures), 6 figures. On the occasion of 20th anniversary of the first experimental observation of the Leggett collective plasma oscillation in MgB2

Detailed temperature dependence of both superconducting gaps was obtained directly by means of SnS-Andreev spectroscopy. The \Delta \sigma,\pi(T) -curves were shown to be deviated from standard BCS-like behavior, due to k-space proximity effect between \sigma - and \pi - condensates, which could give a key to experimental determination of interband electron-phonon 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 MgB2 samples (with defects of crystal structure), and Mg(Al)B2 polycrystalline samples (with the local critical temperatures Tc variation 10 K < Tc < 37 K) were studied by means of the "break-junction" technique within the temperature range 4.2 K < T < Tc., Comment: 13 pages; 3 figures; typos are corrected in v3; submitted to Solid State Communications

The break-junction tunneling has been systematically investigated in MgB2. Two types of the break-junction contacts have been exploited on the same samples, which demonstrated tunnel contact like (SIS) and point contact like (SnS) behavior. Both of them have shown the existence of the two distinct energy gaps. We have observed also the peculiarities on the I(V)- characteristics related to Leggett's collective mode assisted tunneling. --> Corresponding author address: kuzmichev@mig.phys.msu.ru, Comment: 14 pages, 6 figures, 1 table; corrected typos and figs

Far-infrared reflectance of a MgB2 film has been measured by Fourier-transform spectroscopy for frequencies 10 cm^{-1}<\nu <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., Comment: 5 pages, figures included

We have measured the temperature dependence of the microwave surface impedance Z_s = R_s + i\Omega\mu_0 of a MgB2 film at a frequency \Omega /2\Pi of 18 GHz employing a dielectric resonator technique. We found that the temperature dependence of the magnetic field penetration depth \Lamda can be fitted by \Lamda(T)= \Lamda(0) [1-(T/Tc)^2] ^(-1/2) with \Lamda(0)=(260 +/- 20)nm. The absolute value of \Lamda(0) was confirmed by direct measurements employing submillimeter wave transmission spectroscopy at 430 GHz. The analysis of the \Lamda(T) data below Tc/2 revealed significant deviations from the quadratic temperature dependence. In contrast, we found that an exponential temperature dependence fits the experimental data within the statistical measurement error for temperature changes of \Lamda of +/- 0.4 nm. This observation indicates thermal excitation of quasiparticles over a finite energy gap of (3.3 +/- 0.3) meV corresponding to \Delta /kTc = 1.2 +/- 0.1. Our results strongly supports a multipgap or a strongly anisotropic gap and the absence of nodes in the gap function., Comment: 8 pages, 4 figures, submitted to PRL

The complex conductivity of a MgB2 film has been investigated in the frequency range 4 cm^{-1}< nu < 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., Comment: 4 pages, 4 figures

The anisotropic conductivity of thin Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ films was measured in the frequency range 8 cm$^{-1}<\nu <$ 40 cm$^{-1}$ and for temperatures 4 K $

The anisotropic conductivity of a series of tilted Nd_{1.85}Ce_{0.15}CuO_{4-\delta} thin films was measured by quasioptical spectroscopy in the frequency range 3cm^{-1}<\nu <40cm^{-1}. Two characteristic features have been observed in the low-temperature transmission spectra. The first one at \nu = 12cm^{-1} was shown to reflect the c-axis plasma frequency of Nd_{1.85}Ce_{0.15}CuO_{4-\delta}. The second feature represents a new mixed ab-plane/c-axis excitation. The frequency of this resonance may be changed in a controllable way by rotating the polarization of the incident radiation., Comment: 3 pages, 3 figures

A complex study of the electron-phonon interaction in thin NbC films with electron mean free path l=2-13 nm gives strong evidence that electron scattering is significantly modified due to the interference between electron-phonon and elastic electron scattering from impurities.