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The structure and physical properties of superconducting compounds Y(La)-Ba(Sr)-Cu-O are studied, the compounds being prepared by the method of cryogenic dispersion of a charge consisting of premix oxides and carbonates. Electrical conductivity and critical current density of the superconductors are measured over a wide temperature range of 10~$mK$ to 300~$K$. Degradation of the superconductor critical parameters in time and structural characteristics, magnetic susceptibility, heat capacity and acoustic properties are studied, and current-voltage characteristics are determined., Comment: 9 pages, 11 figures

Measurement of the current-voltage characteristics of point contacts makes it possible to study the properties of individual crystallites in a superconducting ceramic. The critical current density in the superconducting regions of the ceramic $\rm La_{1.8}Sr_{0.2}CuO_4$, with a size of the order of several tens of angstroms, is found to attain values of $10^8\ A/cm^2$, which are of the same order of magnitude as the pair-breaking current density, as evaluated from the formulas of the standard theory of superconductivity., Comment: 2 pages, 2 figures

A polarity-dependent reversible change in the current-voltage characteristics between states corresponding to different values of the excess current ${{I}_{exc}}$ is observed for bias voltages of several hundred millivolts in $Ag-ReB{{a}_{2}}C{{u}_{3}}{{O}_{7-x}}$ $\left( Re\text{ }=\text{ }Y,\text{ }Ho \right)$ point contacts in the current-carrying state. The observed effect has a high intensity (right up to a complete suppression or recovery of ${{I}_{exc}}$) and is not accompanied by a noticeable variation of the energy gap or the critical temperature in the point contact region. It is also established that the states with intermediate values of $I_{exc}$ are stable for zero bias voltages across the point contacts, at least up to $110 \ K$. It is assumed that the variation of ${{I}_{exc}}$ and the corresponding modification of IVC are due to the oxygen migration processes induced by the electric field and current, resulting in a local variation of the superconducting parameters in the point-contact region., Comment: 6 pages, 4 figures

Electron-phonon interaction in superconducting transition metals (Ta, Mo, W) is investigated by the method of point-contact spectroscopy. The spectra obtained are compared with model densities of the phonon states $F(\omega)$ computed from experiments on neutron scattering, and with the tunnel electron-phonon interaction function $g(\omega)$ for Ta. Close agreement between the appropriate spectra is obtained for Ta and Mo, while a significant discrepancy is observed for W, which is the appearance of a blurred maximum instead of the two expected separated peaks on the point-contact spectra. The absolute intensities of a number of point-contact spectra $G(eV)$ and electron-phonon coupling constants $\lambda$ are calculated. For all three metals the values obtained turned out to be considerably below those expected. The possible reasons for this disagreement are discussed., Comment: 5 pages, 3 figures

Electrical characteristics are measured in submicron YBaCuO-normal metal point contacts with a direct conductivity. In a microwave field, such contacts reveal a nonstationary Josephson effect which can occur in the current concentration region not only in the initial weak bonds in YBaCuO but also the weak bonds of a phase-slip center type, formed as a result of a significant current injection (as well as a microwave field). The existence of internal weak bonds in the HTS-electrode contact area produces a narrow minimum in the differential resistance near zero biases. Moreover, the effect of magnetic flux quantization is observed for separate granules in magnetic fields stronger than the first critical field., Comment: 5 pages, 3 figures

The point-contact spectra of tetragonal $\rm YBa_2Cu_3O_{7-\delta}$ with $T_c=50-60~K$ are studied at various temperatures. The single-electron energy ($2\Delta_0/kT_c\simeq 17$) and its temperature dependence are determined. The characteristic energies of quasiparticle excitations interacting with conduction electrons in this high-$T_c$. superconductor are found to correlate with the characteristic phonon energies. The possibility of a pair gap, which increases with temperature, is revealed near the critical temperature., Comment: 4 pages, 3 figures

The maximum value of the energy gap $\Delta\simeq 8\ meV$ and the ratio $2\delta/kT_c\simeq~2.5$ are determined for a high-temperature superconductor $\rm Bi_2Sr_2CaCu_2O_{8+y}$ by using point contacts. It was found that the high-temperature superconductor is transformed under the effect of current injection of quasiparticles to a new modified state with a reduced gap, which is stable in a wide range of injection intensity., Comment: 4 pages, 3 figures

Point-contacts formed by the high-temperature superconductor $\rm YBa_2Cu_3O_{7-\delta}$ prepared by cryochemical technique, and a noble metal Ag or Cu) are investigated. The maximum value of the energy gap $\Delta\simeq 40~meV$ and the ratio $2\Delta/kT_c\simeq 12$ are obtained. It is found that along certain crystallographic axes of the superconductor under investigation, the electrical resistivity $\lesssim 10^{-5} \Omega\cdot cm$., Comment: 5 pages, 3 figures

$YBaCuO-Ag$ pressure point contacts with direct conduction are investigated. The excess (relative to the normal state) conductivity mainly caused by fluctuational pairing of electrons above $T_c$ is measured in the temperature interval 100-200~$K$. The superconductivity above 120~$K$ is found to be of the two-dimensional type. The obtained preliminary results indicate the presence of small amount of an unknown phase with $T'_c\gtrsim 200~K$ in $YBaCuO$., Comment: 5 pages, 5 figures

Electrical characteristics $(I(eV), dV/dI(eV), d^{2}V/dI^{2}(eV))$ are studied comprehensively for pressed submicron point contacts between fresh fractures of LaSrCuO ceramic pellets and a normal metal (Cu). Point contacts (PC) with the direct-type conductivity are investigated, which are characterized by a considerable excess current due to Andreev reflection of quasiparticles at an NS boundary of high transparency. Basic requirements for point contacts suitable for PC measurements are formulated. The models of investigated point contacts permitting the interpretation of the observed characteristics are considered. The results of measurement of the energy gap reveal that LaSrCuO is a superconductor with a strong coupling since the maximum value of the ratio $2\Delta/kT\simeq 11$. It is found that superconducting properties may vary appreciably over distances of several tens of angstroms, resulting in some cases in the simultaneous emergence of two gaps corresponding to two critical temperatures. The $dV/dI(eV)$ and $d^{2}V/dI^{2}(eV)$ curves display some high-intensity singularities whose position on the eV-axis is practically independent of temperature and correlates with the phonon spectrum obtained in neutron scattering experiments. This points to a strong electron-phonon interaction in LaSrCuO., Comment: 17 pages, 19 figures

The energy gap has been measured and the characteristic phonon energies determined for the superconducting compound $La_{1.8}Sr_{02}CuO_4$ from the current-voltage characteristics of point contacts with a copper counterelectrode. The temperature dependence of the energy gap was determined. pacs 73.40.Jn, 74.25.Kc, 74.45.+c, 73.40.-c, 74.20.Mn, 74.70.Ad, 74.72.-h, Dn, Comment: 3 pages, 3 figures

Superconducting (SC) superlattices (SL) of the epitaxial semiconductor $PbTe-PbS$/(001) $KCl$ with two-dimensional ordered misfit dislocation (MD) networks across heteroboundaries (HB) are studied, and the obtained results are compared with the corresponding data for $YBaCuO$. An analysis of the fluctuational region showed that the pairing of electrons in SL takes place at the sites of dislocation networks. In the region of SC fluctuations, the IVC derivatives of point-contacts (PC) based on SL and HTS display a metal-insulator transition associated with delocalization of carriers from the vicinity of the structural elements responsible for the emergence of SC. The critical current density in a superlattice is determined, and a mechanism is proposed for formation of excess current on the IVC of PC based on SL and HTS. The values of the energy gap $\Delta_0$ at low temperatures in units of $kT_c$ are found to be anomalously large for SL and HTS. For anisotropic three-dimensional SL with a strong bond between two-dimensional SC planes, two gaps emerge from the MD. For quasi-two-dimensional SL with a weak bond between two-dimensional SC planes, the $\Delta(T)$ dependence is nonmonotonic, and the fluctuation region for these SL and for HTS reveals a gap that is practically independent of temperature. The IVC derivatives of a $Cu$-SL PC reveal an oscillating structure which is restricted to an energy of $90\ meV$ and is apparently associated with the electron-phonon interaction. It is assumed that there is a similarity between the SC mechanisms in SL and HTS. pacs 73.40.Jn, 74.25.Kc, 74.45.+c, 74.50.+r., 73.40.-c, 74.78.-w, 74.78.Fk, 74.20.Mn, 74.40.+k.-n, 74.45.+c, 74.62.Dn, 74.70.Ad, Comment: 21 pages, 14 figures

The energy gap of superconducting $PbTe-PbS$ semiconductor superlattices has been analyzed with the help of point contacts for the first time below the critical temperature, $T_{c}=3.9~K$, and in the fluctuation region $T>T_{c}$. The size of the gap and its dependence on the temperature and field are determined by the position of the point contact inside the superlattice. \pacs{73.40.Jn, 74.25.Kc, 74.45.+c, 74.50.+r., 73.40.-c, 74.78.-w, 74.78.Fk, 74.20.Mn, 74.40.+k.-n, 74.45.+c, 74.62.Dn, 74.70.Ad, Comment: 4 pages, 3 figures

The volt-ampere curves and their second derivatives were studied for niobium point contacts at low temperatures in the voltage range corresponding to the characteristic phonon energies. It was found that while for the dirty contacts in the normal state no PC spectra of phonons could be detected, in the superconducting state there were singularities in the I-V curves corresponding to maxima either in the first or in the second derivatives. The singularities observed were due to the energy dependence of the excess current. We suppose that the origin of these singularities is due to the inelastic transitions of electrons between chemical potentials of Cooper pairs at both sides of the contact, which differ in energy by $eV$. These transitions are possible if $\xi \left(0 \right)\gtrsim d \sim{{\Lambda}_{\varepsilon}}$ ($\xi (0)$ being the coherence length, $d$, the contact diameter, ${{\Lambda}_{\varepsilon}}={{({{l}_{i}}\cdot {{l}_{\varepsilon}})}^{1/2}}$, where ${{l}_{i}}$ and ${{l}_{\varepsilon}}$ being the elastic and inelastic electron mean free paths, respectively)., Comment: 5 pages, 3 figures

The nonlinearities of current-voltage characteristics (IVC) of superconducting niobium point contacts for voltages of the order of the characteristic phonon frequencies are investigated. It is shown that in limiting by dirty contacts ${{l}_{1}}\ll d$ , (${{l}_{1}}$ is the momentum mean-free path of electrons), features are observed on the IVC corresponding to maxima in the first or second derivatives and are situated at energies close to the characteristic phonon energies. These features are due to the energy dependence of the excess current, since the nonlinearity of the IVC in the normal state is several times smaller in absolute magnitude, and its second derivative is monotonic. For a contact diameter $d$ of the order of several tens of angstroms (the contact resistance is of the order of 100 $\Omega $), the thermal effects can be neglected. It is assumed that in the region of the constriction, the characteristic lengths of the system satisfy the relation $\xi \gtrsim d>{{\Lambda }_{\varepsilon }}$ (${{\Lambda }_{\varepsilon }}$ is the diffusion length for energy relaxation of the electrons, $\xi$ is the coherence length) which makes possible inelastic transitions of electrons between pair chemical potentials differing in energy by an account $eV$ ($V$ is the voltage drop across the contact), Comment: 8 pages, 4 figures

A qualitative study of the electron-phonon interaction (EPI) spectra in $2H-NbS{{e}_{2}}$ is carried out by the method of point-contact spectroscopy in the superconducting state. Anisotropy of the EPI spectra for contacts oriented in the principal crystallographic directions (along the $c$ axis and parallel to the basal surface) was observed. Approximate values of the positions of the main features in the phonon density of states were established for both directions. The additional low-frequency peaks observed in the spectra for separate crystallographic orientations are associated with the collective oscillations of the amplitude and phase of charge density waves., Comment: 9 pages, 5 figures

The method of microcontact spectroscopy in the superconducting state was used to investigate weak nonlinearities of the current-voltage characteristics of point contacts made of $N{{b}_{3}}Sn$ single crystals. The nature of the spectrum of the electron-phonon interaction was found to vary considerably from contact to contact, indicating considerable deviations of the composition of the surface of $N{{b}_{3}}Sn$ from stoichiometry. A correlation was established between the nature of the spectrum and the magnitude of the gap singularities of the current-voltage characteristics. In the case of "dirty" high-resistance contacts with strong gap singularities the microcontact spectra were reasonably reproducible, which made it possible to relate them sufficiently closely to the microcontact function of the electron-phonon interaction in the bulk material. It was found that microcontact spectroscopy of this interaction was possible in the superconducting state not only in dirty $S-c-S$ contacts, but also in dirty $S-c-N$ contacts., Comment: 5 pages, 3 figures

When there is a deviation from the inequality $d\ll {{l}_{\varepsilon }}$ ($d$ is the contact diameter, and ${{l}_{\varepsilon }}$ is the energy relaxation length of the electrons), structural features are produced on the current-voltage characteristics of $S-c-N$ contacts at characteristic phonon energies because of the decrease in $\Delta $ due to the accumulation of nonequilibrium phonons with low group velocities near the contact., Comment: 4 pages, 3 figures

The current-voltage characteristics (IVC) of $S-c-N$ point contacts of superconductors with a small coherence length ${{\xi}_{0}}$ reveal steps with discrete values of the differential resistance. This peculiarity is associated with a transition of the contact region of the superconductor to a spatially inhomogeneous state under the influence of the current injection of nonequilibrium quasiparticles penetrating the superconductor to a depth ${{l}_{E}}$ . The role of the relaxation of the disbalance between the occupancies of electron- and hole-like branches of the quasiparticle spectrum is manifested in the displacement of the position of the singularities on the IVC towards higher energies upon an increase in the magnetic field and/or temperature. This effect was observed in superconductors with different ratios of the contact diameter $d$ and ${{\xi}_{0}}$ or ${{l}_{E}}$ in the series $Ta\ (d\ll {{\xi}_{0}},\ {{l}_{E}})\to NbS{{e}_{2}}$, $N{{b}_{3}}Sn\ (d\gtrsim {{\xi}_{0}},\ {{l}_{E}})\to YB{{a}_{1.25}}S{{r}_{0.75}}C{{u}_{3}}{{O}_{7-\delta}}(d\gg {{\xi}_{0}},\ {{l}_{E}})$. Apparently, a jumplike displacement of the boundary between regions with suppressed and equilibrium values of the energy gap near the contact is responsible for the oscillations observed on the IVC of point contacts between single crystals of high- temperature superconductors (HTS) and a normal metal. The resistance-periodic step-like structure of the IVC allows us to estimate the penetration depth ${{l}_{E}}$ of the electric field in $NbS{{e}_{2}}$ and $YBaSrCuO$., Comment: 6 pages, 6 figures

The point-contact spectra of tantalum in the superconducting state, with $Ta$, $Cu$, and $Au$ counterelectrodes, have been studied. We discovered some new distinctive features, whose position on the $eV$ axis is determined by the critical power required for the injection of nonequilibriumquasiparticles. At this level of power the band gap $\Delta $ decreases abruptly in the vicinity of the contact. A correction to the point-contact spectrum, with the sign opposite to that of the usual correction, arises in the region of phonon energies. The maxima in the $Ta$ spectrum become sharper and their position on the energy axis becomes stabilized near the values $e{{V}_{ph}}=7.0$, 11.3, 15.5, and 18 $meV$, which correspond to low phonon group velocities $\partial \omega /\partial q\simeq 0$ in $Ta$. This is confirmed by the existence of corresponding flattenings on the dispersion relations $\omega (q)$ of lattice vibrations. Slow phonons are created near the $N-S$ interface in quasiparticle recombination and relaxation processes and cause a decrease in $\Delta $ and an increase in the differential resistance in the vicinity of $e{{V}_{ph}}$. An excess quasiparticle charge is accumulated in the region of the contact, producing a correction to the resistance, which decreases as $eV$, $T$, and $H$ increase. These mechanisms are particularly effective in dirty contacts, thus permitting phonon spectroscopy in the superconducting state even when the current flow occurs in a nearly thermal mode., Comment: 13 pages, 16 figures

Tantalum is studied by the method of point-contact (PC) spectroscopy: the parameter ${{\lambda }_{pc}}$ and the absolute intensity of the PC EPI function are determined, and the form of the EPI function is determined more accurately. Both homocontacts $Ta-Ta$ and heterocontacts $Ta-Cu$ and $Ta-Au$ were studied. It was found that the contributions of copper and gold to the spectrum of the heterocontact are not noticeable, though the forms of the PC spectra of $Ta$ in homo- and heterocontacts are different. The intensities of the spectra of $Ta-Ta$ homocontacts and $Ta-Cu$ heterocontacts turned out to be close to one another. Simple expressions, enabling numerical calculations of the PC characteristics for geometrically symmetric heterocontacts in the approximation of a spherical Fermi surface, are derived, based on the theory. It is shown that the use of the free-electron approximation in the numerical estimates leads to an error. It is proposed that the effective mass approximation be used for such estimates., Comment: 12 pages, 4 figures

A phase transition of the region of the superconductor near the point contact into a new nonequi- librium state at the critical density of nonequilibriumquasiparticles is observed., Comment: 3 pages, 3 figures