Experimental study of the electron–phonon interaction (EPI) spectra of zirconium was carried out using Yanson's point-contact (PC) spectroscopy. The EPI spectral function and constant were determined. Both homocontacts (Zr–Zr) and heterocontacts (Zr–Cu, Zr–Ag, and Zr–Au) were studied. No contribution by copper, silver or gold to the heterocontact spectra was detected. The positions of five phonon features of the EPI function were observed at energies 5–6, 9–11, 13–15, 18–20, and 27–29 meV. The observed diversity of the PC spectra was attributed to the anisotropy of Zr.

The Andreev reflection spectra were studied for the first time in the magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4. It is found that an external magnetic field (up to 0.7 Hc2) is a strong stimulator of superconductivity in contrast with traditional superconductors with a singlet pairing. The ratio 2Δ/kBTc ≈4.0 obtained for some contacts is higher than the value 3.52 which is typical of conventional superconductors with a weak electron-phonon interaction. It has been proposed that in Dy0.6Y0.4Rh3.85Ru0.15B4 a triplet mechanism of superconducting pairing is realized.

The Andreev reflection spectra dI/dV(V) of the magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4 have been investigated. Pronounced stimulation of superconductivity by an external magnetic field has been observed for the first time. The effect showed up as enhancement of the gap structure (and hence the gap itself) in the spectra and its shift towards higher voltages with an increasing field. In the intermediate fields the structure also behaved strangely: instead of the usual smooth decrease with an increasing field, the gap features dropped abruptly near the critical point Hc2. Of interest is also the abnormally high relative gap value 2Δ/kBTc ≈ 4 (as compared to conventional singlet superconductors) which was found for some contacts from a comparison of experimental spectra and the modified Blonder–Tinkham–Klapwijk theory. We attribute the features revealed in the point-contact spectroscopic investigations of Dy0.6Y0.4Rh3.85Ru0.15B4 in a magnetic field to the triplet-type Cooper pairing in the compound because only in this case one can expect the stimulation of superconductivity in the stationary magnetic fields up to ∼0.7Hc2.

Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 having an unexpectedly low superconducting transition temperature Tc~11.3 K among related FeAs compounds on a base Sm and Gd surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as against the expected value, is attributed to the divalent properties of Eu ions when in the compound investigated along with the weakly magnetic Eu3+ ions may be present and the strongly magnetic Eu2+ ones that is a strong destructive factor for superconductivity. Most of the spectra measured showed features that corresponds to two energy gaps whose values varied from contact to contact within 2{\Delta}s/kTc=2.2{\pm}4.7 and 2{\Delta}1/kTc=5.1{\pm}11.7 for small and large gap, respectively. The corresponding variations for single-gap spectra are 2{\Delta}/kTc = 2.6{\pm}6.4. The relatively large size of crystallites (no less than ~25 \mu{m}) and the large number of contacts measured (several tens) suggest with a high degree of probability that the spectra obtained account quite fully for the gap distribution practically in all crystallographic directions. The data obtained and the absence of zero gaps in the measured spectra evidence in favor of the anisotropic s- or s\pm-symmetry of the order parameter in EuAsFeO0.85F0.15 that was revealed in other similar compounds with higher Tc. Thus, the character of the gap function {\Delta}(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides.

Polycrystalline samples of a new superconducting EuAsFeO0.85F0.15 compound with critical temperature Tc=11K were prepared by solid state synthesis. Its electric and magnetic properties have been investigated in magnetic fields from 0.1 to 140000 Oe. Critical magnetic fields Hc1, and Hc2 were measured and hence the magnetic penetration depths and the coherence length have been estimated. The temperature dependence Hc2 (T) exhibits clear hyperbolic - type behavior starting with the lowest fields. The data derived were used to estimate probable high Tc and Hc2 in compounds doped with rare-earths having small atomic radii.

The point-contact Andreev-reflection measurements on. the new paramagnetic superconductor Mo3Sb7 were carried out. It was found that the order parameter Delta is strongly anisotropic varying over a very wide range depending on the contact orientation when the maximum.Delta(max) exceeds at least 40 times the minimum Delta(min) in spite of the invariable T-c that cannot be typical of conventional superconductors. For these and other reasons we suggest that Mo3Sb7 is not a trivial BCS (s-wave) superconductor but rather has some unconventional pairing symmetry.

In this work for the first time we give the evidence that compound Dy1-xYxRh4B4 with the tetragonal body-centered crystal structure LuRu4B4 is the ferrimagnetic at temperatures T-Cur < 40 K, ferrimagnetic superconductor at T-C < 10 K and antiferromagnetic superconductor at T-N < 3 K. No reentrant behavior was found down to T = 0.32 K. For the first time by means of a method of microcontact spectroscopy of the Andreev reflection in point contact Ag-Dy(0.8)y(0.2)Rh(4)B(4) the value and temperature and field dependences of superconducting gap parameter A (T, H) in Dy(0.8)y(0.2)Rh(4)B(4) were determined. The value of the ratio 2 Delta(0)/kT(C) is about 4. Some unusual features of Delta(T,H) dependences were observed, which give the evidence that the Dy1-xYxRh4B4 is a candidate for spin-triplet Cooper pairing of charges with the parallel spins.

Point-contact (PC) Andreev-reflection experiments on the new paramagnetic superconductor Mo3Sb7 are reported for which we have observed strong distinctions in the PC spectra measured for different contact axis orientations. Analysis of the PC spectra in the framework of the Blonder–Tinkham–Klapwijk theory has shown that the gap parameter Δ is strongly anisotropic, varying over a very wide range depending on the contact orientation: the maximum Δmax can exceed the minimum Δmin by at least 40 times. For these and other reasons we suggest that Mo3Sb7 is not a trivial BCS (s-wave) superconductor but rather has (s+g)-wave or another unconventional pairing symmetry.

Andreev reflection in contacts based on the magnetic superconductor Mo3Sb7 in a magnetic field is investigated by the point-contact method. It is found that the behavior of the order parameter in the magnetic field Δ(H) is essentially dependent on the value of Δ, which varies widely (Δ(0,0)≈0.01–0.31meV) for different contacts at practically invariant Tc. At high Δ the dependence Δmax(H) deviates slightly from the theoretical prediction applicable under point-contact conditions. As the order parameter decreases, the deviation starts to increase and becomes very large when Δ approaches its minimum. Proceeding from the results obtained in this study and from the temperature measurements on Mo3Sb7 performed previously, we are inclined as before to assign the compound to the class of superconductors with an anisotropic gap function. Therefore, in no way can it be considered as a conventional BCS-type superconductor. The upper critical field Hc2≃16.5kOe found here is close to that obtained from magnetization measurements in other studies. According to an estimate made, the pair-breaking effect of the Pauli paramagnetism is rather weak in Mo3Sb7.

Direct evidence for superconductivity in the new magnetic compound PrAg6In6 is revealed for the first time. The distinct Andreev-reflection current is observed in metallic point contacts (PC) based on this compound. The data obtained provide reason enough to suggest that the rise of superconductivity depends strongly on the local magnetic order varying over the sample volume. The triangular-shaped PC spectra (dV/dI(V)) in the vicinity of the zero-bias voltage suggest an unconventional type of superconducting pairing. As follows from the temperature and magnetic field dependences of the PC spectra, the superconducting energy gap structure transforms into the pseudogap one as the temperature or the magnetic field increases.

For the first time we present direct evidence for superconductivity in the ternary magnetic compound YFe4Al8 with the ThMn12 type structure, found via point-contact (PC) experiments on contacts between a silver needle and single-crystal YFe4Al8, which reveal a distinct Andreev-reflection current. The spectra measured prove the existence of a normal–superconducting interface and exhibit a triangular-like shape in the vicinity of zero bias voltage, implying an unconventional type of superconductivity. The derived dependences of the order parameter versus temperature Δ(T) and magnetic field Δ(H) are presented. Δ(T) follows BCS theory, whereas Δ(H) does not satisfy any theoretical predictions. In some cases there exists noticeable superconductivity enhancement by a weak magnetic field. The data obtained imply a very inhomogeneous distribution of superconductivity over the sample volume in spite of its single-crystal structure. We assume that the reason is associated with inherent magnetic inhomogeneities of this material. The highest values for the critical temperature Tc, upper critical magnetic field Hc2, and ratio 2Δ(0)/kTc are 7.4 K, 5 T, and 7.2, respectively.

: The influence of electric fields and currents has been investigated in the high-T c superconductors YBaCuO and BiSrCaCuO using a point-contact geometry with Ag as the counterelectrode, which reveal switching transitions between states of a different resistance. The origin of this effect in point contacts is associated with electromigration of the oxygen, driven by the electric field as well as by the current-induced “electron wind”. The switching effect preserves its basic features at elevated temperatures up to room temperature and in high magnetic fields up to 10 T.

Point contacts (PC) based on the magnetic superconductor HoNi2B2C with a superconducting onset temperature Tc∼8.5 K have been investigated in order to determine the temperature and magnetic field dependences of the superconducting order parameter. The temperature dependence of the order parameter satisfies the Bardeen–Cooper–Schrieffer (BCS) theory only below the BCS transition temperature TcBCS=5.5–5.8 K, which exceeds slightly the Néel temperature TN∼5.0 K. At higher temperatures, above TN, the PC spectra dV/dI(V) indicate an anomalous, most likely gapless, superconducting state. At magnetic fields well below Hc2, a considerable increase (∼30%) in the characteristic voltage of the gap-related structure in the PC spectra is observed which points to an improved spin alignment with a decreasing influence of the pair breaking in modest fields.

Andreev-reflection spectra of superconducting-normal contacts with HoNi2B2C show a continuous increase of the superconducting order parameter at the antiferromagnetic phase transition TN = 5 K without re-entrant behaviour below the superconducting critical temperature Tc = 9 K. A change is found in the superconducting ground state at Tc* = 6.5 K (zero magnetic field), and the magnetic-field–temperature phase diagram corresponding to the two superconducting states is reconstructed.

A comparative study has been made of the current—voltage characteristics of RNi2B2C-Ag point contacts with R = Y, Er, and Ho, at various temperatures and magnetic fields. At low temperatures the double-minimum structure in the differential resistance dV/dI, characteristic for Andreev-reflection processes, can be well described by the Blonder—Tinkham—Klapwijk (BTK) model, yielding an energy gap Δ = 2.42 ± 0.07, 1.7 ± 0.2, and 1.04 ± 0.06meV for, respectively, the Y-, Er-, and Ho-based compounds. For the Y- and Er-based compounds, the temperature dependence of the spectra is in quantitative agreement with the BTK prediction. However for the Ho-based compound, an anomalous temperature dependence with two superconducting transitions at 8.5 and 6.5K has been observed. The ratio 2Δ/kBTc amounts about 3.7 for all compounds (providing one takes Tc* ⋍ 6.5 K for the critical temperature of the Ho compound), corresponding to a moderate electron—phonon coupling strength.

An Andreev-reflection study is presented on point contacts with the high-Tc superconductor (Hg0.7Cr0.3)Sr2CuO4 with a critical temperature near 60 K. The normal-metal—superconductor point contacts show a clear superconducting gap structure in the dV/dI(V) spectra with gap parameter Δ=10–15 meV yielding 2Δ/kBTc= 3.4–5.1. The temperature dependence of the Andreev-reflection spectra was analyzed within the Blonder—Tinkham—Klapwijk model by the introduction of a broadening parameter Γ.

An essentially nonlinear dependence of the differential resistance dV/dl on the bias V has been established in bismuth-based microbridges. At low temperatures when the mean drift velocity of electrons nuBAR becomes of the order of the Fermi velocity nu(F) a decrease of the differential resistance is observed with the increasing bias. Drop of resistance is due to ballistic charge carrier transit within the region of microbridge at large biases. For the first time the phonon spectrum of Bi has been observed in microbridges.

A polarity-dependent reversible change in the current-voltage characteristics between states corresponding to different values of the excess current Iexc is observed for bias voltages of several hundred millivolts in YBaCuO-Ag point contacts in the current-carrying state.