E. V. Khristenko

Critical currents, phase slip centers, and phase slip lines in superconducting films in the absence of external magnetic field

The temperature dependence of the current–voltage characteristics of high-quality thin films of tin from 7 to 50 μm thick is investigated in the absence of external magnetic field. For the first time on the same samples phase slip centers are observed near Tc, where the films are narrow channels, and phase slip lines are observed at lower temperatures, where the films become wide. The critical current exhibits temperature crossover, which affects its absolute value, but in a certain temperature interval a temperature dependence of the form (1−T/Tc)3/2 is maintained. When the temperature is decreased further, the critical current at which vortices due to the self-field of the current can enter the sample depends linearly on temperature and corresponds to the Aslamazov–Lempitsky theory. The temperature at the start of crossover with decreasing temperature coincides with the temperature at which the film width is equal to four times the penetration depth of a weak magnetic field perpendicular to the film pla...

Observation of enhancement of superconductivity by an external electromagnetic field and of rf phase-slip lines in wide tin films

The enhancement of superconductivity by an external electromagnetic field and rf phase-slip lines are observed in wide tin films. The temperature, frequency, and power dependences of the critical current are investigated and demonstrate the phenomena observed. It is shown that this class of nonequilibrium effects is common to both narrow and wide films.

Andreev reflection study of the new magnetic superconductor Mo3Sb7: evidence of gap anisotropy

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.

Enhancement of superconductivity in tin films of different widths by an external electromagnetic field

The current-voltage characteristics of high-quality thin tin films, ranging in width from 7 to 50 μm, irradiated by microwave radiation are investigated. The behavior of the critical current Ic and of the maximum current Im for the existence of a stationary flow of the characteristic vortices of the transport current (formation current of the first phase-slip line) is analyzed. The regimes of enhancement and suppression of the currents Ic and Im by a microwave field are studied and analyzed. The criteria for separating films into narrow and wide are established. It is shown that the superconductivity enhancement effect is a general effect and is observed for uniform (narrow films) and nonuniform (wide films) distributions of the superconducting current over the film width.

Superconducting and magnetic properties of a new superconductor: EuAsFeO0.85F0.15

Polycrystalline samples of the new superconducting compound EuAsFeO0.85F0.15, with critical temperature Tc=11K, are prepared by solid state synthesis. Its electric and magnetic properties are investigated in magnetic fields from 0.1to140000Oe. The critical magnetic fields Hc1 and Hc2 are measured, and thence the magnetic penetration depth λ and the coherence length ξ are 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.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.

Resistive states of superconducting channels in an alternating electromagnetic field (Review)

The processes that determine the structure and properties of the resistive state of superconducting channels having a uniform distribution of the current and order parameter over their transverse cross section are discussed. Particular attention is devoted to the resistive state arising when an external alternating electromagnetic field is applied to the superconductor. The current, temperature, frequency, and power characteristics of the resistive state are examined. The necessary information about the NS boundary and phase-slip centers is given, and the known results for high-Tc superconducting materials are presented.

Point-contact Andreev reflection spectroscopy of a magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4

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 b...

Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO0.85F0.15: Evidence of strong anisotropy in the order parameter

Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 with an unexpectedly low superconducting transition temperature Tc ≈ 11.3 K compared to related FeAs compounds based on Sm and Gd surrounding Eu in lanthanide series. The nearly fivefold lower Tc, compared to the expected value, is attributed to the divalent properties of Eu ions; the weakly magnetic Eu3+ ions may be present and the highly magnetic Eu2+ ions are strong suppressors of superconductivity. Most of the measured spectra have features corresponding to two energy gaps whose values varied, from contact to contact, over 2Δs/kTc = 2.2 −4.7 and 2Δ1/kTc = 5.1−11.7 for small and large gaps, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6−6.4. The relatively large crystallite sizes (at least ∼25 μm) and the large number of measured contacts (several tens) suggest, with a high degree of probability, that these spectra account quite fully for the gap distribution in essentially all the...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.

Enhancement of the superconducting order parameter in the compound Dy0.8Y0.2Rh4B4 at the phase transition of its magnetic subsystem from the antiferromagnetic to the ferrimagnetic state

The dependence of the superconducting order parameter Δ on magnetic field H in the compound Dy0.8Y0.2Rh4B4 is investigated experimentally for the first time at temperatures both above and below the antiferromagnet-ferrimagnet phase transition in the magnetic subsystem of the compound at TN≈2.7K. It is shown that this phase transition has a substantial influence on the charge subsystem of the superconductor and the parameter Δ. The parameter Δ turns out to be larger in the presence of the uncompensated magnetic moment of the ferrimagnetic state than in its absence in the antiferromagnetic state. In Dy0.8Y0.2Rh4B4 the charge and spin subsystems are substantially coupled, and the mechanism of superconducting pairing may be unconventional.

Andreev reflection study of the new magnetic superconductor Mo3Sb7 in magnetic field

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 m...