L. A. Ishchenko

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.

Superconductivity, negative magnetoresistance, and anisotropy of the conductivity of YFe4Al8 and ScFe4Al8 single crystals in the frequency range 0–108 Hz

The anisotropy of the electrical resistivity of YFe4Al8 and ScFe4Al8 single crystals is investigated for the first time in the frequency range 0–108 Hz and temperature range 300–4.2 K. It is found that the conductivity is higher along the growth axis of the crystals than in the perpendicular direction, while the parameters governing the antiferromagnetic ordering, negative magnetoresistance, and superconductivity effects observed in these single crystals are isotropic. Some effects known previously for polycrystalline high-Tc superconductors containing magnetic impurities are observed in the single crystals investigated and are found to be anisotropic in them.

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.

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...

Structural and high-frequency (0–110 MHz) resistive characteristics of MgB2 in the temperature range 5–300 K

The structure and resistivity of polycrystalline MgB2 and its impedance at frequencies of 9–110 MHz are determined in the temperature range 5–300 K. It is shown that the lattice type and symmetry of the superconducting phase of MgB2 remain unchanged over this temperature range. At the superconducting transition temperature Tc=39.5 K a structural instability is observed which is accompanied by an overshoot of the measured lattice parameters. It is concluded that the strain of the crystals upon a change in temperature is of a substantially anisotropic character. Measurements of the temperature and frequency dependence of the surface resistance Rs(T,f ) in the superconducting state reveal a transition from the Pippard nonlocal limit at T≪Tc to the London local limit near Tc. At T/Tc<0.76 the value of Rs(T) is well described by an exponential dependence exp(−Δ(T)/kT) in accordance with the BCS theory.

Suppression of superconductivity of Dy0.6Y0.4Rh3.85Ru0.15B4 in inclined magnetic fields

The electrical resistance of polycrystalline Dy0.6Y0.4Rh3.85Ru0.15B4 is measured for the first time near the superconducting transition temperature (Tc1/2 ≈ 6.66 K) in various inclined magnetic fields (φ = 0, 45, and 90°) and plots are made of R(T) and the upper critical field in dimensionless units h*(t). These dependences exhibit strong anisotropy with respect to the transport current through a sample when the orientation of the magnetic field is changed from φ = 0 to 90°. This appears to be related to texturing of the polycrystals. An analysis of the R(T) and h*(t) curves shows that the magnetic field suppresses the superconducting state more strongly at φ = 45° than at the other angles of inclination. The h*(t) dependences are analyzed in terms of the Werthamer-Helfand-Hohenberg theory and the Maki parameter α and the spin-orbital scattering parameter λSO are estimated. It is shown that for φ = 0 and 90°, α = 0 and λSO = 0 and only the orbital contribution is observed, while for φ = 45°, α = 4.2 and λ...

Low-temperature specific heat of magnetic superconductors Dy0.6Y0.4Rh3.85Ru0.15B4 and Dy0.6Y0.4Rh4B4

Specific heat CM(T) of polycrystalline Dy0.6Y0.4Rh4B4 and Dy0.6Y0.4Rh3.85Ru0.15B4 was studied in the temperature range of 0.5–9 K and magnetic fields 0–10 kOe for the first time. It was found that the λ-anomaly in the specific heat exists at Tc ≈ 6 K for Dy0.6Y0.4Rh4B4 and at Tc ≈ 6.6 K for Dy0.6Y0.4Rh3.85Ru0.15B4. It is suppressed in a magnetic field and shifted to lower temperatures. Partial substitution of Rh by Ru enhances superconductivity, presumably, due to stronger inner magnetism of the dysprosium sublattice in Dy0.6Y0.4Rh4B4 as compared with Dy0.6Y0.4Rh3.85Ru0.15B4. Furthermore, it was observed that the molar heat capacity CM(T) of Dy0.6Y0.4Rh3.85Ru0.15B4 increases with decreasing temperature for T < 4 K. In Dy0.6Y0.4Rh4B4, an increase in CM(T) with decreasing temperature is accompanied by the appearance of a maximum at Tmax = 1.5 K, which might be a manifestation of the magnetic phase transition in the dysprosium subsystem at this temperature.