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

Abstract

The Andreev reflection spectra dI/dV(V) of the magnetic superconductor Dy 0.6 Y 0.4 Rh 3.85 Ru 0.15 B 4 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 a grooving field, the gap features dropped abruptly near the critical point H c2. Of interest is also the abnormally high relative gap value 2∆/k B T c ≈ 4 (as compared to conventional singlet superconductors) which was found for some contacts from a comparison of experimental spectra and the modified Blonder-Tinkham-Klapwiyk theory. We attribute the features revealed in the point-contact spectroscopic investigations of Dy 0.6 Y 0.4 Rh 3.85 Ru 0.15 B 4 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 ~ 0.7H c2 .

Anisotropy of electric resistance and upper critical field in magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4

“…Using the data of Fig.1 and accepting for H c2 (T ) the values of the external magnetic field and the temperature, at which R(H, T ) = 0.5R N , we plotted the experimental temperature dependencies of the upper critical field depicted by circles and squares in Fig.2. In contrast to the previous Andreev spectroscopy data [3,6,7], we did not found any non-monotony in the behavior of H c2 (T ) that possibly reflects certain ambiguity in the interpretation of the results of the point-contact measure- ments in nonhomogeneous samples; a certain role in this difference may be also played by the admixture of ruthenium in our samples. Dashed curves in Fig.1 show the dependencies H c2 (T ) calculated from the equation of the Werthamer-Gelfand-Hohenberg (WHH) theory [11]:…”

“…2. In contrast to the previous Andreev spectroscopy data [3,6,7], we did not found any non-monotony in the behavior of H c2 (T ) that possibly reflects certain ambiguity in the interpretation of the results of the point-contact measure- ments in nonhomogeneous samples; a certain role in this difference may be also played by the admixture of ruthenium in our samples. Dashed curves in Fig.…”

“…This could be explained either by the effect of anisotropy (may lead to increase in H c2 by 20 − 30% [12]) or by the presence of strong coupling in the superconducting condensate (can enhance H c2 by 1.3 times [13]), which are beyond the frameworks of the WHH theory. The possibility of the strong coupling in this material follows from the results of the Andreev spectroscopy of the superconducting gap ∆ [7], according to which the ratio 2∆/kT c can reach 4 or even higher values, larger than the value 3.52 for conventional superconductors with the weak coupling.…”

“…Measurements of the specific heat of Dy 0.8 Y 0.2 Rh 4 B 4 (T M = 30.5 K, T c = 5.9 K) indicate emergence of another magnetic phase transition below 2.7 K [3]. Recently, anomalies of some physical quantities, unusual for systems with conventional superconductivity, were discovered in Dy 1−x Y x Rh 4 B 4 : the paramagnetic Meissner effect [4,5] and non-monotonic temperature dependencies of the upper critical magnetic field H c2 and the superconducting gap [3,6,7].…”