Point contacts are used to investigate the anisotropy of the superconducting energy gap in LuNi2B2C in the ab plane and along the c axis. It is shown that the experimental curves should be described assuming that the superconducting gap is non-uniformly distributed over the Fermi surface. The largest and the smallest gaps have been estimated by two-gap fitting models. It is found that the largest contribution to the point-contact conductivity in the c direction is made by a smaller gap and, in the ab plane by a larger gap. The deviation from the one-gap BCS model is pronounced in the temperature dependence of the gap in both directions. The temperature range, where the deviation occurs, is for the c direction approximately 1.5 times more than in the ab plane. The Γ parameter, allowing quantitatively estimate the gap anisotropy by one-gap fitting, in c direction is also about 1.5 times greater than in the ab plane. Since it is impossible to describe satisfactorily such gap distribution either by the one-or two-gap models, a continuous, dual-maxima model of gap distribution over the Fermi surface should be used to describe superconductivity in this material.

FeSe single crystals have been studied by soft point-contact Andreev-reflection spectroscopy. Superconducting gap features in the differential resistance dV /dI(V ) of point contacts such as a characteristic Andreev-reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing dV /dI within the extended two-gap Blonder-Tinkham-Klapwijk model allows to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field indicating a weak decrease of the superconducting gaps. Analyzing the dV /dI(V ) spectra for 25 point contacts results in the averaged gap values ∆L = 1.8±0.4 meV and ∆S =1.0±0.2 meV and reduced values 2 ∆L /kBTc=4.2± 0.9 and 2 ∆S /kBTc=2.3±0.5 for the large (L) and small (S) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent decreasing with both temperature and magnetic field. No signatures in the dV /dI spectra were observed testifying a gapless superconductivity or presence of even smaller gaps.

Point contacts (PC) Andreev reflection dV /dI spectra for the antiferromagnetic (TN ≃6 K) superconductor (Tc ≃11 K) ErNi2B2C have been measured for the two main crystallographic directions. Observed retention of the Andreev reflection minima in dV /dI up to Tc directly points to unusual superconducting order parameter (OP) vanishing at Tc. Temperature dependence of OP was obtained from dV /dI using recent theory of Andreev reflection including pair-breaking effect. For the first time existence of a two superconducting OPs in ErNi2B2C is shown. A distinct decrease of both OPs as temperature is lowered below TN is observed.Introduction. -The family of quaternary nickel borocarbides superconductors RNi 2 B 2 C, where R is a rare-earth element or Y, has attracted worldwide attention both because of a relatively high critical temperature T c , up to 16 K for R=Lu, and especially from the point of view of competition between superconducting and magnetic ordered states in the case of R=Tm, Er, Ho, Dy, where energy scales for the antiferromagnetic and superconducting order can be varied over a wide range (see, e. g., Refs. [1,2] and further Refs. therein). The compound with R=Er and T c ≃11 K is interesting for two reasons [1]: below (T N ≃6 K) incommensurate antiferromagnetic order with spin density wave occurs and weak ferromagnetism develops below T WFM ≃2 K [3]. Both phenomena are, in general, antagonistic to superconductivity, so that competition between superconducting and the magnetic state should take place in this compound. Additionally, the superconducting ground state in borocarbide superconductors is expected to have a multiband nature [4,5] with a complex Fermi surface and different contributions to the superconducting state by different Fermi surface sheets. Therefore, determining the influence of these magnetic states on a possible multiband superconducting ground state or multiband order parameter (OP) in ErNi 2 B 2 C is

We have analyzed about hundred voltage-dependent differential resistance dV /dI(V ) curves of metallic point contacts between c-axis oriented MgB2 thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with a single-gap features result in more broad maximum at 3.5 meV. The double-gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps ∆S = 2.45 ± 0.15 meV and ∆L = 7.0 ± 0.45 meV in MgB2 with gap ratio ∆L/∆S = 2.85 ± 0.15. Our observations prove further a widely discussed multigap scenario for MgB2, when two distinct gaps are seen in the clean limit, where a single averaged gap is present in the dirty one.