N. L. Bobrov

Superconducting energy gap distribution in c-axis oriented MgB2 thin film from point contact study

We have analyzed about a 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 single-gap features result in a 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: ΔS=2.45±0.15 meV and ΔL=7.0±0.45 meV in MgB2 with the gap ratio ΔL/ΔS=2.85±0.15. Our observations further prove a widely discussed multigap scenario for MgB2, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.

Competition of multiband superconducting and magnetic order in ErNi2B2C observed by Andreev reflection

Point contacts (PC) Andreev reflection dV/dI spectra for the antiferromagnetic (TN6 K) superconductor (Tc11 K) ErNi2B2C have been measured for the two main crystallographic directions. The observed retention of the Andreev reflection minima in dV/dI up to Tc directly points to an unusual superconducting order parameter (OP) vanishing at Tc. The temperature dependence of the OP was obtained from dV/dI using the recent theory of Andreev reflection including the pair-breaking effect. For the first time the existence of two superconducting OPs in ErNi2B2C is shown. A distinct decrease of both OPs as temperature is lowered below TN is observed.

Point-contact spectroscopy of the nickel borocarbide superconductors RNi2B2C (R = Y, Dy, Ho, Er, Tm, Lu)

An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi 2 B 2 C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron-boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R = Dy, Ho, Er and Tm, while for R = Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R = Ho and near 6 meV in R = Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R = Y, Lu. At low temperature the SC gap in R = Ho exhibits a standard single-band BCS-like dependence, which vanishes above T*c ≃ 5.6 K < T c ≃ 8.5 K, where a specific magnetic ordering starts to play a role. For R = Tm (Tc ∼ 10.5 K) a decrease of the SC gap is observed below 5 K.

Reconstruction of the electron-phonon interaction function in Ta, 2H-NbSe2 and MgB2 from spectra of S–c–N microcontacts

Using Ta, 2H-NbSe2 and MgB2 as an example it is shown that it is possible to reconstruct qualitatively a function of the electron-phonon interaction from point-contact spectra in a superconducting state. The limits and the restrictions of this method are also shown. The results obtained are compared with available literature data.

Observation of an anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi2B2C

The point-contact spectra of the Andreev reflection dV∕dI curves of the superconducting rare-earth nickel borocarbide ErNi2B2C (Tc≈11K) have been analyzed in the “one-gap” and “two-gap” approximations using the generalized Blonder-Tinkham-Klapwijk model and the Beloborod’ko model allowing for the pair-breaking effect of magnetic impurities. Experimental and calculated curves have been compared not only in shape, but in magnitude as well, which provides more reliable data for determining the temperature dependence of the energy gap (or superconducting order parameter) Δ(T). The anisotropic effect of antiferromagnetic ordering at TN≈6K on the superconducting gap/order parameter has been determined: as the temperature is lowered, Δ decreases by ∼25% in the c-direction and only by ∼4% in the ab-plane. It is found that the pair-breaking parameter increases in the vicinity of the magnetic transitions, the increase being more pronounced in the c-direction. The efficiency of the models was tested for providing Δ(...

Superconducting Gap and Electron-Phonon Interaction in MgB2 Thin Film Studied by Point Contacts

The compound MgB2 with graphite-like planes of boron atoms has attracted much attention as the superconductor with, at the presen,t the highest Tc ≃40 K [1] for binary systems. The observation of a boron isotope effect [2, 3] and examination of electron-phonon coupling [4, 5, 6, 7, 8, 9, 10] in MgB2 are in accordance with the expectations for conventional BCS superconductivity mediated by electron-phonon interaction (EPI). Investigations of the order parameter by tunneling [11, 12, 13, 14, 15, 16, 17, 18] and point-contact techniques [18, 19, 20, 21, 22, 23, 24] confirm that MgB2 is most likely an s-wave superconductor. In all cases the spectra show unambiguous features of an energy gap Δ in the density of states (DOS), albeit the results are controversial as to the gap width. Values of Δ ranging from 1.5 to 8meV have been reported, pointing out the possibility of an anisotropic or distributed (nonhomogeneous) energy gap or even multiple gaps. The latter scenario has been recently recalled by Liu et al. [8] for MgB2. The Fermi surface consisting from nearly cylindrical hole sheets arising from quasi-2D boron bands and three dimensional tubular network [4] was considered. The different character of the sheets raises the possibility that each has a distinct gap. The existence of two different energy gaps with the ratio approximately 1:3 being respectively smaller (for the 3D gap) and larger (for the 2D gap) than the standard weak coupling BCS value Δ=1.76kBTc ≃6 meV was predicted in [8] .

The Superconducting gap behavior in the antiferromagnetic Nickel-Borocarbide compounds RNi2B2C (R=Dy, Ho, Er, Tm) studied by point-contacts spectroscopy

An general survey of the superconducting (SC) gap study in the title compounds by point-contact (PC) spectroscopy is presented. The SC gap was determined from dV/dI of PCs employing the well-known theory of conductivity for normal metal-superconductor PCs accounting Andreev reflection. The theory was modified by including pair-breaking effects considering the presence of magnetic rare-earth ions. A possible multiband structure of these compounds was also taken into account. The PC study of the gap in the Er-compound (TN ≊ 6 K Tc ≊ 6.5 K) the SC gap has a BCS-like dependence in the AF state. The SC gap for R = Ho (TN ≊5.2 K < Tc ≊ 8.5 K) exhibits below T* ≊5.6 K a single-band BCS-like dependence vanishing above T*, where a specific magnetic order occurs. The difference in the SC gap behavior in the title compounds is attributed to different AF ordering.

Point-Contact Study of the Superconducting Gap in the Magnetic Rare-Earth Nickel-Borocarbide RNi2B2C (R = Dy, Ho, Er, Tm) Compounds

We present an overview of the efforts in point-contact (PC) study of the superconducting (SC) gap in the antiferromagnetic (AF) nickel-borocarbide compounds RNi2B2C (R = Dy, Ho, Er, Tm), for which the energy scales of AF and SC order, measured by the Neel temperature T N and the critical temperature T c, respectively, can be varied over a wide range. The SC gap was determined from the experimental dV ∕ dI curves of PCs employing the well-known BTK theory of conductivity for normal metal-superconductor PCs accounting Andreev reflection. Additionally, the mentioned theory including pair-breaking effect due to magnetic impurities was employed and a multiband structure of the title compounds was taken into consideration. Recent directional PC study of the SC gaps gives evidence for the anisotropic two-band (two-gap) nature of SC-ty in R = Er (T N ≈ 6 K T c ≈ 6. 5 K) the SC gap has BCS-like dependence in the AF state. Distinct features of the SC gap behavior in the mentioned magnetic superconductors are discussed.