The superconducting gap function of Sr2RuO4 was investigated by means of quasiparticle reflection and transmission at the normal conductor-superconductor interface of Sr2RuO4-Pt point contacts. We found two distinctly different types of dV /dI vs V spectra either with a double-minimum structure or with a zero-bias conductance anomaly. Both types of spectra are expected in the limit of high and low transparency, respectively, of the interface barrier between a normal metal and a spin-triplet superconductor. Together with the temperature dependence of the spectra this result strongly supports a spin-triplet superconducting order parameter for Sr2RuO4. 74.70.Dd, 71.20.Lp, 73.40.Jn, 74.80.Fp

We performed point-contact spectroscopy on the binary superconductor MgB2. The differential conductance shows gap-related structures which vary in width and position from contact to contact. The distribution of energy gaps shows a distinct accumulation around 1.7 and 7 meV which is associated with the occurrence of a small and a large energy gap in MgB2. While with increasing T the structure in dI/dV associated with the small gap is present up to TC, in magnetic field it is suppressed well below Bc2.PACS numbers: 74.70. Ad, 74.80.Fp, 74.20.Rp The observation of superconductivity at 39 K in the simple binary compound MgB 2 [1] was a surprise for most condensed matter scientists and immediately raises the question of the mechanism of superconductivity in this material. Following a basic approach to superconductivity, two key issues have to be considered: the orderparameter symmetry and the coupling mechanism. For the latter, a significant boron isotope effect was observed in MgB 2 [2] which is consistent with a phonon-mediated BCS mechanism where the boron phonon modes are playing an important role. A moderatly strong electronphonon coupling constant λ el−ph ≈ 0.8 was derived from measurements of the specific heat [3] in good agreement with recent theoretical predictions [4][5][6].The order-parameter symmetry has already been investigated by tunneling spectroscopy and point-contact spectroscopy (PCS) [7][8][9][10][11][12][13][14][15]. Although the spectra show unambiguous features of an energy gap in the density of states, probably with s-wave symmetry of the order parameter, the results are controversal on the gap width. Values of 2∆/k B T c ranging from 1.2 to 4.7 have been reported, raising the possibility of an anisotropic energy gap or even multiple gaps. Careful analysis of specific-heat measurements [16,17] and photoemission spectroscopy [18] support this scenario as well.We present measurements of the differential conductance dI/dV of normal conductor/superconductor point contacts between a Pt tip and a MgB 2 pellet. The MgB 2 pellet was prepared in an Ar atmosphere from magnesium filings (> 99.9 %, Chempure) and powdered boron (99 %, Ventron). The powders were mixed in the ratio Mg:B = 1.1:2 and pressed into a pellet. After wrapping the pellet in a Ta foil it was enclosed in an Fe cylinder and inserted in a high-pressure furnace. The pellet was then heated to 1073 K for 1.5 h and to 953 K for 2h under an Ar pressure of 58 MPa. The DC magnetization of the pellet (Fig. 1) indicates an onset superconducting transition temperature T theoretical shielding signal is reached in 1 mT applied field in a zero-field cooled (zfc) -field heated (fh) temperature cycle. The Meissner signal remains weak, i. e. of the order of a few percent. Upon reducing the applied field to 10 µT, a kink in the transition curve at 35 K reveals the granular structure of the sample: below T onset c first the individual grains become superconducting, below 35 K intergranular shielding currents cause shielding of the whole pellet.PCS spe...

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