We present the results of the first directional point-contact spectroscopy experiments in highquality MgB2 single crystals. Due to the directionality of the current injection into the samples, the application of a magnetic field allowed us to separate the contributions of the σ and π bands to the total conductance of our point contacts. By using this technique, we were able to obtain the temperature dependency of each gap independent of the other. The consequent, strong reduction of the error on the value of the gap amplitude as function of temperature allows a stricter test of the predictions of the two-band model for MgB2.PACS numbers: 74.50.+r, 74.80.Fp, 74.70.Ad During the last year, the consensus has been growing within the scientific community on the fact that most of the features of MgB 2 discovered so far can be properly explained by admitting that two band systems are present in this new superconductor: quasi-2D σ bands arising from hybrid sp 2 orbitals in the boron planes, and 3D π bands that stem from the out-of-plane p z orbitals [1,2]. The unusual consequence of this band structure is that two different energy gaps can be observed in clean limit [2,3,4]: ∆ σ (the larger) and ∆ π (the smaller). Both gaps are expected to close at the same temperature T c because of an inter-band pair-scattering mechanism [5] but, while ∆ σ (T ) should approximately follow a BCSlike curve, a marked reduction of ∆ π (T ) with respect to a BCS-like behavior is expected at T > ∼ 20 K [3,4]. So far, one of the most convincing experimental supports of this model has been the observation of two gaps by tunneling [6] and point-contact spectroscopy [7] in polycrystal samples and films. However, a direct and accurate test of the predictions of the two-band model has been so far impossible due to the lack of high-quality single crystals large enough to be used for directioncontrolled point-contact and tunnel spectroscopy.In this Letter, we present the results of the first directional point-contact measurements in large single crystals of MgB 2 . We injected current along the ab plane or along the c axis, and applied a magnetic field either parallel or perpendicular to the ab planes. This allowed us to separate the partial contributions of the σ and π bands to the total conductance, and to fit them obtaining the temperature dependency of each gap with great accuracy. We will show that all the results of this technique confirm very well the predictions of the two-band model.The high-quality MgB 2 single crystals used for our point-contact experiments were produced at ETH (Zürich) by starting from a mixture of Mg and B. This mixture was put into a BN container and the crystals were grown at a pressure of 30-35 kbar in a cubic anvil device. The thermal process includes a one-hour heating up to 1700-1800 • C, a plateau of 1-3 hours, and a final cooling lasting 1-2 hours. MgB 2 plate-like crystals up to 200 µg in weight and 1.5 × 0.9 × 0.2 mm 3 in size can be obtained by using this technique, even though the crystals used in our me...

Uranium-lead ages from an ion probe were taken for zircons from the ore-bearing Noril'sk I intrusion that is comagmatic with, and intrusive to, the Siberian Traps. These values match, within an experimental error of +/-4 million years, the dates for zircons extracted from a tuff at the Permian-Triassic (P-Tr) boundary. The results are consistent with the hypothesis that the P-Tr extinction was caused by the Siberian basaltic flood volcanism. It is likely that the eruption of these magmas was accompanied by the injection of large amounts of sulfur dioxide into the upper atmosphere, which may have led to global cooling and to expansion of the polar ice cap. The P-Tr extinction event may have been caused by a combination of acid rain and global cooling as well as rapid and extreme changes in sea level resulting from expansion of the polar ice cap.

Directional point-contact Andreev-reflection measurements in Ba(Fe(1-x)Co(x))2As2 single crystals (T(c) = 24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The point-contact Andreev-reflection spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Ω(b)(T) is obtained, very similar to the spin-resonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s ± Eliashberg model by using an electron-boson spectral function peaked at Ω(0) = 12 meV ≃ Ω(b)(0).

Single crystals of Mg 1-x Al x B 2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x < 0.1 were obtained as a result of optimization of already developed MgB 2 crystal growth procedure. Systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg 1-x Al x B 2 crystals was found to be slightly different than that one observed for polycrystalline samples since, even a very small substitution of 1-2% of Al leads to the decrease of T c by about 2-3 K. X-ray and high resolution transmission electron microscopy investigations indicate on the appearance of second precipitation phase in the crystals with x > 0.1. This is in a form of non-superconducting MgAlB 4 domains in the structure of superconducting Mg 1-x Al x B 2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, H c2 , for H//c for the samples with small x, significant reduction of the H c2 anisotropy at lower temperatures, and decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with T c in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing T c down to 20 K. It may be related to an appearance of the precipitation phase in the Mg 1-x Al x B 2 structure. 74.70.Ad, 74.62.Dh, 81.10.-h, 74.25.Ha

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides REFeAsO 1-x F x (RE = rare earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on LaFeAsO 1-x F x gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, ∆ 1 = 2.8-4.6 meV, which scales with the local T c of the contact; a larger unconventional OP that gives conductance peaks at 9.8-12 meV, apparently closes below T c and decreases on increasing the T c of the contact; a pseudogaplike feature (i.e. a depression in the conductance around zero bias), that survives in the normal state up to T * ~ 140 K (close to the Néel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations (AF SF) coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in LaFeAsO 1-x F x .