Point-contact spectroscopy was originally developed for the determination of the electron-phonon spectral function in normal metals. However, in the past 20 years it has become an important tool in the investigation of superconductors. As a matter of fact, point contacts between a normal metal and a superconductor can provide information on the amplitude and symmetry of the energy gap that, in the superconducting state, opens up at the Fermi level. In this paper we review the experimental and theoretical aspects of point-contact spectroscopy in superconductors, and we give an experimental survey of the most recent applications of this technique to anisotropic and multiband superconductors.

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...

Point-contact Andreev reflection spectroscopy (PCAR) has proven to be one of the most powerful tools in the investigation of superconductors, where it provides information on the order parameter (OP), a fundamental property of the superconducting state. In the past 20 years, successive improvements of the models used to analyze the spectra have continuously extended its capabilities, making it suited to study new superconductors with "exotic" properties such as anisotropic, nodal and multiple OPs. In Fe-based superconductors, the complex compound-and doping-dependent Fermi surface and the predicted sensitivity of the OP to fine structural details present unprecedent challenges for this technique. Nevertheless, we show here that PCAR measurements in Fe-based superconductors carried out so far have already greatly contributed to our understanding of these materials, despite some apparent inconsistencies that can be overcome if a homogeneous treatment of the data is used. We also demonstrate that, if properly extended theoretical models for Andreev reflection are used, directional PCAR spectroscopy can provide detailed information not only on the amplitude and symmetry of the OPs, but also on the nature of the pairing boson, and even give some hints about the shape of the Fermi surface.

a b s t r a c tA review of our investigations on single crystals of LnFeAsO 1Àx F x (Ln = La, Pr, Nd, Sm, Gd) and Ba 1Àx -Rb x Fe 2 As 2 is presented. A high-pressure technique has been applied for the growth of LnFeAsO 1Àx F x crystals, while Ba 1Àx Rb x Fe 2 As 2 crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O, F) position in superconducting LnFeAsO 1Àx F x crystals. Resistivity measurements on LnFeAsO 1Àx F x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba 1Àx Rb x Fe 2 As 2 simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 Â 10 9 A/m 2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO 1Àx F x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba 1Àx Rb x Fe 2 As 2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO 1Àx F x . Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy ($20 meV).

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).