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.

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).

The experimental critical temperatures and gap values of the superconducting pnictides of both the 1111 and 122 families can be simultaneously reproduced within the Eliashberg theory by using a three-band model where the dominant role is played by interband interactions and the order parameter undergoes a sign reversal between hole and electron bands (s±-wave symmetry). High values of the electron-boson coupling constants and small typical boson energies (in agreement with experiments) are necessary to obtain the values of all the gaps and to correctly reproduce their temperature dependence.PACS numbers: 74.70. Dd, 74.20.Fg, 74.20.Mn The recently discovered Fe-based pnictide superconductors [1,2,3] have aroused great interest in the scientific community. They have indeed shown that high T c superconductivity does not uniquely belong to cuprates but can take place in Cu-free systems as well. Nevertheless, as in cuprates, superconductivity occurs upon charge doping of a magnetic parent compound above a certain critical value. However, important differences exist: the parent compound in cuprates is a Mott insulator with localized charge carriers and a strong Coulomb repulsion between electrons; in the pnictides, on the other hand, it is a bad metal and shows a tetragonal to orthorhombic structural transition below ≈ 140 K, followed by an antiferromagnetic (AF) spin-density-wave (SDW) order [4]. Charge doping gives rise to superconductivity and, at the same time, inhibits the occurrence of both the static magnetic order and the structural transition. The Fermi surface consists of two or three hole-like sheets around Γ and two electron-like sheets around M . Up to now, the most intensively studied systems are the 1111 compounds, ReFeAsO 1−x F x (Re = La, Sm, Nd, Pr, etc.) and especially the 122 ones, hole-or electron-doped AFe 2 As 2 (A = Ba, Sr, Ca). The huge amount of experimental work already done in 122 compounds is due to the availability of rather big high-quality single crystals.Most of the present research effort is spent clarifying the microscopic pairing mechanism responsible for superconductivity. The conventional phonon-mediated coupling mechanism cannot explain the observed high T c within standard Migdal-Eliashberg theory and the inclusion of multiband effects increases T c only marginally [5]. On the other hand, the magnetic nature of the parent compound seems to favor a magnetic origin of superconductivity and a coupling mechanism based on nestingrelated AF spin fluctuations has been proposed [6]. It predicts an interband sign reversal of the order parameter between different sheets of the Fermi surface (s± symmetry). The number, amplitude and symmetry of the superconducting energy gaps are indeed fundamental physical quantities that any microscopic model of superconductivity has to account for. Experiments with powerful techniques such as ARPES, point-contact spectroscopy, STM etc., have been carried out to study the superconducting gaps in pnictides (for a review see [7]). Although results are so...

By using an electrochemical gating technique with a new combination of polymer and electrolyte, we were able to inject surface charge densities n2D as high as 3.5 × 10 15 e/cm 2 in gold films and to observe large relative variations in the film resistance, ∆R/R , up to 10% at low temperature. ∆R/R is a linear function of n2D -as expected within a free-electron model -if the film is thick enough (≥ 25 nm), otherwise a tendency to saturation due to size effects is observed. The application of this technique to 2D materials will allow extending the field-effect experiments to a range of charge doping where giant conductance modulations and, in some cases, even the occurrence of superconductivity are expected.PACS numbers: 73.61.At, 82.47.Uv Since the Sixties, the possibility to modulate the transport properties of various materials by means of the so-called field effect (FE) has attracted much interest. Apart from the nowadays obvious application in semiconductor-based electronic devices such as FETs (field-effect transistors), the technique has been widely used also for more exotic purposes. It has allowed enhancing the critical temperature of some superconductors [1][2][3], inducing metallic behavior in insulators [4] or even a superconducting phase transition in materials like SrTiO 3 [5] ZrNCl [6] and KTaO 3 [7]. In the standard FET configuration, the maximum density of the induced surface charge, σ max , is of the order of 10 13 charges cm −2 if suitable dielectrics are used. Only with a polymeric gating technique [8,9] electric fields as high as 100MV/cm, and surface carrier concentrations of 10 14 /cm 2 [6] have been achieved. The present record, to the best of our knowledge, is 4.5 × 10 14 cm −2 [10]. The reason of this order-of-magnitude improvement with respect to the conventional FETs is the formation of the electric double layer (EDL) at the interface between the electrolyte solution and the sample surface. The EDL acts as a parallel-plate capacitor with extremely small distance between the plates (of the order of the polymer molecule size) [6] and thus very large capacitance.Here, we will show that a new polymeric electrolyte solution (PES) allows further extending the surface charge density to some units in 10 15 charges cm −2 , for applied voltages of the order of a few Volts (5 V at most), which marks a significant improvement with respect to the present state of the art. In particular, we will apply this technique to Au films.The FE in metals has been devoted little attention, either because of its little practical interest or because often believed to be unobservable. Indeed, in the semiclassical, metallic limit, the electronic screening length (the Thomas-Fermi radius) is less than one atomic diameter. Nonetheless, a modulation of the conductivity of metal films (including Au) has been obtained already in the Sixties [11,12] with a conventional gating technique. These and the following measurements of the same kind [1,[13][14][15] have evidenced a number of unexpected properties and differenc...

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 .

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