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We fabricate ion-gated field-effect transistors (iFET) on mechanically exfoliated multilayer MoS$_2$. We encapsulate the flake by Al$_2$O$_3$, leaving the device channel exposed at the edges only. A stable Li$^+$ intercalation in the MoS$_2$ lattice is induced by gating the samples with a Li-based polymeric electrolyte above $\sim$ 330 K and the doping state is fixed by quenching the device to $\sim$ 300 K. This intercalation process induces the emergence of anomalies in the temperature dependence of the sheet resistance and its first derivative, which are typically associated with structural/electronic/magnetic phase transitions. We suggest that these anomalies in the resistivity of MoS$_2$ can be naturally interpreted as the signature of a transition to a charge-density-wave phase induced by lithiation, in accordance with recent theoretical calculations., Comment: 8 pages, 4 figures

We have irradiated with Au ions (having an energy of 250 MeV) epitaxial thin films of BaFe2(As(1-x)P(x))2 with x @ 0.2 (optimal doping), grown by MBE on MgO single-crystal substrates. We have studied the effects of irradiation (for three different fluences, F=2.4×10^11 cm^-2, 4.8×10^11 cm^-2 and 7.3×10^11 cm^-2) on the surface morphology, on the resistivity and on the critical temperature. We have found that irradiation progressively destroys the very clear and interconnected growth terraces typical of the pristine surface, leading first to their smoothing - accompanied by the appearance of localized defects and cracks - and then to a completely disordered surface. However, the resistivity increases only slightly and the critical temperature decreases very little (i.e. by about 2%) going from the pristine film to the most-irradiated one. The latter result is confirmed by local measurements of the critical temperature carried out by point-contact Andreev reflection spectroscopy.

We investigated the Josephson effect between the conventional superconductor Pb0.7In0.3 and Ba0.4K0.6 (FeAs)2 single crystals by performing directional point-contact experiments. The junctions were irradiated with microwaves and the behavior of the Shapiro steps as a function of the square root of the microwave power was studied by means of the RSJ model extended to the nonautonomous case with an rf current-source term. The analysis reveals that, for current injection along the ab plane, the current-phase relation deviates from being simply described by the standard sin(ϕ) (as we indeed observed along the c axis) and a dominant sin(2ϕ) component is observed. The results are compared with a theory of the dc Josephson effect, formulated by using a method based on the calculation of temperature Green’s function in the junction within the tight-binding model (Burmistrova et al., Phys. Rev. B 91, 214501, 2015). The comparison shows that the experimental results here presented strongly favor an s ±-wave symmetry of the order parameter in this compound.

In a recent paper, we presented the first directional point-contact Andreev reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure and discussed the pressure dependence of the gaps and the critical temperature. While T c exhibits a well-known smooth dependence and a broad maximum at about 0.6 GPa, both gaps increase very sharply in a small pressure range between 0.5 and 0.6 GPa, leading to a doubling of the ratio 2Δ2/k B T c and a quadruplication of the ratio 2Δ1/k B T c , Δ1 and Δ2 being the small and large gap, respectively. This peculiar behavior is likely to be related to a sharp change in the lattice structure that, in turn, produces a 2D-3D topological transition in the hole-like Fermi surface sheet. In this work, we show that, within an effective three-band Eliashberg theory for the electron-spin fluctuation coupling (s± symmetry), these results can be rationalized as being due to a large increase of the electron-boson coupling, which mimics the effects of a boost mechanism for the coupling related to the underlying Lifshitz transition.

We fabricate electric double-layer field-effect transistor (EDL-FET) devices on mechanically exfoliated few-layer graphene. We exploit the large capacitance of a polymeric electrolyte to study the transport properties of three, four and five-layer samples under a large induced surface charge density both above and below the glass transition temperature of the polymer. We find that the carrier mobility shows a strong asymmetry between the hole and electron doping regime. We then employ ab-initio density functional theory (DFT) calculations to determine the average scattering lifetime from the experimental data. We explain its peculiar dependence on the carrier density in terms of the specific properties of the electrolyte we used in our experiments., 6 pages, 3 figures

The design and realization of a cryogenic rig for point-contact spectroscopy measurements in the needle-anvil configuration is presented. Thanks to the use of two piezoelectric nano-positioners, the tip can move along the vertical ($z$) and horizontal ($x$) direction and thus the rig is suitable to probe different regions of a sample \textit{in situ}. Moreover, it can also form double point-contacts on different facets of a single crystal for achieving, e.g., an interferometer configuration for phase-sensitive measurements. For the latter purpose, the sample holder can also host a Helmholtz coil for applying a small transverse magnetic field to the junction. A semi-rigid coaxial cable can be easily added for studying the behavior of Josephson junctions under microwave irradiation. The rig can be detached from the probe and thus used with different cryostats. The performance of this new probe has been tested in a Quantum Design PPMS system by conducting point-contact Andreev reflection measurements on Nb thin films over large areas as a function of temperature and magnetic field., Comment: 7 pages, 7 figures, published in Rev. Sci. Instrum

Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superconductor LaFeAsO0.9F0.1 with resistive T con ∼ 27 K. The observation of two pairs of peaks in the low-temperature Andreev-reflection spectra clearly indicates the presence of two order parameters. The behavior of the two gaps as a function of temperature, obtained by fitting the conductance curves by means of the generalized two-band Blonder-Tinkham-Klapwijk model, shows some anomalies. A theoretical analysis performed within the two-band Eliashberg theory with a generic electron-boson coupling can reproduce the low-temperature value of the two gaps but generally fails in giving a satisfactory fit of their overall temperature dependence, indicating the rich and complex physics of these newly discovered superconductors.

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO0.8F0.2 state-of-the-art polycrystals, with critical temperature Tc≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB2 and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to Tc, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ1=6.15±0.45 meV and Δ2=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same Tc. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.

We summarize the results of point-contact Andreev-reflection (PCAR) spectroscopy in MgB2 doped by chemical substitutions, either magnetic (Mn) or non-magnetic (Al, C), obtained by us and by other groups in the last four years. Despite the variety of samples used (crystals and polycrystals of various origin) and some minor differences in the experimental techniques, these measurements have directly provided a complete and consistent picture of the effects of chemical substitutions on the gaps of MgB2 shedding light on other relevant parameters (scattering rates, DOSs) affected by doping. In Al-doped crystals and polycrystals, the gap amplitudes Δσ and Δπ – obtained through a two-band Blonder-Tinkham-Klapwijk (BTK) fit of the Andreev-reflection conductance curves – decrease on increasing the Al content x (i.e. on decreasing the critical temperature of the contacts T c A ), but remain clearly distinct with no evidence of gap merging even when T c A is of the order of 10 K. The case of C-substituted MgB2 is less clear: the merging of the two gaps at x ≃ 0.13 was observed in single crystals grown at ETH, while the persistence of two gaps up to x = 0.10 – with apparently no tendency to merging at higher x – was found in other crystals and polycrystals. Finally, in Mn-doped single crystals the effects of pair-breaking magnetic scattering strongly suppress the critical temperature and the gaps, which however remain clearly distinct from each other down to Tc ≈ 10 K. In all cases, the analysis of the Δσ and Δπ data as a function of T c A (or of the doping content x) within the two-band Eliashberg theory gives interesting information about the effects of substitutions on the different scattering channels (interband and intraband, magnetic or non-magnetic).

We present theory of dc Josephson effect in contacts between Fe-based and spin-singlet $s$-wave superconductors. The method is based on the calculation of temperature Green's function in the junction within the tight-binding model. We calculate the phase dependencies of the Josephson current for different orientations of the junction relative to the crystallographic axes of Fe-based superconductor. Further, we consider the dependence of the Josephson current on the thickness of an insulating layer and on temperature. Experimental data for PbIn/Ba$_{1-x}$K$_{x}$(FeAs)$_2$ point-contact Josephson junctions are consistent with theoretical predictions for $s_{\pm}$ symmetry of an order parameter in this material. The proposed method can be further applied to calculations of the dc Josephson current in contacts with other new unconventional multiorbital superconductors, such as $Sr_2RuO_4$ and superconducting topological insulator $Cu_xBi_2Se_3$., 16 pages, 14 figures

In the first part of the present paper we discuss the fabrication and the characterization of an MgB 2 -based SQUID magnetometer with a directly coupled large-area pick-up loop, made on an MgB 2 film deposited by an all in situ technique. The coarse structure of the SQUID was defined by optical lithography and Ar-ion milling, while the two nanobridges acting as weak links in the superconducting loop were made by focused ion beam (FIB) milling. The device was characterized at different temperatures and showed Josephson quantum interference up to 20 K as well as a noise level already compatible with the recording of an adult magnetocardiogram. In the second part, concerning the fundamental physics of MgB 2 , we present the results of very recent point-contact measurements on Mg 1− x Mn x B 2 single crystals with 34.1 ⩾ T c ⩾ 13.3 K (i.e. 0.37% ⩽ x ⩽ 1.5%). The experimental conductance curves were fitted with the generalized two-band BTK model and their behaviour in magnetic fields was studied to check if both the order parameters (OPs) of the σ and π bands were present in the whole doping range. The dependence of the OPs (evaluated through the fit) on the Andreev critical temperature of the junctions is analyzed in the framework of the two-band Eliashberg theory by including the effects of magnetic impurities. The results give an evidence of a dominant effect of the magnetic impurities on the σ-band channel.

We report the results of directional point-contact measurements in Mg(B\(_{1-x}\)C\(_{x})_{2}\) single crystals. The amplitudes of the gaps, \(\Delta_{\pi}\) and \(\Delta_{\sigma}\), were determined for each C content by fitting the experimental low-temperature normalized conductance curves of our “soft” point contacts with the BTK model generalized to the two-band case. We found that, on increasing the carbon content, \(\Delta_{\sigma}\) decreases almost linearly with \(T_{c}\) and \(\Delta_{\pi}\) slightly increases until, at \(x=0.132\) (where \(T_{c}=19\) K), they assume the same value \(\Delta =3.2 \pm 0.9\) meV. This result is confirmed by the temperature and magnetic-field dependence of the conductance curves at this C content, which do not show any evidence of two distinct gap values. In particular, the Δ versus T curve follows very well a standard BCS curve, with a gap ratio \(2\Delta /k_{B} T_{c}=3.9\). These experimental findings are compared to the theoretical predictions of the two-band model in the Eliashberg formulation.

The temperature dependence of electric transport properties of single-layer and few-layer graphene at large charge doping is of great interest both for the study of the scattering processes dominating the conductivity at different temperatures and in view of the theoretically predicted possibility to reach the superconducting state in such extreme conditions. Here we present the results obtained in 3-, 4- and 5-layer graphene devices down to 3.5 K, where a large surface charge density up to about 6.8x10^14 cm^(-2) has been reached by employing a novel polymer electrolyte solution for the electrochemical gating. In contrast with recent results obtained in single-layer graphene, the temperature dependence of the sheet resistance between 20 K and 280 K shows a low-temperature dominance of a T^2 component - that can be associated with electron-electron scattering - and, at about 100 K, a crossover to the classic electron-phonon regime. Unexpectedly this crossover does not show any dependence on the induced charge density, i.e. on the large tuning of the Fermi energy., Comment: 13 pages, 6 color figures

We performed point-contact Andreev reflection spectroscopy measurements in Ca(Fe_{1-x}Co_x)2As2 single crystals with effective x=0.060 +- 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5-6 meV. Their fit with the three-dimensional Blonder-Tinkham-Klapwijk model (making use of a analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Fermi surface pockets centered at the Z point in the Brillouin zone. These results nicely fit into the theoretical picture for the appearance of nodal superconductivity in 122 compounds., Comment: 11 pages, 3 figures

The field-effect technique, popular thanks to its application in common field-effect transistors, is here applied to metallic thin films by using as a dielectric a novel polymer electrolyte solution. The maximum injected surface charge, determined by a suitable modification of a classic method of electrochemistry called double-step chronocoulometry, reached some units in 10^15 charges/cm^2. At room temperature, relative variations of resistance up to 8%, 1.9% and 1.6% were observed in the case of gold, silver and copper, respectively and, if the films are thick enough (> 25 nm), results can be nicely explained within a free-electron model with parallel resistive channels. The huge charge injections achieved make this particular field-effect technique very promising for a vast variety of materials such as unconventional superconductors, graphene and 2D-like materials., 6 pages, 6 figures. Applied Surface Science, in press

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds., 7 pages, 4 figures. Journal of Superconductivity and Novel Magnetism, in press

In this paper we propose a model to reproduce superconductive and normal properties of the iron pnictide LiFeAs in the framework of the four-band spm wave Eliashberg theory. A confirmation of the multiband nature of the system rises from the experimental measurements of the superconductive gaps and resistivity as function of temperature. We found that the most plausible mechanism is the antiferromagnetic spin fluctuation and the estimated values of the total antiferromagnetic spin fluctuation coupling constant in the superconductive and normal state are lambda{tot}=2.00 and lambda{tot,tr}=0.77., Comment: 5 pages, 3 figures

A century on from its discovery, a complete fundamental understanding of superconductivity is still missing. Considerable research efforts are currently devoted to elucidating mechanisms by which pairs of electrons can bind together through the mediation of a boson field different than the one associated to the vibrations of a crystal lattice. PuCoGa_5, a 5f-electron heavy-fermion superconductor with a record critical temperature T_c=18.5 K, is one of the many compounds for which the short-range, isotropic attraction provided by simple electron-phonon coupling does not appear as an adequate glue for electron pairing. Here, we report the results of point-contact spectroscopy measurements in single crystals of PuCoGa_5. Andreev reflection structures are clearly observed in the low-temperature spectra, and unambiguously prove that the paired superconducting electrons have wavefunction with the d-wave symmetry of a four-leaf clover. A straightforward analysis of the spectra provide the amplitude of the gap and its temperature dependence, \Delta(T). We obtain \Delta(T -> 0) = 5.1 \pm 0.3 meV and a gap ratio, 2\Delta/k_B T_c = 6.5 \pm 0.3, indicating that the compound is in the regime of strong electron-boson coupling. The gap value and its temperature dependence can be well reproduced within the Eliashberg theory for superconductivity if the spectral function of the mediating bosons has a spin-fluctuations-like shape, with a peak energy of 6.5 meV. Electronic structure calculations, combining the local density approximation with an exact diagonalization of the Anderson impurity model, provide a hint about the possible origin of the fluctuations., Comment: Main article with 4 figures, and supplementary info with 7 figures

By using an electrochemical gating technique with a new combination of polymer and electrolyte, we were able to inject surface charge densities n_2D as high as 3.5 \times 10^15 e/cm^2 in gold films and to observe large relative variations in the film resistance, DeltaR/R', up to 10% at low temperature. DeltaR/R' is a linear function of n_2D - 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., 5 pages, 5 figures, RevTeX

Polycrystalline samples and single crystals of SmFeAs1−xPxO1−y were synthesized and grown employing different synthesis methods and annealing conditions. Depending on the phosphorus and oxygen content, the samples are either magnetic or superconducting. In the fully oxygenated compounds, the main impacts of phosphorus substitution are to suppress the Neel temperature TN of the spin density wave (SDW) state and to strongly reduce the local magnetic field in the SDW state, as deduced from muon spin rotation measurements. On the other hand, the superconducting state is observed in the oxygen-deficient samples only after heat treatment under high pressure. Oxygen deficiency as a result of synthesis at high pressure brings the Sm-O layer closer to the superconducting As/P-Fe-As/P block and provides additional electron transfer. Interestingly, the structural modifications in response to this variation of the electron count are significantly different when phosphorus is partly substituting arsenic. Point contact spectra are well described with two superconducting gaps. Magnetic and resistance measurements on single crystals indicate an in-plane magnetic penetration depth of ∼200 nm and an anisotropy of the upper critical field slope of ∼4–5.

The experimental critical temperatures and the gap values of the superconducting materials LaFeAsO1−xFx, SmFeAsO1−xFx and Ba1−xKxFe2As2, which are exponent of the most important families (1111 and 122) of iron pnic- tides, can be reproduced by an effective s-wave three-band Eliashberg model in which the dominant coupling is in the interband channel and the order parameter is nodeless but undergoes a sign reversal between hole and electron bands (s± symmetry). In particular, high values of the electron- boson coupling constants and small typical boson energies (in agreement with neutron diffraction experiments) are nec- essary to reproduce the exact Tc, the experimental gap values and their temperature dependence. The same parameters al- low fitting the experimental temperature dependence of the upper critical field and predicting the presence of character- istic structures related to the electron-boson coupling in the Andreev-reflection spectra.

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., 27 pages, 14 figures. Review article to appear in a special issue of Reports on Progress in Physics

Directional point-contact Andreev-reflection measurements in $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$ single crystals (${T}_{c}=24.5\text{ }\text{ }\mathrm{K}$) 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 ${\ensuremath{\Omega}}_{b}(T)$ 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 $s\ifmmode\pm\else\textpm\fi{}$ Eliashberg model by using an electron-boson spectral function peaked at ${\ensuremath{\Omega}}_{0}=12\text{ }\text{ }\mathrm{meV}\ensuremath{\simeq}{\ensuremath{\Omega}}_{b}(0)$.

Massive carpets of well packed, vertically aligned and very long multiwall carbon nanotubes were synthesized by an efficient thermal Chemical Vapour Deposition process. Electrical properties of the material were evaluated, both in terms of "global" characteristics (bulk resistivity) and in terms of "local" properties (Scanning Tunnel Spectroscopy measurements) for as-grown and annealed at different temperatures samples. The behaviour of bulk resistivity as a function of temperature was evaluated in the range 3-300 K, with a four-probe technique. The resistivity shows a linear dependence with the square root of temperature in the investigated range. From the electrical analyses, it was found that the quality of the MWNTs was improved by the annealing process, since the resistivity decreases. Heat transport properties were evaluated by the laser flash technique in order to study thermal diffusivity. Moreover high temperature behavior of the specific heat capacity of single and multi-wall carbon nanotubes, was measured up to 800 K with a Differential Scanning Calorimeter.

Point-contact Andreev-reflection spectroscopy measurements were performed in ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ polycrystals with $x=0.09$ $({T}_{c}\ensuremath{\simeq}42\text{ }\text{K})$ and $x=0.20$ $({T}_{c}\ensuremath{\simeq}52\text{ }\text{K})$. In all cases the experimental conductance curves reproducibly exhibit low-energy peaks and higher-energy shoulders (at 4--6 and 16--20 meV, respectively, for $x=0.20$), which indicate the presence of two nodeless superconducting gaps. While the single-band Blonder-Tinkham-Klapwijk model can only reproduce a small central portion of a given conductance curve, the two-gap one accounts remarkably well for the shape of the whole experimental $dI/dV$ vs $V$ curve. The fit of the normalized curves gives ${\ensuremath{\Delta}}_{1}(0)=6.15\ifmmode\pm\else\textpm\fi{}0.45\text{ }\text{meV}$ and ${\ensuremath{\Delta}}_{2}(0)=18\ifmmode\pm\else\textpm\fi{}3\text{ }\text{meV}$ for $x=0.20$, while for $x=0.09$ the values ${\ensuremath{\Delta}}_{1}(0)=4.9\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{meV}$ and ${\ensuremath{\Delta}}_{2}(0)=15\ifmmode\pm\else\textpm\fi{}1\text{ }\text{meV}$ are obtained. In all cases, both gaps close at the same temperature and follow a BCS-like behavior.

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides ${\text{REFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ ($\text{RE}=\text{rare}$ earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, ${\ensuremath{\Delta}}_{1}=2.8--4.6\text{ }\text{meV}$, 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 ${T}^{\ensuremath{\ast}}\ensuremath{\sim}140\text{ }\text{K}$ (close to the N\'eel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$.

A review of our investigations on single crystals of LnFeAsO1-xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2 is presented. A high pressure technique has been applied for the growth of LnFeAsO1-xFx crystals, while Ba1-xRbxFe2As2 crystals were grown using 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 less than full occupation of the (O, F) position in superconducting LnFeAsO1-xFx crystals. Resistivity measurements on LnFeAsO1-xFx crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba1 xRbxFe2As2 simply shifts to lower temperature. Critical current density for both compounds is relatively high and exceeds 2x109 A/m2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO1-xFx crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba1-xRbxFe2As2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO1-xFx. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (~20 meV)., 27 pages, 19 figures, 2 tables, accepted to the special issue of the Physica C on superconducting pnictides

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 sigh 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., 4 pages, 4 color figures, Revised version: Figs. 2, 3, 4 improved and some values in the text changed to correct few mistakes

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO1-xFx (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection experiments performed on LaFeAsO1-xFx (La-1111) polycrystals with Tc ~ 27 K and SmFeAsO0.8F0.2 (Sm-1111) ones with Tc ~ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias, an experimental situation quite similar to that observed by the same technique in pure and doped MgB2. The single-band Blonder-Tinkham-Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO1-xFx (Re = La, Sm): a small gap, Delta1, smaller than the BCS value (2Delta1/kBTc ~ 2.2 - 3.2) and a much larger gap Delta2 which gives a ratio 2Delta2/kBTc ~ 6.5 - 9. In Sm-1111 both gaps close at the same temperature, very similar to the bulk Tc, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to Tc. The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive up to T* ~ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound., 10 pages, 7 color figures, Special Issue of Physica C on Superconducting Pnictides

We present a systematic characterization of Mg1−xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li content actually enters the structure. Also the transition temperature is only weakly affected by Li substitution, but its relationship with the residual resistivity and magnetoresistivity data is compatible with a picture where π bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth λ by an inductance technique. Data fits based on the standard Bardeen–Cooper–Schrieffer two-band model yield the zero-temperature limit of both λ and superconducting gaps: we find that λ(0) increases weakly and quickly saturates with increasing x, whereas Δπ(0) and Δσ(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the π band. Point contact spectroscopy results on samples obtained from the same batch are in full agreement with magnetic penetration depth data on the undoped sample, but give superconducting gaps almost insensitive to Li substitution, showing at most a small increase in Δπ(0) that may be related to an inhomogeneous distribution of Li content.

We studied the effects of co-doping with Li and Al on the energy gaps of MgB2 by performing point-contact Andreev-reflection spectroscopy (PCAR) in polycrystalline Mg1−x(AlαLi1−α)xB2 samples with x≤0.4. Even though the lattice parameters and the critical temperature of the compound simply scale with the effective Al content αx, irrespective of the Li concentration, the energy gaps do not. In particular, for a given effective Al content, the comparison with Mg1−y(Al)yB2 with y = αx shows that the σ bandgap is practically the same while the π bandgap is higher. A clear gap merging is observed in the most doped sample (x = 0.4) when Tc

The effect of Li substitution for Mg and of Li-C co-substitution on the superconducting properties and crystal structure of MgB2 single crystals has been investigated. It has been found that hole doping with Li decreases the superconducting transition temperature Tc, but at a slower rate than electron doping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li for Mg and C for B decreases more than in the case where C is substituted alone. This means that holes introduced by Li cannot counterbalance the effect of decrease of Tc caused by introduction of electrons coming from C. The possible reason of it can be that holes coming from Li occupy the pi band while electrons coming from C fill the sigma band. The temperature dependences of the upper critical field Hc2 for Al and Li substituted crystals with the same Tc show a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite of much different substitution level. This indicates that the mechanism controlling Hc2 and Tc is similar in both hole and electron doped crystals. Electrical transport measurements show an increase of resistivity both in Li substituted crystals and in Li and C co-substituted crystals. This indicates enhanced scattering due to defects introduced by substitutions including distortion of the lattice. The observed behavior can be explained as a result of two effects, influencing both Tc and Hc2. The first one is doping related to the changes in the carrier concentration, which may lead to the decrease or to the increase of Tc. The second one is related to the introduction of new scattering centers leading to the modification of the interband and/or intraband scattering and therefore, to changes in the superconducting gaps and to the reduction of Tc.

We present the first results of directional point-contact spectroscopy in high quality CaC6 samples both along the ab plane and in the c-axis direction. The superconducting order parameter \Delta(0), obtained by fitting the Andreev-reflection (AR) conductance curves at temperatures down to 400 mK with the single-band 3D Blonder-Tinkham-Klapwijk model, presents two different distributions in the two directions of the main current injection, peaked at 1.35 and 1.71 meV, respectively. By ab-initio calculations of the AR conductance spectra, we show that the experimental results are in good agreement with the recent predictions of gap anisotropy in CaC6., Comment: 4 pages, 4 color eps figures. New results at 400 mK added; figures and discussion modified accordingly. Phys. Rev. Lett., accepted

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg_{1-x}Al_{x}B_{2} single crystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-band Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes Delta_{sigma} and Delta_{pi}. The gap values agree rather well with other PCAR results in Al-doped crystals and polycrystals up to x=0.2 reported in literature, and extend them to higher Al contents. In the low-doping regime, however, we observed an increase in the small gap Delta_{pi} on increasing x (or decreasing the local critical temperature of the junctions, T_{c}^{A}) which is not as clearly found in other samples. On further decreasing T_{c}^{A} below 30 K, both the gaps decrease and, up to the highest doping level x=0.32 and down to T_{c}^{A}= 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashberg theory shows that this gap trend can be explained as being mainly due to the band filling and to an increase in the interband scattering which is necessary to account for the increase in Delta_{pi} at low Al contents (x < 0.1). We suggest to interpret the following decrease of Delta_{pi} for T_{c}^{A} < 30 K as being governed by the onset of inhomogeneity and disorder in the Al distribution that partly mask the intrinsic effects of doping and is not taken into account in standard theoretical approaches., 22 pages, 9 eps figures, Elsevier style. Theoretical details added in appendix. Characterization of crystals included

We summarize and compare the results obtained by point-contact spectroscopy (PCS) in high-quality single crystals and polycrystals of MgB2 with partial Al or C substitutions or irradiated with neutrons. We determined the values of the gaps Delta(sigma) and Delta(pi) by fitting the conductance curves of point contacts in the Andreev-reflection regime with the generalized BTK model. We always studied the temperature and magnetic-field dependencies of the conductance curves, so as to determine the critical temperature of the contact, T-c(A) and highlight whether one or two gaps were present. The dependence of the gaps on TcA shows a dominant effect of the band filling not only in Mg1-xAlxB2 and Mg(B1-xCx)(2) but also, unexpectedly, in irradiated MgB2. The increase in interband scattering is instead smaller than expected in Al-doped and irradiated MgB2 but is very large in C-doped single crystals, probably for extrinsic effects. (c) 2007 Elsevier B.V. All rights reserved.

We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Al x B2 crystals and polycrystals with x up to 0.32, recently grown at ETH (Zurich) and University of Genova, respectively. The differential conductance curves of our “soft” point contacts were measured as a function of temperature from 4.2 K up to the Andreev critical temperature T , at which the normal-state conductance is recovered. The gap amplitudes, Δ σ and Δ π , were obtained through a two-band BTK fit of the normalized conductance curves. The results extend previous data to the range of heavy Al doping and show that contrary to expectations, no gap merging is observed up to x=0.32 (i.e., down to T =9 K). The analysis of the gap trends within the two-band Eliashberg theory shows that band filling is the main effect of Al doping, even though a small increase in interband scattering might be necessary to account for the experimental data in single crystals.

We review recent results of point-contact spectroscopy (PCS) in doped (with Al, C or Mn) and neutron-irradiated samples of the two-band superconductor MgB2. The aim of these measurements was to clarify the role of inter- and intra-band scattering, and to test the theoretical expectations about the effects of substitutions and disorder on two-band superconductivity. The gap amplitudes, Δπ and Δσ, were obtained as a function of the critical temperature of the junctions (or of the doping content) by fitting the conductance curves with the two-band BTK model. The analysis of the resulting curves within the Eliashberg theory allowed us to extract information on the partial DOSs, as well as on the scattering rates within and between the two bands. The results are discussed and compared to theoretical expectations.

We present the first results of directional point-contact measurements in Mg1-xMnxB2 single crystals, with x up to 0.015 and bulk Tc down to 13.3 K. The order parameters Deltasigma and Deltapi were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk model. BothDeltapi and Deltasigma decrease with the critical temperature of the junctions TAC, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principles band-structure calculations.

Point-contact Andreev-reflection spectroscopy (PCARS) has demonstrated to be one of the most effective experimental tools for the investigation of fundamental properties of superconductors such as the superconducting gap and the electron–phonon (or, more generally, electron–boson) coupling. By reviewing relevant examples reported in literature and presenting new results, in this paper we show that when the direction of the interface with respect to the crystallographic axes can be controlled (as in single crystals and epitaxial films) PCARS can provide invaluable information about the anisotropy of the pairing wavefunction or — in the case of multiband superconductors — on the number, amplitude and symmetry of the energy gaps. Moreover, the analysis of PCARS results within a suitable 3D generalization of the BTK model allows obtaining qualitative information about the topology of the Fermi surface.

Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-based superconductor LaFeAsO1−x F x in order to investigate the energy gaps. The Andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Their values, as determined by fitting the conductance curves measured in junctions with local T c =27.3–28.6 K within the two-band BTK model, are Δ 1=2.75–3.8 meV for the small gap and Δ 2=7.9–10.2 meV for the larger one, respectively. The absence of zero-bias conductance peaks and considerations regarding the nonperfectly directional current injection in PCAR experiments and the polycrystalline nature of the samples rule out the possibility of nodal gaps in the superconductor. Moreover, the conductance curves show pseudogap-like features coexisting, at low temperature, with superconductivity and disappearing close to the Neel temperature of the parent compound, T N≈140 K.