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Electrically-conducting diamond is a promising candidate for next-generation electronic, thermal and electrochemical applications. One of the major obstacles towards its exploitation is the strong degradation that some of its key physical properties - such as the carrier mobility and the superconducting transition temperature - undergo upon the introduction of disorder. This makes the two-dimensional hole gas induced at its surface by electric field-effect doping particularly interesting from both a fundamental and an applied perspective, since it strongly reduces the amount of extrinsic disorder with respect to the standard boron substitution. In this short review, we summarize the main results achieved so far in controlling the electric transport properties of different field-effect doped diamond surfaces via the ionic gating technique. We analyze how ionic gating can tune their conductivity, carrier density and mobility, and drive the different surfaces across the insulator-to-metal transition. We review their strongly orientation-dependent magnetotransport properties, with a particular focus on the gate-tunable spin-orbit coupling shown by the (100) surface. Finally, we discuss the possibility of field-induced superconductivity in the (110) and (111) surfaces as predicted by density functional theory calculations., Comment: 13 pages, 8 figures

We report on the first successful growth of single crystals of the noncentrosymmetric superconductor Mo$_3$Al$_2$C obtained by means of a cubic-anvil, high-pressure and high-temperature technique. Composition, structure, and normal-state transport properties of the crystals were studied by means of X-ray diffraction, energy-dispersive X-ray spectroscopy, magnetic susceptibility and resistivity measurements as a function of temperature. Variations in critical temperature ($T_c$) between 8.6 and 9.3 K were observed, probably due to the slightly different carbon stoichiometry of the samples. Single-crystal X-ray refinement confirmed the high structural perfection of the grown crystals. Remarkably, the refined Flack parameter values for all the measured crystals using a $P4_132$ space-group model were consistently close to either 0 or 1, hence indicating that the considered crystals belong to two enantiomorphic space groups, $P4_132$ and $P4_332$. An anomaly in the resistivity is observed at about 130 K, most likely associated with the onset of a charge-density-wave phase. The superconducting properties (and in particular the symmetry, the amplitude and the temperature dependence of the superconducting gap) were studied by using - for the first time in this compound - point contact Andreev-reflection spectroscopy. The results confirm that Mo$_3$Al$_2$C is a moderately strongly-coupled superconductor with $2\Delta/k_BT_c \simeq 4$ and unambiguously prove that the order parameter has an s-wave symmetry despite the asymmetric spin-orbit coupling arising from the lack of inversion symmetry., Comment: 12 pages, 12 figures, revtex format

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe2As2, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the holelike Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line., Scientific Reports, 6, ISSN:2045-2322

The temperature dependence of the resistivity of epitaxial Ba(Fe_(1-x)Co_x)2As2 thin films (with nominal doping x = 0.08, 0.10 and 0.15) has been analyzed and compared with analogous measurements on single crystals taken from literature. The rho(T) of thin films looks different from that of single crystals, even when the cobalt content is the same. All rho(T) curves can be fitted by considering an effective two-band model (with holes and electrons bands) in which the electrons are more strongly coupled with the bosons (spin fluctuations) than holes, while the effect of impurities is mainly concentrated in the hole band. Within this model the mediating boson has the same characteristic energy in single crystals and thin films, but the shape of the transport spectral function at low energy has to be very different, leading to a "hardening" of the electron-boson spectral function in thin films, associated with the strain induced by the substrate., Comment: 13 pages, 4 figures

The dependence of the superconducting gaps in epitaxial Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the 2D Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps \Delta_{S} and \Delta_{L} show similar monotonic trends as a function of the local critical temperature T_{c}^{A} (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e. to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T_{c} according to the empirical law \Omega_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength \lambda_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly decreases to \lambda_{tot}= 1.82 in the overdoped samples (x = 0.15)., Comment: 8 pages, 5 color figures

Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1-xCox)2As2 single crystals (Tc=24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The PCAR spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy 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+- Eliashberg model by using an electron-boson spectral function peaked at Omega_0 = 12 meV ~ Omega_b(0)., Comment: 4 pages, 4 color figures. 2 authors added; new data included (Fig.4) and discussed. Accepted in PRL

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, Delta1 = 2.8-4.6 meV, which scales with the local Tc of the contact; a larger unconventional OP that gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases on increasing the Tc 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 Neel 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., 19 pages, 12 figures - one figure and some insets added, minor changes to the text

Point-contact Andreev-reflection measurements were performed in SmFeAsO_{0.8}F_{0.2} polycrystals with T_c \simeq 53 K. The experimental conductance curves reproducibly exhibit peaks around \pm 6 mV and shoulders at V \sim 16-20 mV, indicating the presence of two nodeless superconducting gaps. While the single-band Blonder-Tinkham-Klapwijk fit can only reproduce a small central portion of the conductance curve, the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV. The fits of the normalized curves give Delta_1(0) = 6.15 \pm 0.45 meV and Delta_2(0) = 18 \pm 3 meV. Both gaps close at the same temperature and follow a BCS-like behavior., Comment: 4 pages, 4 eps color figures

We report on recent results of point-contact spectroscopy measurements in Mg^{11}B_2 polycrystals irradiated at different neutron fluences up to 1.4 x 10^{20} cm^{-2}. The point contacts were made by putting a small drop of Ag paint -- acting as the counterelectrode -- on the cleaved surface of the samples. The gap amplitudes were extracted from the experimental conductance curves, showing Andreev-reflection features, through a two-band Blonder-Tinkham-Klapwijk fit and reported as a function of the Andreev critical temperature of the junctions, TcA. The resulting Delta_sigma(TcA) and Delta_pi(TcA) curves show a clear merging of the gaps when TcA ~ 9K that perfectly confirms the findings of specific-heat measurements in the same samples. Anomalous contacts with TcA > Tc (being Tc the bulk critical temperature) were often obtained, particularly in samples irradiated at very high fluences. Their fit gave a different dependence of Delta_pi on TcA. The possible origin of these contacts is discussed in terms of local current-induced annealing and/or nanoscale inhomogeneities observed by STM in the most irradiated samples., 12 pages, 10 figures, revtex4. One figure removed; Figs. 4 and 8 changed. Text changed (particularly Sect. V)

We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3 K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model. Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the junctions T_c^A, 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-principle band structure calculations., 4 pages, 5 eps figures. New theoretical results added, text and some figures changed. References added

We report the results of the first directional point-contact measurements in Mg(B_{1-x}C_{x})_2 single crystals with 0.047, 5 pages, 5 figures. One figure and one panel added; text and discussion updated

We study the effects of C substitutions in MgB_2 within the two-band model in the Eliashberg formulation. We use as input the B-B stretching-mode frequency and the partial densities of states N_{sigma}(EF) and N_{pi}(EF), recently calculated for Mg(B_{1-x}C_{x})_2 at various x values from first-principles density functional methods. We then take the prefactor in the Coulomb pseudopotential matrix, mu, and the interband scattering parameter, Gamma^{sigma pi}, as the only adjustable parameters. The dependence on the C content of Tc and of the gaps (Delta_{sigma} and Delta_{pi}) recently measured in Mg(B_{1-x}C_{x})_2 single crystals indicate an almost linear decrease of mu on increasing x, with an increase in interband scattering that makes the gaps merge at x=0.132. In polycrystals, instead, where the gap merging is not observed, no interband scattering is required to fit the experimental data., Comment: 7 pages, 8 figures, RevTex4. Detailed discussion of the results added

In this Letter, the study of the effect of Al substitution on the upper critical field, Bc2, in AlxMg1-xB2 samples is presented. We find a straightforward correlation between Bc2 and the sigma-band gap, Delta_sigma, evaluated by point-contact measurements. Up to x=0.2 Bc2 can be well described within a clean limit model and its decrease with x is directly related to the suppression of Delta_sigma. For larger doping we observed the crossover to the dirty regime driven mostly by the strong decrease of Delta_sigma rather than by the increase of the sigma-band scattering rate, Comment: 5 pages, 4 figures

We present the results of point-contact spectroscopy measurements in polycrystalline RuSr_2GdCu_2O_8 samples previously characterized from both the structural and the electrical point of view. AC susceptibility and resistivity measurements, as well as SEM and optical microscopy, indicate that the polycrystalline material is made up of small, weakly connected grains. Possibly because of this morphology, point-contact measurements turned out to be rather difficult. Best reproducibility of the conductance curves was achieved in point contacts obtained by pressing Au or Pt-Ir tips on the surface of the samples. The low-temperature conductance curves reproducibly show a clear zero-bias cusp that might be interpreted as due to Andreev reflection at the normal metal-superconductor interface in the case where the superconducting order parameter has a pure d-wave symmetry - even though alternative explanations cannot be ruled out. In this paper we show that these curves can indeed be fitted, in the framework of the generalised BTK model, by using pure d-wave symmetry and a gap of the order of 6 meV. Finally, we present the temperature dependence of the conductance curves from 4.2 K up to the critical temperature of the junctions., Comment: 7 pages, 5 figures, Proceedings of the 6th EUCAS Conference (14-18 September 2003, Sorrento - Italy)- Added acknowledgements

We present the results of directional point-contact measurements in MgB_2 single crystals, in magnetic fields up to 9 T parallel to the c axis. By fitting the conductance curves of our point contacts -- showing clear Andreev-reflection features -- with a generalized Blonder-Tinkham-Klapwijk model, we were able to extract the values of the two gaps Delta_sigma and Delta_pi. The comparison of the resulting Delta_sigma(B) and Delta_pi(B) curves to the theoretical predictions of the two-band model in dirty limit, recently appeared in literature, allows the first direct test of this model and gives a clear and quantitative proof that the pi band, even in the best single crystals, is in the moderate dirty limit., 4 pages, 3 figures, revtex4

We made point-contact measurements with Au tips on La_{2-x}Sr_{x}CuO_{4} samples with 0.08 < x < 0.20 to investigate the relationship between superconducting gap and pseudogap. We obtained junctions whose conductance curves presented typical Andreev reflection features at all temperatures from 4.2 K up to T_c^A close to the bulk T_c. Their fit with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The doping dependence of the low temperature dominant isotropic gap component Delta_{s} follows very well the T_{c} vs. x curve. These results support the separation between the superconducting (Andreev) gap and the pseudogap measured by angle-resolved photoemission spectroscopy (ARPES) and tunneling., 4 pages, 5 eps figures, 1 table. SNS 2001 Conference

We present and discuss the results of measurements of the ab-plane resistivity of La_{2-x}Sr_{x}CuO_{4} single crystals with small Sr contents (x=0.052-0.075). The resistivity was obtained in the temperature range between 4.2 K and 300 K by using an AC Van der Pauw technique. When the temperature is lowered, the rho_{ab}(T) curves show a transition from a linear behaviour to a semiconducting-like one. The deviation from the linearity occurs at a doping-dependent temperature T_{ch}. This effect can be interpreted as due to a progressive pinning of preexistent charge stripes. In the same framework, the appearance of anomalous sharp peaks in the rho_{ab}(T) curves of superconducting samples just above T_{c} may be explained by a transition from a nematic to a more ordered smectic electronic stripe phase., 4 RevTex pages, 4 EPS figures, submitted to Phys. Rev. B

We present the results of the first directional point-contact spectroscopy experiments in high-quality MgB_2 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 sigma and pi 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 MgB_2., 4 pages, 5 eps figures. References added, Fig.1 changed, some text added

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

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