Single crystals of ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ 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 the optimization of an already developed ${\mathrm{MgB}}_2$ crystal growth procedure. A 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 ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ 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 the appearance of the second precipitation phase in the crystals with $x>0.1$. This is in a form of nonsuperconducting ${\mathrm{MgAlB}}_4$ domains in the structure of a superconducting ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, $H_{c2}$, for $H\Vert c$ for the samples with small $x$, a significant reduction of the $H_{c2}$ anisotropy at lower temperatures, and a 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 ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ structure.

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\ifmmode\times\else\texttimes\fi{}1015\,\,e/cm2 in gold films and to observe large relative variations in the film resistance, $\Delta${}R/R$'${}, up to 10% at low temperature. $\Delta${}R/R$'${} is a linear function of n2D\char22{}as expected within a free-electron model\char22{}if the film is thick enough ($\ge${}25\,\,nm); otherwise, a tendency to saturation due to size effects is observed. The application of this technique to 2D materials might allow extending the field-effect experiments to a range of charge doping where large conductance modulations and, in some cases, even the occurrence of superconductivity are expected.

Directional point-contact Andreev-reflection 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 point-contact Andreev-reflection 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\ifmmode\pm\else\textpm\fi{} Eliashberg model by using an electron-boson spectral function peaked at $\Omega${}0=12\,\,meV$\simeq${}$\Omega${}b(0).

We report the results of the first directional point-contact measurements in $\mathrm{Mg}{\left({\mathrm{B}}_{1-x}{\mathrm{C}}_x\right)}_2$ single crystals with $0.047⩽x⩽0.132$. The two-gap superconductivity typical of ${\mathrm{MgB}}_2$ persists up to $x=0.105$. In this region, the values of the gaps ${\Delta }_{\sigma }$ and ${\Delta }_{\pi }$ were determined by fitting the Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk (BTK) model, and confirmed by the single-band BTK fit of the $\sigma$- and $\pi$-band conductances, separated by means of a magnetic field. At $x=0.132$, when $T_c=19\mathrm{K}$, we clearly observed the merging of the two gaps into one of amplitude $\Delta \simeq 3\mathrm{meV}$.