The anisotropy $\gamma$ of the superconducting state of high quality single crystals of ${\mathrm{MgB}}_2$ was determined, using torque magnetometry with two different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from $\gamma \simeq 6$ at 15 K to 2.8 at 35 K. Reversible torque data near $T_c$ reveal a field dependent anisotropy, increasing nearly linearly from $\gamma \simeq 2$ in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by nonlocal effects of anisotropic pairing and/or the $\overrightarrow{k}$ dependence of the effective mass tensor.

The growth of carbon-substituted magnesium diboride $\mathrm{Mg}{\left({\mathrm{B}}_{1-x}{\mathrm{C}}_x\right)}_2$ single crystals with $0⩽x⩽0.15$ is reported, and the structural, transport, and magnetization data are presented. The superconducting transition temperature decreases monotonically with increasing carbon content in the full investigated range of substitution. By adjusting the nominal composition, $T_c$ of substituted crystals can be tuned in a wide temperature range between 10 and 39 K. Simultaneous introduction of disorder by carbon substitution and significant increase of the upper critical field $H_{c2}$ is observed. Comparing with the nonsubstituted compound, $H_{c2}$ at 15 K for $x=0.05$ is enhanced by more than a factor of 2 for $H$ oriented both perpendicular and parallel to the $ab$ plane. This enhancement is accompanied by a reduction of the $H_{c2}$-anisotropy coefficient $\gamma$ from 4.5 (for the nonsubstituted compound) to 3.4 and 2.8 for the crystals with $x=0.05$ and 0.095, respectively. At temperatures below 10 K, the single crystal with larger carbon content shows $H_{c2}$ (defined at zero resistance) higher than 7 and 24 T for $H$ oriented perpendicular and parallel to the $ab$ plane, respectively. Observed increase of $H_{c2}$ cannot be explained by the change in the coherence length due to the disorder-induced decrease of the mean free path only.

Iron-chalcogenide single crystals with the nominal composition ${\text{FeSe}}_{0.5}{\text{Te}}_{0.5}$ and a transition temperature of $T_c\simeq 14.6\text{K}$ were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal x-ray and neutron powder diffraction, superconducting quantum interference device and torque magnetometry, and muon-spin rotation (μSR). Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of ${\text{Fe}}_{1.045}{\text{Se}}_{0.406}{\text{Te}}_{0.594}$. Additionally, a minor hexagonal ${\text{Fe}}_7{\text{Se}}_8$ impurity phase was identified. The magnetic penetration depth $\lambda$ at zero temperature obtained by means of μSR was found to be ${\lambda }_{ab}\left(0\right)=491\left(8\right)\text{nm}$ in the $ab$ plane and ${\lambda }_c\left(0\right)=1320\left(14\right)\text{nm}$ along the $c$ axis. The zero-temperature value of the superfluid density ${\rho }_s\left(0\right)\propto {\lambda }^{-2}\left(0\right)$ obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron pnictides. The temperature dependences of both ${\lambda }_{ab}$ and ${\lambda }_c$ are well described by a two-gap $s+s$-wave model with the zero-temperature gap values of ${\Delta }_S\left(0\right)=0.51\left(3\right)\text{meV}$ and ${\Delta }_L\left(0\right)=2.61\left(9\right)\text{meV}$ for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter ${\gamma }_{\lambda }\left(T\right)={\lambda }_c\left(T\right)/{\lambda }_{ab}\left(T\right)$ increases with decreasing temperature, in agreement with ${\gamma }_{\lambda }\left(T\right)$ observed in the iron-pnictide superconductors.