We demonstrate the existence of a novel superconducting state in high quality two-component ${\mathrm{MgB}}_2$ single crystalline superconductors where a unique combination of both type-1 (${\lambda }_1/{\xi }_1<1/\sqrt{2}$) and type-2 (${\lambda }_2/{\xi }_2>1/\sqrt{2}$) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.

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.

We report observation of Leggett’s collective mode in a multiband ${\mathrm{MgB}}_2$ superconductor with $T_c=39\mathrm{K}$ arising from the fluctuations in the relative phase between two superconducting condensates. The novel mode is observed by Raman spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed mode frequency is consistent with theoretical considerations based on first-principles computations.