Magnetic field effect on the pressure-induced superconducting state in the hole-doped two-leg ladder compoundSr2Ca12Cu24O41

We report electrical resistivity on a single crystal of the hole-doped two-leg ladder compound ${\mathrm{Sr}}_2{\mathrm{Ca}}_{12}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$, which becomes superconducting with $T_c\sim 5\mathrm{K}$ only at pressures above $\sim 3.0\mathrm{GPa}$. Measurements were performed at nearly hydrostatic pressures up to $5.7\mathrm{GPa}$ and low temperatures down to $100\mathrm{mK}$ under static magnetic fields up to $20\mathrm{T}$ parallel to the $a$ axis (along the ladder rungs) and up to $7\mathrm{T}$ parallel to both the $b$ axis (perpendicular to the ladder plane) and the $c$ axis (along the ladder legs). A clear difference in the resistive upper critical field $H_{c2}\left(T\right)$ is observed among these three directions, confirming that this system has a highly anisotropic superconducting ground state. Also, $H_{c2}\left(T\right)$ parallel to the ladder plane is found to exceed the Pauli limit by a factor of more than 2, suggesting either a strong spin-orbit scattering or spin-triplet pairing. Furthermore, it is implied, from measurements of resistivity versus angle of magnetic field in the $bc$ plane, that another superconducting phase is stable below around $3\mathrm{K}$ only when the magnetic field is applied exactly along a certain direction that is $\pm 35°$ from the ladder direction.