New muon-spin-rotation measurement of the temperature dependence of the magnetic penetration depth in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$

We report muon-spin-rotation (μSR) measurements of the a-b magnetic penetration depth (${\lambda }_{\mathit{a}\mathit{b}}$) in the vortex state of high quality single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$. Contrary to earlier μSR studies on powders and crystal mosaics, 1/${\lambda }_{\mathit{a}\mathit{b}}^2$ shows a strong linear temperature dependence below 50 K which weakens with increasing magnetic field. These results support recent microwave cavity measurements in zero field and provide further evidence for unconventional pairing of carriers.