On the possible secondary component of the order parameter observed in London penetration depth measurements

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We discuss the effect of a secondary component of the superconducting order parameter on the superfluid density in the cuprates. If we assume a main dₓ₂₋y₂ gap, the most stable realization of a mixed order parameter has a time-reversal breaking dₓ₂₋y₂+ dxy symmetry. In this state the nodes are removed and the temperature dependence of the superfluid density changes from the linear behavior of a pure d-wave to a more rounded shape at low temperature. The latter is compatible with the behavior experimentally observed in the in-plane magnetic field penetration depth of optimally doped La₂₋ₓSrₓCuO₂ and YBa₂Cu₃O₇₋δ.