Strong pressure dependence of the magnetic penetration depth in single crystals of the heavy fermion system CeCoIn$_5$ studied by muon spin rotation

The pressure dependence ($0-1$ GPa) of the in-plane magnetic penetration depth ($\lambda_a$), the penetration depth anisotropy ($\gamma=\lambda_c/\lambda_a$) and the temperature dependence of $1/\lambda^2_i$ ($i=a,c$) were studied in single crystals of the heavy fermion system CeCoIn$_5$ by means of muon spin rotation. A strong decrease of $\lambda_a$ with pressure was observed, while $\gamma$ and $\lambda_i^2(0)/\lambda_i^2(T)$ are pressure independent. A linear relationship between $1/\lambda_a^2$(270 mK) and Tc was also found. The large decrease of $\lambda_a$ with pressure is the signature of an increase of the number of superconducting quasiparticles by a factor of about 2.