  The formation of heavy fermion bands can occur by means of the conversion of
a periodic array of local moments into itinerant electrons via the Kondo effect
and the huge consequent Fermi-liquid renormalizations. Leggett predicted for
liquid $^3$He that Fermi-liquid renormalizations change in the superconducting
state, leading to a temperature dependence of the London penetration
depth~$\Lambda$ quite different from that in the BCS theory. Using Leggett's
theory, as modified for heavy fermions, it is possible to extract from the
measured temperature dependence of $\Lambda$ in high quality samples both
Landau parameters $F_0^s$ and $F_1^s$; this has never been accomplished before.
A modification of the temperature dependence of the specific heat
$C_\mathrm{el}$, related to that of $\Lambda$, is also expected. We have
carefully determined the magnitude and temperature dependence of $\Lambda$ in
CeCoIn$_5$ by muon spin relaxation rate measurements to obtain $F_0^s = 36 \pm
1$ and $F_1^s = 1.2 \pm 0.3$, and find a consistent change in the temperature
dependence of electronic specific heat $C_\mathrm{el}$. This, the first
determination of $F_1^s$ with a value~$\ll F_0^s$ in a heavy fermion compound,
tests the basic assumption of the theory of heavy fermions, that the frequency
dependence of the self-energy is much more important than its momentum
dependence.
