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In a <heavy fermion material>, interactions of narrow, often $f$-derived states with conduction electrons produce low-energy <quasiparticles> with very large thermodynamic <effective mass>. A coherent <Kondo lattice> can form a narrow low-energy band, so the large mass is an interaction-renormalized response rather than a larger bare electron mass.
Two concrete experimental signatures are an enormous linear <electronic heat capacity coefficient> and a strongly enhanced quadratic low-temperature <electrical resistivity>. Calorimetry separates $C=\gamma T+\beta T^3$ by plotting $C/T$ against $T^2$; the intercept measures the electronic coefficient rather than a lattice contribution. Four-terminal transport tests the <Fermi liquid> law $\rho=\rho_0+AT^2$. The <low-temperature calorimetry and resistivity of CeAl3> found $\gamma=1.62\,\mathrm{J\,mol^{-1}\,K^{-2}}$ and $A=35\,\mathrm{\mu\Omega\,cm\,K^{-2}}$ below $0.2\,\mathrm K$, making both signatures quantitative.<4f-Virtual-Bound-State Formation in CeAl3 at Low Temperatures by Klaus Andres and others>[Abstract: specific heat and electrical resistivity below 0.2 K, with gamma and the quadratic resistivity coefficient]{ref}
An enhanced nearly temperature-independent spin susceptibility and a large quantum-oscillation <cyclotron effective mass> provide additional tests. Not every heavy-fermion compound has the same ordered ground state; magnetism or superconductivity can intervene before a simple normal Fermi-liquid limit is reached.
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