Nonvanishing Energy Scales at the Quantum Critical Point of CeCoIn₅

Heat and charge transport were used to probe the magnetic field-tuned quantum critical point in the heavy-fermion metal CeCoIn₅. A comparison of electrical and thermal resistivities reveals three characteristic energy scales. A Fermi-liquid regime is observed below T_FL, with both transport coefficients diverging in parallel and T_FL→0 as H→H_c, the critical field. The characteristic temperature of antiferromagnetic spin fluctuations, T_SF, is tuned to a minimum but finite value at H_c, which coincides with the end of the T-linear regime in the electrical resistivity. A third temperature scale, T_QP, signals the formation of quasiparticles, as fermions of charge e obeying the Wiedemann-Franz law. Unlike T_FL, it remains finite at H_c, so that the integrity of quasiparticles is preserved, even though the standard signature of Fermi-liquid theory fails.