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Intact Quasiparticles at an Unconventional Quantum Critical Point

We report measurements of in-plane electrical and thermal transport properties in the limit $T \rightarrow 0$ near the unconventional quantum critical point in the heavy-fermion metal $β$-YbAlB$_4$. The high Kondo temperature $T_K$ $\simeq$ 200 K in this material allows us to probe transport extremely close to the critical point, at unusually small values of $T/T_K < 5 \times 10^{-4}$. Here we find that the Wiedemann-Franz law is obeyed at the lowest temperatures, implying that the Landau quasiparticles remain intact in the critical region. At finite temperatures we observe a non-Fermi liquid T-linear dependence of inelastic scattering processes to energies lower than those previously accessed. These processes have a weaker temperature dependence than in comparable heavy fermion quantum critical systems, and suggest a new temperature scale of $T \sim 0.3 K$ which signals a sudden change in character of the inelastic scattering.