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A quantum critical point (QCP) arises at a continuous transition betweencompeting phases at zero temperature. Collective excitations at magnetic QCPsgive rise to metallic properties that strongly deviate from the expectations ofLandau's Fermi liquid description, the standard theory of electron correlationsin metals. Central to this theory is the notion of quasiparticles, electronicexcitations which possess the quantum numbers of the bare electrons. Here wereport measurements of thermal and electrical transport across thefield-induced magnetic QCP in the heavy-fermion compound YbRh$_2$Si$_2$. Weshow that the ratio of the thermal to electrical conductivities at thezero-temperature limit obeys the Wiedemann-Franz (WF) law above the criticalfield, $B_c$. This is also expected at $B < B_c$, where weak antiferromagneticorder and a Fermi liquid phase form below 0.07 K ($B = 0$). However, at thecritical field the low-temperature electrical conductivity suggests anon-Fermi-liquid ground state and exceeds the thermal conductivity by about10%. This apparent violation of the WF law provides evidence for anunconventional type of QCP at which the fundamental concept of Landauquasiparticles breaks down. These results imply that Landau quasiparticlesbreak up, and that the origin of this disintegration is inelastic scatteringassociated with electronic quantum critical fluctuations. Our finding bringsnew insights into understanding deviations from Fermi-liquid behaviourfrequently observed in various classes of correlated materials.

Thermal and Electrical Transport across a Magnetic Quantum Critical Point