A quantum critical point (QCP) develops in a material at absolute zero when anew form of order smoothly emerges in its ground state. QCPs are of greatcurrent interest because of their singular ability to influence the finitetemperature properties of materials. Recently, heavy-fermion metals have playeda key role in the study of antiferromagnetic QCPs. To accommodate the heavyelectrons, the Fermi surface of the heavy-fermion paramagnet is larger thanthat of an antiferromagnet. An important unsolved question concerns whether theFermi surface transformation at the QCP develops gradually, as expected if themagnetism is of spin density wave (SDW) type, or suddenly as expected if theheavy electrons are abruptly localized by magnetism. Here we reportmeasurements of the low-temperature Hall coefficient ($R_H$) - a measure of theFermi surface volume - in the heavy-fermion metal YbRh2Si2 upon field-tuning itfrom an antiferromagnetic to a paramagnetic state. $R_H$ undergoes anincreasingly rapid change near the QCP as the temperature is lowered,extrapolating to a sudden jump in the zero temperature limit. We interpretthese results in terms of a collapse of the large Fermi surface and of theheavy-fermion state itself precisely at the QCP.
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