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