  Motivated by recent Hall-effect experiment in YbRh$_2$Si$_2$, we study ground
state properties of a Kondo lattice model in a two-dimensional square lattice
using variational Monte Carlo method. We show that there are two types of phase
transition, antiferromagnetic transition and topological one (Fermi surface
reconstruction). In a wide region of parameters, these two transitions occur
simultaneously without the breakdown of Kondo screening, accompanied by a
discontinuous change of the Hall coefficient. This result is consistent with
the experiment and gives a novel theoretical picture for the quantum critical
point in heavy fermion systems.
