Magnetic-Field Induced Quantum Critical Point in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi/><mml:mi mathvariant="normal">Y</mml:mi><mml:mi mathvariant="normal">b</mml:mi><mml:mi mathvariant="normal">R</mml:mi><mml:msub><mml:mi mathvariant="normal">h</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:msub><mml:mi mathvariant="normal">i</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math>

We report low-temperature calorimetric, magnetic, and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh(2)Si(2) ( T(N)=70 mK) as a function of magnetic field B. While for fields exceeding the critical value B(c0) at which T(N)-->0 the low-temperature resistivity shows an AT2 dependence, a 1/(B-B(c0)) divergence of A(B) upon reducing B to B(c0) suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.