Divergence of the magnetic Grüneisen ratio at the field-induced quantum critical point in YbRh2Si2.

The heavy-fermion metal YbRh2Si2 is studied by low-temperature magnetization M(T) and specific-heat C(T) measurements at magnetic fields close to the quantum critical point (H_{c}=0.06 T, H perpendicularc). Upon approaching the instability, dM/dT is more singular than C(T), leading to a divergence of the magnetic Grüneisen ratio Gamma_{mag}=-(dM/dT)/C. Within the Fermi-liquid regime, Gamma_{mag}=-G_{r}(H-H_{c};{fit}) with G_{r}=-0.30+/-0.01 and H_{c};{fit}=(0.065+/-0.005) T which is consistent with scaling behavior of the specific-heat coefficient in YbRh2(Si0.95Ge0.05)_{2}. The field dependence of dM/dT indicates an inflection point of the entropy as a function of magnetic field upon passing the line T;{ small star, filled}(H) previously observed in Hall and thermodynamic measurements.