Metamagnetic transitions, electronic structures and quantum critical point of CeRu₂ (Si_{1 - x} Ge_x)₂

We have studied the electronic structures of CeRu₂ (Si_{1 - x} Ge_x)₂ single crystals, whose ground state changes from paramagnetic state to antiferromagnetic state with increasing Ge concentration across the quantum critical point, via the dHvA effect. It has been reported that the effective masses of the up and down spin conduction electrons in CeRu₂ Si₂ are considerably different, although their Fermi surfaces are nearly the same. Such anomalous conduction electron state is found to exist above the metamagnetic transition fields in both the samples of the paramagnetic and antiferromagnetic ground state.