| Quantum critical properties of strongly correlated metals in heavy fermion systems are investigated. Based on an extended dynamic mean field theory of the Kondo lattice model, two types of quantum phase transitions are found to exist in these materials: the conventional spin density wave transition and a novel locally critical quantum phase transition where the local dynamics is also critical. The associated quantum impurity model, the Bose-Fermi Kondo model, is extensively studied with an epsilon-expansion renormalization group analysis and a large N method. A local quantum critical point is identified in all these approaches, when the bosonic bath has a sub-ohmic spectrum; the results guarantee that a self-consistent solution of the locally critical type is a robust solution to the Kondo lattice model. Quantum critical properties such as thermodynamics are also theoretically investigated for both pictures. A universally diverging Gruneisen ratio is discovered at any quantum critical point, which can be used to characterize different classes of quantum phase transitions. |
