  Quantum critical behavior in heavy electron materials is typically brought
about by changes in pressure or magnetic field. In this communication, we
develop a simple unified model for the combined influence of pressure and
magnetic field on the effectiveness of the hybridization that plays a central
role in the two-fluid description of heavy electron emergence. We show that it
leads to quantum critical and delocalization lines that accord well with those
measured for CeCoIn5, yields a quantitative explanation of the field and
pressure induced changes in antiferromagnetic ordering and quantum critical
behavior measured for YbRh2Si2, and provides a valuable framework for
describing the role of magnetic fields in bringing about quantum critical
behavior in other heavy electron materials.
