  Quantum phase transitions in a system of N bosons with angular momentum L=0,2
(s,d) and a single fermion with angular momentum j are investigated both
classically and quantum mechanically. It is shown that the presence of the odd
fermion strongly influences the location and nature of the phase transition,
especially the critical value of the control parameter at which the phase
transition occurs. Experimental evidence for the U(5)-SU(3) (spherical to
axially-deformed) transition in odd-even nuclei is presented.
