  Single-electron transistors attached to ferromagnetic leads can undergo a
continuous quantum phase transition as their gate voltage is tuned. The
corresponding quantum critical point separates a Fermi liquid phase from a
non-Fermi liquid one. Here, we expound on the physical idea proposed earlier.
The key physics is the critical destruction of the Kondo effect, which
underlies a new class of quantum criticality that has been argued to apply to
heavy fermion metals. Its manifestation in the transport properties is studied
through an effective Bose-Fermi Kondo model; the bosonic bath, corresponding to
the spin waves of the ferromagnetic leads, describes a particular type of
sub-Ohmic dissipation. We also present results for general forms of sub-Ohmic
dissipative bath, and consider in some detail the case with critical paramagons
replacing spin waves. Finally, we discuss some delicate aspects in the
theoretical treatment of the effect of a local magnetic field, particularly in
connection with the frequently employed Non-Crossing Approximation.
