We discuss quantum phase transitions in the pseudogap Kondo problem, which describes a magnetic moment coupled to conduction electrons with a power-law density of states, rho(omega) ~ |omega|^r. We show that different perturbative expansions, together with renormalization group techniques, provide effective low-energy field theories for the relevant critical fixed points. In particular, we review expansions near the lower-critical and upper-critical dimensions of the problem, being r=0 and r=1, respectively.
Categories: cond-mat.str-el cond-mat.stat-mech