Quantum criticality of the two-channel pseudogap Anderson model: universal scaling in linear and non-linear conductance

The quantum criticality of the two-lead two-channel pseudogap Anderson impurity model is studied. Based on the non-crossing approximation (NCA) and numerical renormalization group (NRG) approaches, we calculate both the linear and nonlinear conductance of the model at finite temperatures with a voltage bias and a power-law vanishing conduction electron density of states, rho(c)(omega) alpha | omega-mu(F)|(r) (0 < r < 1) near the Fermi energy mu(F). At a fixed leadimpurity hybridization, a quantum phase transition from the two-channel Kondo (2CK) to the local moment (LM) phase is observed with increasing r from r = 0 to r = r(c) < 1. Surprisingly, in the 2CK phase, different power-law scalings from the well-known root T or root V form is found. Moreover, novel power-law scalings in conductances at the 2CK-LM quantum critical point are identified. Clear distinctions are found on the critical exponents between linear and non-linear conductance at criticality. The implications of these two distinct quantum critical properties for the non-equilibrium quantum criticality in general are discussed.