Modified perturbation theory applied to Kondo-type transport through a quantum dot under a magnetic field

Linear conductance through a quantum dot is calculated under a finite magnetic field using the modified perturbation theory. The method is based on the second-order perturbation theory with respect to the Coulomb repulsion, but the self-energy is modified to reproduce the correct atomic limit and to fulfill the Friedel sum rule exactly. Although this method is applicable only to zero temperature in a strict sense, it is approximately extended to finite temperatures. it is found that the conductance near electron-hole symmetry is suppressed by the application of the magnetic field at low temperatures. Positive magnetoconductance is observed in the case of large electron-hole asymmetry.