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.KEYWORDS: quantum dot, Anderson model, Kondo effect, perturbation theory, conductance, magnetic fieldTo date, numerous theoretical as well as experimental studies have been carried out on the tunneling transport of an electron through a quantum dot connected to the leads.1, 2) The combination of single electron charging and energy level quantization causes the Coulomb blockade or Coulomb oscillation phenomena, which reflects the particle character of an electron. Furthermore, a dot with an odd number of electrons resembles a magnetic impurity coupled to the conduction electrons in a metal, and hence, the Kondo-type phenomenon, which reflects the wave character of electrons, has been predicted by several researchers 3, 4, 5) and several observations of Kondo-assisted tunneling were reported recently. 6,7,8,9) The Kondo effect in a quantum dot system has brought up new and interesting issues for physics, e.g., the tunable Kondo effect or the nonequilibrium Kondo effect.Several theoretical methods have been devised to explain Kondo-type transport through a quantum dot using the impurity Anderson model: second-order perturbation theory (SOPT), 10,11,12) modified perturbation theory, 13,14,15) slave boson method, 16) noncrossing approximation (NCA), 17,18,19) quantum Monte Carlo (QMC)
20)and numerical renormalization group (NRG). 21) Yeyati et al. 13) proposed an interpolative scheme that reproduces the correct atomic limit in addition to the weak correlation limit, and applied it to transport through a dot. We have reinvestigated their method and modified it to create a more natural scheme by introducing the effective energy level within a dot to fulfill the Friedel sum rule exactly.15) The main features of a dot were successfully calculated by this method. Our scheme, however, was applicable only to zero temperature in the strict sense of the Friedel sum formula.In this letter, we approximately extend our previous study to finite temperatures, and show the temperature * E-mail: takagi@krishna.th.phy.saitama-u.ac.jp * * E-mail: saso@phy.saitama-u.ac.jp and external magnetic field dependence of the linear conductance through a quantum dot in the Kondo regime. We also investigate the spin susceptibility and specific heat of the dot, and compare them with the Bethe Ansatz solution of the Anderson model to confirm the accuracy of our method...
