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| Author(s): |
Vivaldo Leiria Campo Júnior
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Carlos. |
| Institution: | Universidade de São Paulo (USP). Instituto de Física de São Carlos (IFSC/BT) |
| Defense date: | 1999-04-30 |
| Examining board members: |
Luiz Nunes de Oliveira;
Lidério Citrângulo Ioriatti Junior;
Nelson Studart Filho
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| Advisor: | Luiz Nunes de Oliveira |
| Abstract | |
In this work a renormalization-group calculation of the low-temperature AC conductance in the linear response regime through a nanostructure coupled to metallic leads is presented. This system shows a competition between two effects: the Coulomb blockade and the Kon¬do effect. Our model considers one-dimensional leads which are connected to form a ring, in which a current is induced by a magnetic flux oscillating at the frequency . We start from a nearest-neighbor tight-binding model for the leads and in this way the potential vector is easily incorporated in the model Hamiltonian by twisting boundary conditions. A potential barrier between the leads and the nanostructure is simulated in terms of a tunneling rate between the nanostructure and the adjacent sites in the leads, which is smaller than the one between neighbors sites in the leads. The capacity of the nanostructure is small, which implies that substantial energy changes are associated with each electron transfered to the nanostructure. As a consequence, the model Hamiltonian maps onto the spin-degenerate Anderson Hamiltoni¬an with correlation U between the electrons. A gate voltage Vg controls the impurity (i.e., nanostructure) energy 0. Plotted as a function of , the conductivity shows two Coulomb-blockade peaks, at the energy needed to add an electron to and to remove an electron from the nanostructure, respectively. In the Kondo regime 0 > 0 > -U (i.e., for gate voltages such that the isolated nanostructure would have a spin-degeneration ground state), an addition (Kondo) peak appears near = 0. Plotted as functions of Vg, the static conductivity shows a broad peak in the Kondo regime and drops rapidly to zero for voltages resulting in a non-degenerate nanostructure ground state. A relation between the conductance and the spectral density of the impurity is obtained and used to interpret the numerical results. (AU) | |