Transport properties of molecular junctions and equivalent devices

• Grant number: 18/21501-1
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• Start date: April 01, 2019
• End date: September 26, 2019
• Field of knowledge: Physical Sciences and Mathematics - Physics - Condensed Matter Physics
• Principal Investigator: Luiz Nunes de Oliveira
• Grantee: Diêgo Guedes Sobrinho
• Host Institution: Instituto de Física de São Carlos (IFSC). Universidade de São Paulo (USP). São Carlos , SP, Brazil

Abstract

A molecular junction is a device comprising a molecule that bridges two electrodes to which it is electrically coupled. From the scientific viewpoint, the molecular junction is equivalent to the single-electron transistor (SET), in which a quantum dot bridges two otherwise independent two-dimensional electron gases. A gate potential controls the electronic occupation of the quantum dot. Experimentally, a potential difference between the electron gases is applied, and the resulting current is measured as a function of temperature and of the gate potential. If the number of dot electrons is odd, the antiferromagnetic coupling between the dot magnetic moment and the gas spins gives rise to the Kondo effect. Here, we focus the SET and accurate experimental measurements of its electrical conductance as a function of temperature and applied gate potential reported in the literature. The goal is threefold: (i) develop a procedure to calculate the thermal dependence of the conductance through a SET at given gate potential, (ii) apply the procedure to a realistic simulation of an experimental device, and (iii) compare the theoretical results with experimental data, quantitatively. The proposed procedure combines density-functional theory (DFT) with renormalization-group concepts to overcome obstacles that previous studies of the problem have encountered. Instead of applying the DFT machinery to the ground state of the Hamiltonian describing the device, we will apply DFT to the fixed-point Hamiltonian that approximately describes the system at high temperatures and the numerical renormalization-group (NRG) method to describe the crossover betwen the high- and the low-temperature regimes. The NRG method determines the thermal dependence of the conductance, which can then be compared with accurate measurements published a decade ago that have, so far, defied theorists. Our work will initially focus the linear conductance. Towards the end of the two-year period, we will rely on time-dependent density function (TD-DFT) to slowly create a finite potential difference between the two gases and compute the resulting electrical current.