Investigation On DC Electronic Transport In Hybrid Multiterminal Quantum Dot Systems

With the remarkable advances in nanofabrication and the emergence of new materials,it is possible to create mesoscopic electronic systems formed by various quantum dot bridging several different electrodes,which we refer to as hybrid multiterminal quantum dot systems.Due to the different electrodes,the Coulomb interaction,and the electron interference in such hybrid systems,a few elementary transport processes coexist and the underlying interplay or competition between them would lead to abundant novel transport phenomena.Therefore,it is helpful to study the DC electronic transport in hybrid multiterminal quantum dot systems for both the development of functional nano electronic devices and the further comprehension of fundamental condensed matter physics.This thesis contains theoretical investigations of DC electronic transport in three hybrid multiterminal quantum dot systems.The main tool we employed is the nonequilibrium Green's function theory.The investigations on the first two systems are devoted to the development of realizing useful quantum electronic devices,and the third is to the prediction of a novel strongly correlated transport phenomenon.The first system we studied is a Cooper pair splitter consisting of two normal leads coupled to a BCS superconducting lead via double noninteracting quantum dots,which has been realized recently with carbon nanotube.This device was proposed to split the entangled constituent electrons of a Cooper pair in the superconductor into the two normal leads.The resultant nonlocal entangled electron pairs are useful in a few subfields such as quantum information processing.We find that a unitary Cooper pair splitting efficiency can be readily achieved by utilizing temperature difference,rather than bias voltage,between the two normal leads and tuning the two dot levels.This prediction is significantly better than the efficiencies obtained by electric manner in experiments so far.In addition,we have discussed how to obtain high-quality nonlocal electron pairs preserving entanglement.The second system is proposed by us,which consists of a suspended carbon nanotube quantum dot coupled to a metallic lead.The intrinsic spin-orbit coupling and the vibrational modes in the carbon nanotube quantum dot are two relevant elements in this system.Provided that the vibrational modes is coupled to a thermal phonon bath held at a fixed temperature,the quantum dot is coupled to a phononic lead besides the metallic one.When this system is exposed to an external longitudinal magnetic field,it is predicted to be a natural thermoelectric unipolar spin battery that can generate pure spin current.On one hand,the builtin spin flip mechanism in the spin battery is a consequence of the spin-vibration interaction resulting from the interplay between the spin-orbit coupling and the vibrational modes.On the other hand,utilizing thermoelectric effect,the temperature difference between the metallic lead and the phononic lead can provide the driving force needed in our spin battery.Our prediction provides a proposal for obtaining controllable pure spin current in spintronics.The third system consists of a vibrating quantum dot embedded between a normal and a BSC superconducting lead.Transport measurements regarding the conventional phonon-assisted inelastic Andreev tunneling in this system have been reported recently.We theoretically investigate the transport in Kondo regime of this system.With a bias applied to the normal lead,we predict a series of Kondo sidebands separated by half a phonon energy in the differential conductance,which are distinct from the sidebands separated by one phonon energy observed outside the Kondo regime.These novel Kondo sidebands are attributed to the KondoAndreev cooperative cotunneling mediated by phonons,which is a emergent manybody transport process due to the interplay of the Kondo effect,the Andreev tunneling,and the mechanical vibrations.