Dynamical Processes And Thermoelectrics In Low Dimensional Transport

Because of the low dimensionality,low dimensional quantum systems usually have properties absent in bulk materials.Typical low dimensional systems such as quantum dots and conducting molecular wires are ingredients in molecular devices.In this thesis,we study transport properties of two low dimensional systems: the double-strand DNA and a junction made of metal,a quantum dot and magnetic insulator.More specifically,we study mutual influence between the molecular dynamics and charge transport in DNA and effects of quantum interface on its thermoelectric transport.For the quantum dot junction,we study its thermospin transport and propose methods to control the thermospin current.These studies are beneficial to design of DNA-based and thermoelectric devices.When a system is away from the equilibrium state,the non-equilibrium Green's functions(NEGF)are useful to calculate time-dependant quantities that reflect properties of the system.Without time-dependent driving,the NEGF reduce to the equilibrium Green's functions,so the NEGF can be considered as the general form of other Green's functions.Thanks to tools like Dyson equation,Wick theorem and other numerical algorithms based on the Green's function method,the NEGF method applies to many problems that are difficult to address by other methods.In the introduction chapter,we present at first an overview of the background and development in the relevant research.With introduction to fundamentals of the NEGF method,we explain how to use it to deal with dynamical effect of the charge transport.We also introduce the slave boson method and the non-crossing approximation used to address strongly correlated systems.We end the chapter with motion of the studies and notes on the arrangement of this thesis.The stack of the ? electrons along the chain makes DNA molecules possible conductor of charges.Since the ? and hydrogen bonds between the bases are relatively weak interations,the structure and configuration of DNA molecules are susceptible to external forces.In Chapter 2,we study effects of the current-induced forces on the dynamics of DNA and its conductivity.Our results suggest that the current can effectively break the base pairs and,in return,the broken base pairs suppress the charge transport through electron-phonon coupling and result in the negative differential resistance effect(NDRE).In the next chapter,we propose a scheme to test the current-induced forces as a mechanism of NDRE in DNA molecules.Different environmental conditions can lead to different dynamics,and conducting property of the DNA molecule varies accordingly.Inter-influence between dynamics and charge transport presents a view point to understand varying conductance of DNA in experimental measurements.Chapter 4 and 5 are devoted to study of transport induced by temperature gradient.In view of the double helix structure with a ? stack channel,in Chapter 4 we study impact of quantum interface on the thermoelectric transport in DNA.Our calculations suggest that the Fano and Dicke resonance can be used to increase the figure of merit.This improvement is significant when the onsite energy of and/or the hopping between the electrons are properly adjusted,which can be realized by means like environmental disorder.In Chapter 5,we study another transport phenomenon in low dimensional systems – spin Seebeck effect in a metal-quantum dot-magnetic insulator junction.Using the slave boson method and the non-crossing approximation,we calculate the Seebeck spin current.In this junction,the spin current can be tuned by a varying gate voltage without changing the temperature gradient.We also find that the coulomb interaction can significantly enhance the spin transport,and an optimal strength for maximizing the tunability of the spin current is presented.In the last chapter,we summarise our findings and major contributions,and also discuss some related topics for further study.