Theoretical Study On Thermal And Electrical Transport Properties Of Tunneling Nanojunction

The research into heat transfer can help people get a better understanding about it,to control heat wisely,and to use energy efficiently.This becomes more and more urgent with the aggravation of the energy problem.Recent years,it becomes one of the focuses of researchers to study heat transport in low dimensional?LD?materials and structures.The LD materials have many unique thermal transport properties compared to three dimensional?3D?bulk materials.Fourier's law of thermal conduction is no longer valid for 1D and2D materials,and the thermal conductivity depends on the size of the system.Also,heat transfer in LD systems is very sensitive to the influence of interface.With the reduction of the size and dimension of the system,influence of the thermal resistance to thermal transport due to the interface becomes more and more important.In the metal-insulator interface,electron-phonon interaction plays an important role in the thermal transport properties of the system.From the perspective of application,with the miniaturization of modern electronic devices,two problems becomes important:?1?The interfacial resistance?for example,the thermal resistance between gold and silicon?blocks the effective heat flow.This becomes an important restriction for designing smaller devices.?2?When the gap distance in devices approaches as small as nanometer scale,the non-contact thermal radiation increases quickly,and this may cause some problems for the devices to work properly.Motivated by the above mentioned facts,this thesis includes the following three part-s.?1?In the study of interfacial effects in LD systems,most works are focused on the metal-insulator interface.Its transport properties are determined by the electron-phonon in-teraction at the interface.Considering the spin Seebeck effect observed in the experiment,we study the thermal transport in a 1D chain influenced by spin-phonon interaction,which is very important for further understanding about the spin Seebeck effect.Related studies are rare.?2?For the thermal radiation in the extreme near distance in nano-junctions,most works are based on the theory of fluctuational electrodynamics,which can not deal with the combined effects of electron tunneling and radiation.Our quantum many-body theory can overcome this difficulty.?3?We propose that the Fano effect should occur for impurity adsorption on ABC stacking trilayer graphene?r-TLG?.We hope to realize high thermal-electrical transport properties based on this quantum interference effect in our further work.Our work is based on the nonequilibrium Green's function?NEGF?method combined with the Feynman diagram rule,which can deal with these interactions conveniently.The details of our work are as follows.Firstly,we study the influence of spin-phonon interaction on the thermal transport prop-erties of a nano-junction.Our model is based on a Heisenberg spin chain.We use the self-consistent Born approximation to include the effect of spin-phonon interaction.We find that the thermal conductance of magnons is influenced by the interaction in the high temperature range.In the low temperature regime,the external magnetic field can restrain the excitation of the magnons and it reduces its thermal conductance.For thermal transport through the interface of a ferromagnetic-insulator junction,we find thermal rectification and negative differential thermal conductance.This is due to the asymmetric spectrum of magnons and phonons near the interface.These effects can be easily tuned by an external magnetic field.It is useful to design magnetic-field-controlled thermal diode.Also,recent studies show that the spin-phonon interaction plays an important role on the spin Seebeck effect.Especially,a new way of spin pumping is realized by a recent experiment.The spin current is driven by the spin-phonon interaction in the ferromagnetic-insulator interface.Our study is helpful to understand this.Secondly,we study the near-field radiation between two vacuum-gapped metal plates.The energy transfer is mediated by the electron-electron interaction.In a very recent ex-periment,measurements for the heat transfer are made within a gap distance of 2 nm.In this case,heat transfer is contributed by electron tunneling as well as electrical-magnetic field radiation.The conventional theory of fluctuational electrodynamics can not handle this situation.Our theory is appropriate for this.We consider the contributions of Coulom-b fluctuation and electron tunneling to the thermal transport.The metal is modeled by a tight-binding cubic lattice.The electron-electron interaction is dealt with by the G₀W₀approximation.Our result is consistent with the conventional theory of fluctuational elec-trodynamics without including electron tunneling.Including electron tunneling,we discuss the competition of coulomb fluctuation and electron tunneling to heat transfer.Finally,we make a systematic study on the properties of magnetic adatom on trilayer graphene.The adsorbtion of magnetic impurity on graphene has received intense studies these years.This has many applications in the design of spin devices based on graphene.The trilayer graphene has different stacking orders and energy band structures.This implies that the adatom adsorbtion on trilayer graphene has many different properties and applications.The trilayer graphene is described by a tight-binding model and the adatom is described by an Anderson impurity model.The electron-spin interaction is dealt with using a self-consistent mean-field theory.Different trilayer graphenes of ABC stacking order?r-TLG?and ABA satcking order?b-TLG?combined with different adsorbtion sites are considered.We find a Fano-shaped density of states?DOS?for the impurity adsorbed on r-TLG.This is due to the intrinsic band strucutres near the Fermi level of the r-TLG.For adatom on b-TLG,the DOS is a simple broadening one with the Lorentzian shape,which is similar to the case of single layer.Also,we discuss the local momentum formation of adatom on TLG under an external electric field.For r-TLG adsorbtion,we find the external field can make the adatom into an in-gap state,which favors its magnetic momentum formation.For b-TLG adsorbtion,the external field induces a band overlap near the Fermi level,which doesn't favor the magnetic momentum formation.The external field can switch on/off the momentum formation of the adatom.This is useful for designing novel spin devices on TLG.Recent experiment has observed the spin-resolved DOS of a hydrogen adatom on graphene.The interesting Fano-shaped DOS we found here is expected to be confirmed by the related experimental technique.