The Studies On Chiral Spin Caloritronics And Its Relizations In DNA-Like Helical Molecular Systems | Posted on:2022-07-30 | Degree:Doctor | Type:Dissertation | Country:China | Candidate:G F Du | Full Text:PDF | GTID:1520306815496004 | Subject:Condensed matter physics | Abstract/Summary: | PDF Full Text Request | As a new research field in spintronics,spin caloritronics has been a hot topic of condensed matter physics in recent years.It deals with the interactions of heat,electrons’ spin and charge currents induced by temperature gradient in magnetic materials or devices.The main research content of this thesis includes how to realize spin Seebeck effect,spin Peltier effect and high spin figures of merit in low-dimensional quantum systems.Our work especially focuses on exploring new mechanisms to generate spin Seebeck effect and pure thermal spin currents based on new materials,such as chiral molecular structures and meanwhile,exploring new spin thermoelectric materials with high thermoelectric conversion efficiency.In our theoretical models and methods,we established the related theoretical models based on tight-binding approximation according to the specific characteristics of material structures and symmetries,and used the equilibrium and non-equilibrium Green’s function theory to study their spin-dependent transport properties and spin-dependent thermoelectric coefficients.The main research contents are listed in the followings:Firstly,both single-helical proteins and double-strand-DNA(ds DNA)molecules have been confirmed experimentally to exhibit high spin polarizations even when they are coupled with nonmagnetic metal leads.However,the geometrical structures in these two classes of helical molecules are much different,such that the long-rang hopping should be considered in the former,resulting in much different thermospin transports in them.Therefore,the spin-dependent thermoelectrics in single helix proteins are more complicated than these in ds DNA molecules.Our theoretical work uncovers that the spin figure of merit in single helical proteins may be much larger than the corresponding charge figure of merit.Moreover,the thermospin transport in single-helical proteins can be manipulated effectively both by the dephasing and by the distance between two neighboring sites in molecules.Consequently,a perfect spin-filter effect(SFE),a well-defined spin-Seebeck effect and even a pure thermal spin current occur.Our theoretical results not only demonstrate that the single-helical proteins are excellent thermoelectric materials with high spin figure of merit,and the devices constructed by us can be designed as a pure spin current generator through controlling their internal parameters.Secondly,double-stranded DNA(ds DNA)molecules have been considered as promising candidates to study chiral spintronics,since the chiral-induced spin selectivity(CISS)in ds DNA was observed in experiments.To develop chiral-based spintronic devices,one of the crucial conditions is to enhance CISS effect or to achieve high spin-polarized transport in helical molecules.Some effective ways such as applying an external gate voltage and using magnetic helix have been put forwards in theory.Considering that the ds DNA molecules are usually flexible in mechanical properties,lattice vibration may be one of important factors to influence the CISS effect.We uncover that the electron-vibration interaction(EVI)not only enhances the CISS effect and the spin polarization in ds DNA,but also induces a series of new spin-splitting transmission modes.In addition,the vibration-induced spin-dependent transmissions are rather robust against the dephasing process,enhancing the CISS effect and spin-polarized transport.Thirdly,the spin-Seebeck effect(SSE)is an effective route to realize pure spin current by using spin-polarized electrons or spin-wave transport in magnetic materials.The above two works mainly focus on how to exhibit spin-polarized transport and spin Seebeck effect in chiral molecular systems.Chiral structures and spin-orbit coupling(SOC)are considered to be two key factors for this phenomenon.Therefore,if we hope to realize the spin Seebeck effect in non-magnetic materials,we should make clear the relationship between the structural chirality and the SOC.To solve this problem,We construct an example of related materials on achiral nanotubes.Our theoretical results reveal that as a temperature gradient is applied along the nanotubes,thermal spin-up and spin-down currents with opposite flowing directions are produced without any accompanying charge current;strain engineering in the radical direction of achiral nanotubes is an effective way to improve and control the SSE.With suitable SOC parameters,the thermally induced spin-up and spin-down currents display multiple oscillation features,indicating that the thermal spin currents can change their flowing directions many times.The nonmagnetic achiral nanotubes with SOC have potential device applications in thermal-spin multi-switcher.Meanwhile,two separated research fields,i.e.,spin orbitronics and spin caloritronics,may be converged together to form a new research subject: spin-orbito-caloritronics.Finally,the SOC induced by structural chirality is an important factor to exhibit spin polarization in organic molecules.However,it is known that the experimentally measured SOC is not larger than a few millielectronvolts in related organic molecular systems,which makes us consider further whether there are other factors to induce the spin polarization in this chiral system.In our work,we adopt the same transmission model as before but remove the SOC from the chiral molecules,and then add a linear part of an atomic SOC between the chiral molecule and both leads,to simulate the fact that chiral molecules commonly contact with the noble metal substrate.We find that the orbital current induced by temperature gradients strongly depends on the degree of freedom of the orbit and thus,we propose a new physical concept: i.e.,the orbit-Seebeck effect(OSE).We also study further the relationship between spin polarization and orbital polarization,and find that by adjusting the strength of SOC between the central chiral molecule and the metal electrodes,the spin polarization and the orbital polarization can be mutually converted.Then,we generate and regulate spin(orbit)filtering effect and spin(orbit)Seebeck effect by adjusting the intrinsic parameters of chiral molecules.This research work puts forwards a new interdisciplinary research field:i.e.,orbit caloritronics.In summary,in this thesis,we mainly study the phenomena of thermal spin transport in chiral organic molecules.There is a great difference in thermal spin transport between single helix protein molecules and ds DNA molecules.Thermoelectric devices relying on single helix protein molecules can be used to realize the mutual conversion between the spin-filtering effect and the spin Seebeck effect,and the thermal spin performance is better than the charge thermoelectric one in the device.Regarding the development of thermal spintronic devices,the key is how to improve the high-spin polarization transport.We propose that the EVI can enhance the CISS effect and the spin-polarized transport.Our theoretical results show that the chiral structure is not a necessary condition for organic molecules to produce spin Seebeck effect.Only the SOC in chiral molecules can also produce spin-polarized transport in the device,resulting in the spin Seebeck effect.Thermoelectric devices composed of chiral molecules produce both thermal spin-polarized transport and thermal orbit polarized transport.Thus,we propose a new physical concept,i.e.,the orbit Seebeck effect.Our theoretical results will develop the research fields of thermal spintronics and thermal orbital electronics. | Keywords/Search Tags: | Spin caloritronics, Spin Seebeck effect, Non-equilibrium Green’s function, Chiral-induced spin selectivity, Spin figure of merit, Spin polarization, Spin polarization effect, Spin-orbit coupling, Orbit polarization effect, Orbit Seebeck effect | PDF Full Text Request | Related items |
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