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postgraduate thesis: Firstprinciples study on transient dynamics of nanodevices
Title  Firstprinciples study on transient dynamics of nanodevices 

Authors  
Issue Date  2015 
Publisher  The University of Hong Kong (Pokfulam, Hong Kong) 
Citation  Yu, Z. [俞之舟]. (2015). Firstprinciples study on transient dynamics of nanodevices. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b5576783 
Abstract  Transient dynamics of nanoscale devices including transient electric current, transient heat current, and transient currentinduced spin transfer torque (STT) have been investigated using the nonequilibrium Green’s function (NEGF) and the complex absorbing potential (CAP) based on the framework of the density functional theory (DFT). For the transient heat current, an exact solution is first obtained using the NEGF approach beyond the wideband limit under the steplike pulse of external bias and gate voltage as well as the dc bias. A timedependent NEGFDFT formalism to study the transient heat current under a steplike pulse of gate voltage is then proposed. In order to speed up the calculation, an algorithm using the CAP is then developed. After replacing the Hamiltonian of leads by the CAP, the effective selfenergy of the Green’s function becomes energy independent. Therefore, the triple energy integration in the exact solution of transient heat current can be reduced to a single energy integral by using the theorem of residue which dramatically reduces the computational complexity. As an example, the NEGFDFTCAP formalism is applied to calculate the transient heat current under an upward gate voltage pulse for the Dithiol benzene molecule connected by two semiinfinite aluminum leads. The enhancement of heat current under the transient gate voltage is observed. The transient electric current and STT are studied for the magnetic layered system under the NEGFDFTCAP framework. Although the Green’s function can be cast into the wideband form within the CAP method, the computational cost of the transient STT is still huge due to the dense mesh of ksampling for the layered system. In order to further increase the computational speed, the [N − 1/N] Padé approximation is introduced to replace the Fermi distribution function. After employing the Padé spectrum decomposition, the energy integrals in the formalism of transient electric current and STT, including that of the Fermi distribution function, can be analytically calculated by the theorem of residue. As an application, the NEGFDFTCAP formalism with the Padé approximation is implemented to study the transient electric current and currentinduced STT of Co/Cu/Co trilayers under an upward pulse of bias with different rotating angles of magnetization direction between two leads. The oscillation behavior is obtained for the transient STT when it approaches the steady state. 
Degree  Doctor of Philosophy 
Subject  Nanoelectronics 
Dept/Program  Physics 
Persistent Identifier  http://hdl.handle.net/10722/221084 
HKU Library Item ID  b5576783 
DC Field  Value  Language 

dc.contributor.author  Yu, Zhizhou   
dc.contributor.author  俞之舟   
dc.date.accessioned  20151026T23:11:56Z   
dc.date.available  20151026T23:11:56Z   
dc.date.issued  2015   
dc.identifier.citation  Yu, Z. [俞之舟]. (2015). Firstprinciples study on transient dynamics of nanodevices. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b5576783   
dc.identifier.uri  http://hdl.handle.net/10722/221084   
dc.description.abstract  Transient dynamics of nanoscale devices including transient electric current, transient heat current, and transient currentinduced spin transfer torque (STT) have been investigated using the nonequilibrium Green’s function (NEGF) and the complex absorbing potential (CAP) based on the framework of the density functional theory (DFT). For the transient heat current, an exact solution is first obtained using the NEGF approach beyond the wideband limit under the steplike pulse of external bias and gate voltage as well as the dc bias. A timedependent NEGFDFT formalism to study the transient heat current under a steplike pulse of gate voltage is then proposed. In order to speed up the calculation, an algorithm using the CAP is then developed. After replacing the Hamiltonian of leads by the CAP, the effective selfenergy of the Green’s function becomes energy independent. Therefore, the triple energy integration in the exact solution of transient heat current can be reduced to a single energy integral by using the theorem of residue which dramatically reduces the computational complexity. As an example, the NEGFDFTCAP formalism is applied to calculate the transient heat current under an upward gate voltage pulse for the Dithiol benzene molecule connected by two semiinfinite aluminum leads. The enhancement of heat current under the transient gate voltage is observed. The transient electric current and STT are studied for the magnetic layered system under the NEGFDFTCAP framework. Although the Green’s function can be cast into the wideband form within the CAP method, the computational cost of the transient STT is still huge due to the dense mesh of ksampling for the layered system. In order to further increase the computational speed, the [N − 1/N] Padé approximation is introduced to replace the Fermi distribution function. After employing the Padé spectrum decomposition, the energy integrals in the formalism of transient electric current and STT, including that of the Fermi distribution function, can be analytically calculated by the theorem of residue. As an application, the NEGFDFTCAP formalism with the Padé approximation is implemented to study the transient electric current and currentinduced STT of Co/Cu/Co trilayers under an upward pulse of bias with different rotating angles of magnetization direction between two leads. The oscillation behavior is obtained for the transient STT when it approaches the steady state.   
dc.language  eng   
dc.publisher  The University of Hong Kong (Pokfulam, Hong Kong)   
dc.relation.ispartof  HKU Theses Online (HKUTO)   
dc.rights  Creative Commons: Attribution 3.0 Hong Kong License   
dc.rights  The author retains all proprietary rights, (such as patent rights) and the right to use in future works.   
dc.subject.lcsh  Nanoelectronics   
dc.title  Firstprinciples study on transient dynamics of nanodevices   
dc.type  PG_Thesis   
dc.identifier.hkul  b5576783   
dc.description.thesisname  Doctor of Philosophy   
dc.description.thesislevel  Doctoral   
dc.description.thesisdiscipline  Physics   
dc.description.nature  published_or_final_version   