First-principles study on transient dynamics of nanodevices

Abstract

Transient dynamics of nanoscale devices including transient electric current, transient heat current, and transient current-induced 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 wide-band limit under the step-like pulse of external bias and gate voltage as well as the dc bias. A time-dependent NEGF-DFT formalism to study the transient heat current under a step-like 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 self-energy 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 NEGF-DFT-CAP formalism is applied to calculate the transient heat current under an upward gate voltage pulse for the Di-thiol benzene molecule connected by two semi-infinite 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 NEGF-DFT-CAP framework. Although the Green’s function can be cast into the wide-band form within the CAP method, the computational cost of the transient STT is still huge due to the dense mesh of k-sampling 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 NEGFDFT-CAP formalism with the Padé approximation is implemented to study the transient electric current and current-induced 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.