Order-Nelectron transport calculations from ballistic to diffusive regimes by a time-dependent wave-packet diffusion method: Application to transport properties of carbon nanotubes

Abstract

We present an order-N ͓O͑N͔͒ calculation method for the quantum electron transport of huge systems up to 80 million atoms. Based on the linear-response Kubo-Greenwood formula, we calculate the conductance through time-dependent diffusion coefficients using the time-dependent wave-packet diffusion approach, which treats the electron wave-packet motion with an O͑N͒ and very high-speed calculation. Combining with molecular-dynamics simulations, we can study the temperature dependence of electron transport properties of materials from atomistic viewpoints from ballistic to diffusive regimes. We apply the present calculation method to transport of the carbon nanotubes ͑CNTs͒ with various lengths at various temperatures. In metallic CNTs, the mean-free paths are in good agreements with recent experiments, which reach about 500 nm at room temperature and increase up to several micrometers at low temperature. We find that the resistance increases almost linearly with temperature and takes larger values than expected in the quasiballistic regime. In semiconducting CNTs, the mobilities are affected strongly by the contacts with metallic electrodes through Schottky barriers. The mobilities are maximally 30 000 cm 2 / V s and cut-off frequencies of 300 GHz at room temperature. These calculated results provide useful information to the design of CNT field-effect-transistor devices.

Conductivity of epitaxial and CVD graphene with correlated line defects

“…5,[22][23][24][25][26][27] This is a computationally efficient method scaling with the number of atoms in the system N , and thus allowing treating very large graphene sheets containing many millions of C atoms.…”

“…We do this numerically, utilizing the quantum mechanical time-dependent real-space Kubo method 5,19,[22][23][24][25][26][27] allowing us to study graphene sheets approaching the realistic dimensions of millions of atoms.…”

Influence of correlated impurities on conductivity of graphene sheets: Time-dependent real-space Kubo approach

“…The transport properties of graphene sheets are calculated on the basis of the time-dependent real-space Kubo formalism where the dc conductivity σ is extracted from the wavepacket temporal dynamics governed by the time-dependent Schrödinger equation. [36][37][38][39][40] This is a computationally efficient method scaling with a number of atoms in the system N , and thus allowing treating very large graphene sheets containing many millions of C atoms. 8,[22][23][24]40 The calculation of the dc conductivity starts from the KuboGreenwood formula…”

“…Among the most popular numerical methods reported in the literature are the recursive Green's function technique [33][34][35] and the time-dependent real-space Kubo method. 8,9,[22][23][24][36][37][38][39][40] The latter method is especially suited to treat large graphene systems containing tens of millions of atoms with dimensions approaching realistic systems.…”

“…In this study, we utilize the timedependent real-space quantum Kubo method, 8,9,[22][23][24][36][37][38][39][40] allowing us to study large graphene sheets containing millions of atoms. We consider models of disorder appropriate for realistic impurities that might exhibit correlations, including the Gaussian potential describing screened charged impurities and the short-range potential describing neutral adatoms.…”