We are interested in developing first principles methods for modeling quantum systems under various conditions that are difficult for existing computational approaches. We are currently working on the following problems.
Quantum Junctions under an External DC Bias: Steady-State Density Functional Theory
When a nanoscale junction is connected with an external DC bias, the system is in a nonequilibrium steady state. In principle, the ground-state density functional theory (GS-DFT) is not applicable. We have proposed a steady-state DFT (SS-DFT) 1 to describe properties of such nonequilibrium quantum systems. We have shown that the steady-state properties of the system can be uniquely determined by two densities, the total electron density and the density of current-carrying electrons. Based on two densities, two mean-field equations, one for equilibrium electrons and another one for current-carrying electrons, can be derived. Such a dual mean field (DMF) approach based on SS-DFT has been implemented into the commonly used computational package SIESTA and applied to various kinds of nanoscale junctions. We show below a case study for a CNT-Benzene-CNT junction under a bias voltage of 0.5 V. In this system, due to the large separation of benzene and CNT, the molecule in middle is in local equilibrium. Calculations using different methods are compared with each other.
Efficient methods for 2D materials: Currently, most of the first principles calculations using planewave expansion of electron wave function on 2D materials are done using a 3D box, with a vacuum layer. This is not efficient. We are interested in developing methods that are more suitable to 2D systems. These include methods for calculating electronic structure, transport properties, etc.
 S. Liu, A. Nurbawono and C. Zhang, Sci. Rep. 5, 15386 (2015)
 S. L. Liu, A. Nurbawono, and C. Zhang, Density Functional Theory for Steady-State Nonequilibrium Molecular Junctions, Sci. Rep. 5, (2015).
 A. Nurbawono, S. L. Liu, and C. Zhang, Modeling Optical Properties of Silicon Clusters by First Principles: From A Few Atoms to Large Nanocrystals, J. Chem. Phys 142, 15 (2015).
 S. L. Liu, Y. P. Feng, and C. Zhang, Communication: Electronic and Transport Properties of Molecular Junctions under A Finite Bias: A Dual Mean Field Approach, J. Chem. Phys 139, 19 (2013).
 C. Zhang, Uniform Electron Gas under An External Bias: The Generalized Thomas-Fermi-Dirac Model and the Dual-mean-field Theory, arXiv preprint arXiv:1311.5680 (2013).
 A. Nurbawono, Y. P. Feng, and C. Zhang, Electron Tunneling through A Hybrid Superconducting-normal Mesoscopic Junction under Microwave Radiation, Phys. Rev. B 82, 1 (2010).
 A. H. Zhang, Z. X. Dai, L. Shi, Y. P. Feng, and C. Zhang, Energy-gap Opening and Quenching in Graphene under Periodic External Potentials, J. Chem. Phys 133, 22 (2010).