Quantum anomalous Hall effect (QAHE) has been experimentally realized in magnetic topological insulator (MTI) thin films fabricated on magnetically doped ( ) Bi, Sb Te 2 3 . In an MTI thin film with the magnetic easy axis along the normal direction (z-direction), orientations of magnetic dopants are randomly distributed around the magnetic easy axis, acting as magnetic disorders. With the aid of the non-equilibrium Greenʼs function and Landauer-Büttiker formalism, we numerically study the influence of magnetic disorders on QAHE in an MTI thin film modeled by a three-dimensional tightbinding Hamiltonian. It is found that, due to the existence of gapless side surface states, QAHE is protected even in the presence of magnetic disorders as long as the z-component of magnetic moment of all magnetic dopants are positive. More importantly, such magnetic disorders also suppress the dissipation of the chiral edge states and enhance the quality of QAHE in MTI films. In addition, the effect of magnetic disorders depends very much on the film thickness, and the optimal influence is achieved at certain thickness. These findings are new features for QAHE in three-dimensional systems, not present in two-dimensional systems. 2 3 systems [ [13][14][15][16]. Different from the conventional QAHE discussed in 2D cases [5,[7][8][9][10][11], the quantized Hall conductance in 3D MTI films is jointly contributed by the top and bottom massive Dirac-like surface states which have opposite signs in their effective masses [12,17]. Furthermore, the gapless side surfaces are still crucial for QAHE in 3D MTI films. Since the top and bottom surfaces are gapped, the chiral edge modes actually propagate through the gapless side surface states. Besides the side surface states, a constant exchange field M is essential for QAHE in MTI films, responsible for breaking the time-reversal symmetry and consequently opening the nontrivial energy surface gap.

Transient current calculation is essential to study the response time and capture the peak transient current for preventing melt down of nano-chips in nanoelectronics. Its calculation is known to be extremely time consuming with the best scaling T N 3 where N is the dimension of the device and T is the number of time steps. The dynamical response of the system is usually probed by sending a step-like pulse and monitoring its transient behavior. Here, we provide a fast algorithm to study the transient behavior due to the step-like pulse. This algorithm consists of two parts: The algorithm I reduces the computational complexity to T 0 N 3 for large systems as long as T < N ; The algorithm II employs the fast multipole technique and achieves scaling T 0 N 3 whenever T < N 2 beyond which it becomes T log 2 N for even longer time. Hence it is of order O(1) if T < N 2 . Benchmark calculation has been done on graphene nanoribbons with N = 10 4 and T = 10 8 . This new algorithm allows us to tackle many large scale transient problems including magnetic tunneling junctions and ferroelectric tunneling junctions that cannot be touched before.PACS numbers: 71.15.Mb

The control and generation of spin-polarized currents (SPCs) without magnetic materials and external magnetic field is a big challenge in spintronics and normally requires spin-flip mechanism. In this work, we propose a novel method to control and generate SPCs in stanene nanoribbons in the quantum spin Hall (QSH) insulator regime by all electrical means without spin-flip mechanism. This is achieved with intrinsic spin-orbit coupling in stanene nanoribbons by tuning the relative phase of spin up and down electrons using a gate voltage, which creates a time delay between them thereby producing alternative SPCs driven by ac voltage. The control and generation of SPCs are demonstrated numerically for ac transport in both transient and ac regime. Our results are robust against edge imperfections and generally valid for other QSH insulators such as silicene and germanene, etc. These findings establish a novel route for generating SPCs by purely electrical means and open the door for new applications of semiconductor spintronics.

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We study light absorption in ZnO nanorod arrays sensitized with CdSe quantum dots as one of the factors affecting solar cell performance in need of improvement given their current performance well below expectations. Light trapping in nanorod arrays (NRAs) as it relates to array density and length as well as quantum dot (QD) loading is studied using the Finite Difference Time Domain model. It is shown that light absorption in such solar cell architecture is a sensitive function of the morphological dimensions and that a higher NRA density does not necessarily correspond to large absorption in the solar cell. Instead, light trapping efficiency depends significantly on the array density, QD axial distribution and refractive index contrast between NR and QDs thus suggesting strategies for improved quantum dot solar cell (QDSC) fabrication. In addition, we present experimental data showing dramatic improvement in photo conversion efficiency performance for relatively short ZnO NRAs (~1 μm) of low NRA density, but whose efficiency improvement can not be solely explained based on our current light trapping estimates from the numerical simulations.