Xiang-Wei Jiang

PDECO: Parallel differential evolution for clusters optimization

The optimization of the atomic and molecular clusters with a large number of atoms is a very challenging topic. This article proposes a parallel differential evolution (DE) optimization scheme for large‐scale clusters. It combines a modified DE algorithm with improved genetic operators and a parallel strategy with a migration operator to address the problems of numerous local optima and large computational demanding. Results of Lennard–Jones (LJ) clusters and Gupta‐potential Co clusters show the performance of the algorithm surpasses those in previous researches in terms of successful rate, convergent speed, and global searching ability. The overall performance for large or challenging LJ clusters is enhanced significantly. The average number of local minimizations per hit of the global minima for Co clusters is only about 3–4% of that in previous methods. Some global optima for Co are also updated. We then apply the algorithm to optimize the Pt clusters with Gupta potential from the size 3 to 130 and analyze their electronic properties by density functional theory calculation. The clusters with 13, 38, 54, 75, 108, and 125 atoms are extremely stable and can be taken as the magic numbers for Pt systems. It is interesting that the more stable structures, especially magic‐number ones, tend to have a larger energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. It is also found that the clusters are gradually close to the metal bulk from the size N > 80 and Pt38 is expected to be more active than Pt75 in catalytic reaction.

A sphere-cut-splice crossover for the evolution of cluster structures

A new crossover operator is proposed to evolve the structures of the atomic clusters. It uses a sphere rather than a plane to cut and splice the parent structures. The child cluster is constructed by the atoms of one parent which lie inside the sphere, and the atoms of the other parent which lie outside the sphere. It can reliably produce reasonable offspring and preserve the good schemata in parent structures, avoiding the drawbacks of the classical plane-cut-splice crossover in the global searching ability and the local optimization speed. Results of Lennard-Jones clusters (30 ≤ N ≤ 500) show that at the same settings the genetic algorithm with the sphere-cut-splice crossover exhibits better performance than the one with the plane-cut-splice crossover. The average number of local minimizations needed to find the global minima and the average number of energy evaluation of each local minimization in the sphere scheme is 0.8075 and 0.8386 of that in the plane scheme, respectively. The mean speed-up ratio for the entire testing clusters reaches 1.8207. Moreover, the sphere scheme is particularly suitable for large clusters and the mean speed-up ratio reaches 2.3520 for the clusters with 110 ≤ N ≤ 500. The comparison with other successful methods in previous studies also demonstrates its good performance. Finally, a further analysis is presented on the statistical features of the cutting sphere and a modified strategy that reduces the probability of using tiny and large spheres exhibits better global search.

Quantum mechanical simulation of electronic transport in nanostructured devices by efficient self-consistent pseudopotential calculation

We present a new empirical pseudopotential (EPM) calculation approach to simulate the million atom nanostructured semiconductor devices under potential bias using periodic boundary conditions. To treat the nonequilibrium condition, instead of directly calculating the scattering states from the source and drain, we calculate the stationary states by the linear combination of bulk band method and then decompose the stationary wave function into source and drain injecting scattering states according to an approximated top of the barrier splitting (TBS) scheme based on physical insight of ballistic and tunneling transports. The decomposed electronic scattering states are then occupied according to the source/drain Fermi-Levels to yield the occupied electron density which is then used to solve the potential, forming a self-consistent loop. The TBS is tested in a one-dimensional effective mass model by comparing with the direct scattering state calculation results. It is also tested in a three-dimensional 22 nm...

Electron energy and angle distribution of GaAs photocathodes

A precise Monte Carlo model is developed to investigate the electron energy and angle distribution of the transmission-mode GaAs (100) photocathode at room temperature. Both distributions are important for high-quality electron sources. The results show that the energy loss (0.1309 eV) and the angle-dependent energy distribution curves fit well with experimental data. It is found that 65.24% of the emission electrons come from Γ valley, 33.62% from L valley, and 1.15% from X valley. The peak of the energy distribution curve is contributed by both Γ and L-valley electrons, while the high-energy part is contributed by Γ-valley electrons rather than L electrons, which is different from previous inference and can be attributed to the narrow energy range of L-valley electrons. However, L-valley electrons have a larger angular spread than Γ-valley electrons and lead to the spread of the emission cone. The further simulation indicates that increasing the hole concentration or the thickness of the first activatio...

Quantum mechanical simulation of nanosized metal-oxide-semiconductor field-effect transistor using empirical pseudopotentials: A comparison for charge density occupation methods

The atomistic pseudopotential quantum mechanical calculations are used to study the transport in million atom nanosized metal-oxide-semiconductor field-effect transistors. In the charge self-consistent calculation, the quantum mechanical eigenstates of closed systems instead of scattering states of open systems are calculated. The question of how to use these eigenstates to simulate a nonequilibrium system, and how to calculate the electric currents, is addressed. Two methods to occupy the electron eigenstates to yield the charge density in a nonequilibrium condition are tested and compared. One is a partition method and another is a quasi-Fermi level method. Two methods are also used to evaluate the current: one uses the ballistic and tunneling current approximation, another uses the drift-diffusion method.

Multiple valley couplings in nanometer Si metal–oxide–semiconductor field-effect transistors

We investigate the couplings between different energy band valleys in a metal-oxide-semiconductor field-effect transistor (MOSFET) device using self-consistent calculations of million-atom Schrodinger-Poisson equations. Atomistic empirical pseudopotentials are used to describe the device Hamiltonian and the underlying bulk band structure. The MOSFET device is under nonequilibrium condition with a source-drain bias up to 2 V and a gate potential close to the threshold potential. We find that all the intervalley couplings are small, with the coupling constants less than 3 meV. As a result, the system eigenstates derived from different bulk valleys can be calculated separately. This will significantly reduce the simulation time because the diagonalization of the Hamiltonian matrix scales as the third power of the total number of basis functions

Quantum confinement modulation on the performance of nanometer thin body GaSb/InAs tunnel field-effect transistors

In this paper, we have presented an atomistic quantum simulation study to investigate the device performances of GaSb/InAs heterojunction tunnel field-effect transistors (TFETs) with nanometer body thicknesses. It is revealed that the thin junction induced quantum confinement effect results in a heterojunction type transition from type-III to type-II as the junction thickness reduces, which can be used as an effective modulation of the TFET device performance. It is found that as the channel thickness decreases, both the ON current and OFF current of the device decrease significantly due to the quantum confinement induced effective band gap enlargement. In addition, the OFF current of the heterojunction GaSb/InAs TFET is always larger than that of the homojunction InAs TFET, which is possibly caused by the GaSb/InAs interfacial state assisted tunneling. It is also revealed that the subthreshold swing of the heterojunction TFET does not change much as the channel thickness is reduced.

SGO: A fast engine for ab initio atomic structure global optimization by differential evolution

Abstract As the high throughout calculations and material genome approaches become more and more popular in material science, the search for optimal ways to predict atomic global minimum structure is a high research priority. This paper presents a fast method for global search of atomic structures at ab initio level. The structures global optimization (SGO) engine consists of a high-efficiency differential evolution algorithm, accelerated local relaxation methods and a plane-wave density functional theory code running on GPU machines. The purpose is to show what can be achieved by combining the superior algorithms at the different levels of the searching scheme. SGO can search the global-minimum configurations of crystals, two-dimensional materials and quantum clusters without prior symmetry restriction in a relatively short time (half or several hours for systems with less than 25 atoms), thus making such a task a routine calculation. Comparisons with other existing methods such as minima hopping and genetic algorithm are provided. One motivation of our study is to investigate the properties of magnetic systems in different phases. The SGO engine is capable of surveying the local minima surrounding the global minimum, which provides the information for the overall energy landscape of a given system. Using this capability we have found several new configurations for testing systems, explored their energy landscape, and demonstrated that the magnetic moment of metal clusters fluctuates strongly in different local minima.

How good is mono-layer transition-metal dichalcogenide tunnel field-effect transistors in sub-10 nm? — An ab initio simulation study

Rigorous ab initio quantum transport simulation based on flexible plane wave method has been presented to predict the performance of mono-layer transition-metal dichalcogenide (TMD) tunnel field-effect transistors (TFETs) at scaling limit. WTe₂-TFET appears as the most promising candidate for both high-performance (HP) and low-operating-power (LOP) transistors with the I_ON as high as 1890 μA/μm, which is very close to the ITRS 2024 requirements for 7nm physical gate length scaling. While other TMD-TFETs are found much less applicable. In addition, performance enhancement based on atomic defect engineering has been proposed. The simulation reveals transition metal vacancy inside the tunneling diode can dramatically increase I_ON while keeping I_OFF unchanged. As an evidence, simulation shows Mo-vacancy engineered mono-layer MoS₂-TFET with 6.3nm physical gate length has 15 times larger I_ON (1324μA/μm) and the same I_OFF (1.2×10-2 μA/μm) compared to the pristine device.

A small box Fast Fourier Transformation method for fast Poisson solutions in large systems

Abstract We present a new divide-and-conquer algorithm to efficiently evaluate the Coulomb interaction in a large system, which is an essential part of self-consistent first-principle calculations. The total Coulomb potential ϕ ( r ) = 1 / | r | is divided into a short range part ϕ S ( r ) and a smooth long range part ϕ L ( r ) . The system is divided into many cuboids, with a small box defined for each cuboid plus a buffer region. For the short range part, the interaction convolution integral is calculated directly using a Fast Fourier Transformation (FFT) in the small box. For the smooth long range part, the convolution integral is evaluated by a global FFT but on a coarse grid. The conversion between the dense grid and coarse grid values is done using small box FFTs with proper mask functions. Using this small box FFT method, the total Coulomb potentials of test quantum dot systems on 480 3 grid and 2400 3 grid are calculated. For the 2400 3 grid case, the calculation is carried out by tens of thousands of processors with a computational speed up close to 10 times when compared with direct global FFT calculations using the FFTW package with the maximumly allowed number of processors. The maximum relative error is 4×10 −5 while the absolute error is less than 0.1 meV.