University of Tsukuba | Center for Computational Sciences

Quantum Condensed Matter Physics
Computational Design of Future Electronics Devices using First-Principles Calculations
A typical trend in first-principles electronic-structure calculations isthat we deal with increasingly complex systems characterized bymany atoms of different chemical nature in open structures and oflow symmetry. This path is supported by the increasing availability ofincreasingly powerful computers spearheaded by high-performancecomputers. The developments of the latter show that we have tocope with massively parallel computer architectures dealing withthousands of cores. This path accelerates in the next 10 years withthe attempt moving from peta-scale to exa-scale computing. Thus
developing electronic structure methods whose applicability scaleswith the available processes becomes a prerequisite.
We develop first-principles calculation code “RSPACE.” RSPACE isbased on the real-space finite-difference method, which is anapproach that has the potential to scale with massively parallelarchitectures and has this potential without compromise on theprecision and it should be superior to conventional plane wave methods. As applications using RSPACE, interface atomicstructures and fabrication procedures are developed using first-principles calculations to realize high-performance and lowenergy-loss electronic devices, e,g., SiC power devices. By investigating the origin of leakage current and carrier scattering atinterfaces, we propose prescriptions to improve device performance and demonstrate the applicability of the interface
structures. Furthermore, we establish a basic technology of computational science to design interface structures andfabrication procedures of future electronic devices.

Time-Dependent Method for Isolate Many-Atom Systems in Intense Laser FieldsIsolate many-electron systems, like atoms, moleculesand clusters, in an intense laser field are the current hot
topics because the processes can be used to generate
atto-second (10-18 s) pulses, image the molecular
structure, and push the femto-chemistry into attochemistry. Unfortunately, such a process cannot beobserved directly from experiments. The detailed
dynamic information can only be dug out by comparing
the first-principles simulation with measurements. The
characteristics of many-atom systems in an intense laser
field are that many continue states are involved and theejected electron can go far away from its parent core andreturn to the core again at a later time. Thus theconventional quantum-chemistry codes, like BigDFT orGAMESS, do not work in the intense laser materialinteractions although they are popularly used for thestructure and spectrum calculations.
To cope with the characteristics of the problem, we develop a time-dependent Schrödinger equation solver for manyelectron systems based on the density functional theory. Real space in a rectangular box is discretized with an equal space
grid and the time-propagation is performed with a second-order-split-operator method by using the fast-fouriertransformation. The advantages of the new method are that the unitary of electron wavefunctions is automatically satisfied
and the numerical precision can be controled by tuning the time-step. More important, the method can be used for a large
system with the number of grid point more than handred millions. The above figure shows a test run of the electron
momentum distribution of N2 molecules in two-color count-rotating circularly polarized laser fields with different aligment
angles between the polarization plane and the molecular axis. The grid points used in the simulation are 1000x1000x100 ina rectangular box. The new method is expected to work in a more complex many-electron system in intense laser fields.
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