Highlights of Research Results and Activities

1.      Current and strain-induced spin polarization in InGaN/GaN superlattices

Main team members : G. Y. Guo (NTU) and Y. F. Chen (NTU, experimentalist)

           It has been proposed theoretically that a transverse spin current, the so called spin Hall current, can be generated in strongly spin-orbit coupling systems by external electric field.  We have investigated the current-induced spin polarization in InGaN/GaN superlattices.  It is found that the degree of polarization changes sign as the direction of the current flow is reversed.  The strain-induced spin Hall effect discovered here paves an alternative way for the creation of spin polarized current, which should be useful for the realization of the future applications in spintronics.

H. J. Chang, T. W. Chen, J. W. Chen, W. C. Hong, W. C. Tsai, Y. F. Chen, and G. Y. Guo, Phys. Rev. Lett. 98, 239902 (2007).

2.      Orbital Ordering and Jahn-Teller Distortion in Perovskite Ruthenate SrRuO₃

Main team members : H. T. Jeng (Sinica) and C. S. Hsue (NTHU)

        Orbital, charge, spin, and lattice degrees of freedom play important roles in the electronic, magnetic, and transport properties of transition-metal oxides.  It was proposed that orbital ordering is closely related to magnetic and crystallographic lattices in perovskite manganites such as La_1-xCa_xMnO₃ in the low temperature insulating charge ordered phase.  We have investigated the electronic structures of SrRuO₃ in the distorted orthorhombic structure using LDA+U.  The obtained band energies agree well with those from photoemission and X-ray absorption spectroscopy.  Our finding unravels the nature of the orbital ordering , the close connection to the JT distortions, and the importance of the on-site correlation U in the relatively extended Ru 4d orbitals.

H. T. Jeng, S. H. Lin, C. S. Hsue, Phys. Rev. Lett. 97, 67002, (2006)

3.     Highly Spin-polarized field emission induced by quantum size effects in ultathin films of Fe on W(001)

Main team members : T. C. Leung (CCU) and C. T. Chan (Hong Kong)

        Nanostructured systems have enhanced effects due to the confinement of quantized states, and , for the particular case of ultrathin supported films, many amazing structural and electronic properties have been discovered.  Although tungsten is the metal of choice for emitter tips, it does not produce spin-polarized field emission currents.  We use density functional calculations to study the spin-polarized field emission from pseudomorphic Fe ultathin films on W(001). We found that nearly completely spin-polarized field emission currents can be realized in two and four Fe layers on W(001) and that these systems have the additional advantages of thermal stability and low work functions.  The thickness dependent field emission properties are traced to spin-polarized quantum well and surface resonance states localized in the Fe layers. 

Bin Li, T. C. Leung, C. T. Chan, Phys. Rev. Lett. 97, 87201 (2006).

4.      OpenMx Study Group Meeting

     It is well known that the computing time of currently popular electronic structure methods based on Density Functional Theroy scale like N^2-3, with N being the number of atoms in the simulation cell.  In the last few years, much effort has been devoted to overcome this problem, and a number of methods have been developed with “ order – N “ [O(N)] scaling, i.e., whose computational cost scales only linearly with the number of atoms.  One of the successful order N ab initio computing code is OpenMx ( Open source package for Material eXplorer ) developed by Dr. Taisuke Ozaki from National Institute of Advanced Industrial Science and Technology in Japan.  In order to familiar with the principles used in the program and how they are implemented so that the program can be properly applied to the systems we want to study, several study group meeting on OpenMx was organized by T. C. Leung (CCU) and held in NCTS (south).  The key members involved in this project are C. S. Hsue (NTHU), W. C. Kuo (NSYS), T. C. Leung (CCU), Y. C. Hsu (NCKU), S. F. Tsay (NSYS), C. Y. Ren (NKNU), and B. R. Wu ( CGU). 

5.      Excited State Study Group Meeting

The conventional DFT method fails to give accurate excitation properties, e.g., band gaps, excitonic binding energies, of solids.  The GW method has been successfully used to predict electronic properties of a variety of different systems, ranging from bulk materials to surfaces, nanotubes, and molecules, whereas the linear optical response is routinely studied by solving the Bethe-Salpeter Equation (BSE) with comparable success. ABINIT is a package whose main program allows one to study the excited states using the GW approximation.  On the other hand, the EXC is an exciton code which uses the output of ABINIT to calculate the dielectric and optical properties for a large variety of systems by solving the Bethe-Salpeter equation.  In order to familiar with the principles used in the program and how they are implemented so that the program can be properly applied to the systems we want to study, Exited State Study Group Meeting was organized by G. Y. Guo (NTU) and held in NTU.  The key members involved in this project are G. Y. Guo (NTU), H. C. Hsueh (TKU), H. T. Jeng ( SINICA), M. Hayashi (CCMS), C. K. Yang (CGU), B. R. Wu ( CGU), T. C. Leung (CCU) and M. C. Lin (FJU).