Program Coordinator: Ching Cheng (NCKU):

In spite of a great diversity of fields covered in condensed matter physics, the local focuses of theoretical condensed-matter research are mainly on nanosystems and surfaces, the two fields which are most exciting in physics as well as relevant to practical industrial applications. Three topics are proposed here in which each has its own distinct objectives while, at the same time, overlapping either in materials, physical properties, or technical interests, deserves promoting interactions as well as calls for coherent running.

 

A. Electro-optical Physics

 

Physics and rationale: The electro-optical physics is on the frontier of current physical researches and covers both fundamental studies and applications. Its development impacts on the prosperity of high technology industry. In fact, the electro-optical industry is the most essential part of Southern Taiwan Science-based Industrial Park.

Based on the expertise and common research interest of the organizers, this program aims on the theoretical and computational studies of (1) the properties of electro-optical materials, (2) the interaction of laser and material surface, and (3) the dynamics of thin-film growth. For the first topic, the emphasis is placed on the electronic, optical, and thermal properties of advanced materials such as carbon nanotubes and AgO. The study of the laser and surface interaction includes (but not limited to) the excitation dynamics of exciton states and surface plasmon, and the surface thermodynamics induced by pulsed laser. The third topic covers the removal of material during pulsed laser ablation that provides the particle and energy sources for thin film growth, the diffusion and transport of atoms and clusters on substrate surface, the thermal effect on the growth and properties of thin film.

In addition to the exchanges and collaborations on the physics and research, this program provides a forum to share and to learn the experiences on the computational and theoretical methods employed in the listed research areas.

Participants : K. R. Chen (NCKU, contact person), Y. T. Lu (NCKU), M. F. Lin(NCKU), Y. C. Tsai (CCU)

Expected achievements : The planned activities listed above are expected to promote the exchange and collaboration within the local community and between local members and outside scientists including both theorists and experimentalists, and to educate and attract more young students and scientists to devote themselves to the research and applications of electro-optical physics. Thus, in addition to resultant more and better publications, it is envisioned that the local research environment in the listed areas will be improved, the collaborated researches will be stimulated, and the capacity and productivity will be enhanced.

 

B. 2D nanosystems

Physics and rationale : Several realizations of quantum phase transition have emerged recently, such as Josephson junction arrays and the gas of ultra cold atoms trapped by an optical lattice . The Bose-Hubbard model captures the relevant physics of these systems. Different from the classical phase transitions, the quantum fluctuation due to Heisenberg's relation may drive the quantum system in the zero temperature from an ordered phase to a disordered one and lead to a macroscopic change.

In this proposal, we aim at a theoretical investigation of the 2D nanosystem. The system consists of bosonic particles with on-site repulsive atom-atom interactions in a 2D square lattice. Through the tunneling mechanism, the boson atoms may hop from one lattice site to near-nearest neighbor. The theoretical studies of the Bose-Hubbard model including mean field, variational and perturbative approaches, and quantum Monte Carlo technique, have shown that the above system will undergo a quantum phase transition from the Mott-insulator phase with a gapped ground state to the super-fluid phase with a gapless one. As the repulsive interaction between atoms decreases until the perfect correlations of the atomic number between lattice sites prevails, the Mott phase transits to the super-fluid phase. The coherence of long-range phase then occurs.

In the experimental set-up, the lattice potential is formed by the gold-cluster arrays. Each cluster consists of 6 Au atoms. Along the line of the force field calculations, we find the minimum of the potential felt by the deposited Rb atoms is situated right above the center of the 6-gold cluster. Since the gold clusters form a square lattice, the local potential minimums will also form a square one. At T=0, the atom will hop from one potential minimum to the next one. The tunneling barrier energy is about 0.08eV. The lattice constant of the cluster lattice is about 20 Å. Based on the WKB tunneling rate formula, the atom hopping rate for the present case is comparable to that for the case of Rb atom in a 3D optical lattice. From the results of quantum Monte-Carlo simulations and variation calculation, the critical value of the ratio J / U at the filling number 1 in the 2D case ranges from 0.12 to 0.14. As we tune the atom-atom interaction by applying a suitable magnetic field, we expect the Mott-superfluid transition will occur in the experiment. We shall investigate the transition in several respects: the tuning parameters by using quantum Monte-Carlo simulation and the large-d expansion, vortex-phase transition by the RG calculations, and explore the physical parameters by ab initio calculations combined with GWA method.

Participants: Yan-Chr Tsai ( 蔡炎熾 ), Chin-Rong Lee ( 李進榮 ), Tsan-Chuen Leung ( 梁贊全 ), Chien-Hua Pao ( 包健華 ), Wang-Chang Su ( 蘇旺昌 ), Ming-Hsien Tu ( 杜明憲 ), C. Y. Lin ( 林財鈺 ).

Expected achievements: The planned activities are expected to stimulate collaboration within the local community and between local members and outside scientists including both theorists and experimentalists (e. g. Prof. Men in CCU). In addition, it also serves as the education for young students and scientists to direct themselves toward the research of condensed-matter physics. Thus, it is expected that this project will result in more and better publications and the local research environment in this area may be improved, and the capacity and productivity will be enlarged.

 

C. Ab initio electronic approaches for nanosystems

Experimental realizations in growing and investigating nanosystems have opened a great opportunity in studying these exciting systems that do not only reveal novel properties but also provide previously unimaginable direction of applications, e.g. field emitters made of carbon nanotubes and optoelectronic nanodevices made of Si, GaN and ZnO nanorods (right figure: top view of ZnO nanorods in various diameters). Ab initio electronic approach, though complicated and computer-time consuming, provides a realistic way in theoretically studying these systems that empirical or phenomenological methods cannot compare with. The proposed studied subjects include : electronic and acoustic and field-emission properties of carbon nanotubes, structural stabilities and electronic properties of Si, GaN, and ZnO nanorods, structural and electronic properties of In nanostructures on Si surfaces. The expected interaction and possible cooperation through this project is not only in physics but also in techniques as some of the studies involve developing computational tools to implement them.

 

Participants : C. Ch0eng (NCKU), T. C. Leung (NCCU), S. F. Tsai (NSYSU), M. H. Tsai (NSYSU)