The Department of Physics
at Chung Yuan Christian University

CHUN-KHIANG CHUA
DEPARTMENT OF PHYSICS, CHUNG YUAN CHRISTIAN UNIVERSITY

CHUANG YUAN CHRISTIAN UNIVERSITY

In 1953, a group of Christian educators and local gentries came together to establish an agricultural and engineering college in Taiwan to train science and engineering talent in the Christian spirit. After several name changes and long preparations, the Chung Yuan Christian College of Science and Engineering (CYCC) was established in October 1955. With "education through honest, diligent pursuit and practical experience" as the school's motto, the college was composed of four departments, namely, the departments of physics, chemistry, chemical engineering, and civil engineering. Upgraded to the status of a full university, CYCC was renamed Chung Yuan Christian University (CYCU) on Aug. 1, 1980. CYCU currently has seven colleges, 28 departments, 31 postgraduate programs, 13 PhD programs, about 15,000 students and 800 faculty members in total. CYCU is among one of the top private universities in Taiwan.

THE DEPARTMENT OF PHYSICS

The Department of Physics was established in 1955 as one of the founding departments of CYCU. In 1974 a graduate program was created in order to offer highly qualified students with master's degrees in applied physics. Since then, the number of faculty members and research fields has been extended. A PhD program was initiated in 1992 to strengthen research activity. This year the university and the department celebrated their 60th anniversary in October. Currently the department has 24 faculty members, and is comprised of one chair professor and 10 full, 10 associate and three assistant professors. The major research fields in the department are condensed-matter physics, photonics and theoretical physics.

CONDENSED-MATTER PHYSICS

The Crystal Growth Group of Kuan-Cheng Chiu is focused on the fabrication and characterization of organic molecular solids, including small organic molecules (C60, Alq3, CuPc, and rubrene) and polymers (polyaniline), whose structural elements contain conjugated π-electron systems that can interact with photons and transport charge carriers. The aims of these research works are expected to clarify some problems of fundamental interest in organic molecular crystal growth and to provide a relationship between the fabrication process and the properties of the as-prepared molecular solid films for use in organic electric and optoelectronic devices.

The Molecular Beam Epitaxy (MBE) Laboratory of Jyh-Shyang Wang works on the epitaxial growth of II-VI compound semiconductor materials for applications in optoelectronic devices. There are currently four master's students in this lab. Over the years, two PhD students and 23 master's degree students have graduated from this lab. They recently studied the growth of Cd(Mn)Te epilayers on Si substrates for the solar cells applications. Their experimental results are useful for developments in highly efficient low-cost tandem solar cells using Si substrates.

Fig. 1: Some research results obtained by Kuan-Cheng Chiu's group.

Fig. 2: The FWHM of X-ray rocking curves obtained by the MBE group declined significantly as the Mn content increased. Raman scattering spectra revealed that the intensity of the Te-Te related defect vibration modes falls significantly as the Mn content increase, even disappearing altogether in the samples with high Mn content.

The research group of the Optoelectronic Semiconductor Lab, led by Ji-Lin Shen, has one postdoctoral fellow, one PhD student, and four master's degree students, who are interested in investigating the optoelectronic properties of semiconductors and nanomaterials. Current research works include the synthesis of graphene quantum dots, and the optical and electrical characterization of graphene quantum dots. The goal of their work is to improve the performance of optoelectronic devices.

Fig. 3: Laser ablation system of Ji-Lin Shen's group.

Fig. 4: Chun-Chuen Yang presenting their result.

Chun-Chuen Yang set up a magnetic and nanophysics laboratory in 2009. Since then, 14 students have received their master's degree based on their works in this lab. The current research topics of the lab are focused on the critical phenomena of multiferroics (RMn₂O₅ family), especially in the quenching of electric and magnetic ordering by the quantum size effect, and in the interplay between crystal structure and its electric and magnetic properties.

The research group of Condensed Matter Physics Laboratory led by Ching-Ling Hsu is focused on the morphology and the electrical characteristics of low-dimensional nanoscale materials. Recent research demonstrates that the electrical percolation threshold in a two-dimensional disordered nanoparticle film constructed from colloidal self-assembly is possible to be less than the value expected by the classical percolarion theory. The non-ohmic properties, which indicates Coulomb blockade in percolated paths, also emerges in the disordered metallic nanoparticle films.

Fig. 5: The Atomic Force Microscopy (AFM) image of a disordered gold nanoparticle film formed by colloidal self-assembly. This film obtained by Ching-Ling Hsu's group shows non-ohmic characteristics in the electrical transport measurement.

Chi-Tsu Yuan is worked on the development of white light-emitting devices based on eco-friendly, cost-effective and solution-processable nanomaterials for solid-state lighting. Two types of nanomaterials, namely cyan carbon nano-dots and red gold nanoclusters, can be fabricated via simple microwave assisted syntheses in an aqueous solution. For practical use, those solution-processable nanomaterials can be transferred to a solid-state phase by incorporating with solid matrices, such as organic polymers or inorganic silica that can preserve their original photoluminescence characteristics in the solid states. The photo-physical properties of such novel nanomaterials can be unraveled using time-resolved, laser scanning confocal microscopy.

The spin physics laboratory of Chii-Bin Wu is devoted to research about the electrons' spin-dependent transport phenomenon. Spin polarized scanning tunneling microscopy at room temperature is the main expertise of the group leader. At present, there is one room temperature ultra high vacuum (UHV) scanning tunneling microscope (STM) from DME in the lab. It is mounted on a UHV preparation chamber. Furthermore, there is also an ambient STM in the lab. The preliminary result from the microscope is the finding that the surface atomic structure of a molybdenum disulfide cyrstal can exhibit a hexagonal atomic structure or a honeycomb structure with the same Pt/Ir probe. This could be explained by the change of the atomic structure at the tip apex, which might be sensitive to the top sulfur atoms at one time (hexagonal structure) and become sensitive to the pz orbital of the underlying molybdenum atoms at another time (honeycomb structure). This indicates the importance of the tip condition in the explanation of the surface atomic structure, even in such simply di-atomic systems.

Another research topic is to study skyrmion systems with micromagnetic simulation software. The phase diagram of skyrmions in homogeneous thin films with Dzyaloshinskii-Moriya interaction (DMI) was obtained using micromagnetic simulation (OOMMF). Finite DMI is necessary in order to generate the skyrmion phase, in which the stripe domains and isolated skyrmions can co-exist. As the field strengthens, the stripe domains shrink to form isolated skyrmions, whose diameter reduces in a larger field. This result provides us the insight to tailor the skyrmion systems for their potential use in spintronics.

Fig. 6: (Above) The surface morphology of MoS2 surface depends on the condition of the Pt/Ir tip. (Below) The magnetic domain patterns of thin films with various DMI strength.

Fig. 7: An SEM image of suspended AuPd nanowires.

Yuan-Liang Zhong's Nano Device and Mesoscopic Physics Group is focused on two-dimensional atomic layered materials and carbon-based materials for studying mescoscale and nanoscale physics, nanodevices and qumatum transport. They have developed novel nanodevices with two dimensional materials, and focused on MoS2 and WSe2 of the transition-metal dichalcogenide (TMDC) materials to study the physics of nano-field-effect transistors (FET), valleytronics and nano-optoelectronic devices. Recently, they used disordered metallic nanowires with suspended structures to study the electron-phonon scattering rate. By decoupling phonons between metallic nanowires and substrates using suspended structures, electron diphase time is measured, which will be useful in the study of electron-phonon scattering mechanisms.

The Optoelectronic Materials and Devices Laboratory of Yu-Chiang Chao is dedicated to the investigation of design, fabrication, and characterization of optoelectronic devices, especially vertical transistors, solar cells and light-emitting diodes. The main goal is to develop high quality optoelectronic materials and devices.

PHOTONICS

The research of Kuo-Pin Chiu is focused on plasmonic nano-photonics and its applications in solar energy harvesting devices. Through optical simulations to analyze the spectra properties and the spatial electric-field distributions of various optical nano-antennas, they intend to realize proper plasmonic nano-antennas, which possess both the features of having high absorption efficiency of sunlight and large asymmetric spatial electric field distribution at the structure apex. Optical nano-antennas with a proper rectifier structure could be incorporated to form geometrical solar cells.

Fig. 8: The calculated attenuated-total-reflection spectra of different-shaped nano-antenna arrays illuminated by light of wavelength 632.8 nm.

Figure 8 shows some results of the calculated attenuated-total-reflection spectra of different-shaped nano-antenna arrays illuminated by light of wavelength 632.8 nm. There are three surface resonant modes, appearing in the results, which are dominated by the periodicity of the nano-antenna array. They have demonstrated that the dominated resonant mode is related to the coupled plasmonic resonance between the nano islands, which may be used to improve the light absorption efficiency of a solar cell.

I-Jen Hsu established the Biophotonics Laboratory in 2003. The research topics in the laboratory are mainly in microscopies with interferometry and spectroscopy, especially in high-resolution profilometry, optical coherence tomography and optical imaging for diagnosis applications.

A new optical technique based on a composite interferometer was invented for the measurement of a surface profile with sensitivity within 1 nm; a prototype of the commercialized instrument has been constructed. Optical techniques for the observation of the dynamic behavior of cells and diagnoses of tumors were also developed.

Fig. 9: (Left) Neural stem cell of rat. (Right) Prototype of profilometer.

Jy-Shan Hsu established the Information Display Laboratory in 2005. The laboratory research is focused on the application of liquid crystals (LCs) based on their physics and optics properties, including liquid crystal displays (LCD), bistable LCDs, and polymer dispersed liquid crystals(PDLC). The basic LC cell parameters, such as cell gap thickness and pretilt angle, can be obtained from their transmittance and phase retardation measurement systems. The current research topics include blue phase, the influence of doping nano-particles on the electro-optical properties of LCs, the purification effect on the degraded liquid crystal by soaking porous metal–organic frameworks (MOFs) in the LC material, and the improvement of PDLC properties.

Fig. 10: Beautiful textures of LCs.

THEORETICAL PHYSICS

The quantum information science group of Li-Yi Hsu studies quantum foundations using an information-based approach. Recently, their research topics include the quantum steering phenomenon and joint measurability of observables and the entropic version of the uncertainty relation.

We have an active research group on high-energy physics (CYCUHEP). The group consists of four faculty members and four post-doctoral researchers. They are also the core members of the CYCU Center for High Energy Physics, where there are two adjunct fellows from the National Taiwan Normal University and the University of Oklahoma.

Kwei-Chou Yang has been working on flavor physics, QCD sum rules and dark matter physics. He is the founding member of the CYCUHEP group.

Members in Chung Wen Kao's theoretic hadronic physics group have been working on the properties of hadrons. In particular, they focus on the structure of the proton, which has been intensively investigated in many labs, such as Jefferson Lab in the US and MAMI in Mainz, Germany.

Their research topics include the spin polarizabilities of the nucleon, the extraction of the strange form factors of the proton, and the phase structure of QCD. Our current research emphasizes the modeling of various fragmentation functions, which are crucial to the extraction of the transversity parton distribution functions. Transversity PDFs will provide precious 3-dimensional information of the inner structure of the proton.

The research of Wen-Yu Wen is focused on several topics, including quantum black holes, integrable models in string theory, and holography in strongly-correlated systems. By studying the quantum aspect of black holes he and his collaborators hope to uncover some secrets of quantum gravity. Recently, they have been inspecting quantum and non-thermal aspects of Hawking radiation and new mechanisms of black hole evolution. They also studied different aspects of AdS/CFT correspondence; for instance, they studied integral models in deformed AdS and looked for stringy excitations in such backgrounds and their dual field theories, and they also studied a typical class of Fermi systems by using charged dilaton in the holographic background.

Chun-Khiang Chua works on flavor physics and dark matter physics. Recently, several topics, including baryonic B decays, three-body B decays, final state interaction in B decays, scalar mesons, leption flavor violating processes, pentaquark decays, Dirac fermionic dark matter and Majorana fermionic dark matter, are studied.