NCTS Taiwan

In this new NCTS phase (2021-2025), we will concentrate on (but not limited to) the following three main research topics that contain fascinating physics and also promise high potential in application.
 

This project will be coordinated by Feng-Chuan Chuang with core participants Yang-Hao Chan, Guang-Yu Guo, Horng-Tay Jeng and others.
 

Graphene, an one-atom-thick two-dimensional (2D) material with carbon atoms arranged in a single-layer honeycomb lattice possessing an unusual band structure with a linear spectrum, exhibits fascinating physical properties such as extraordinary mechanical strength, ballistic transport with charge carriers mimicking massless Dirac fermions, and supreme thermal conductivity. Thus, graphene has attracted enormous attention since it was isolated and characterized for first time in 2004. The current interest in graphene has also stimulated research efforts to fabricate other 2D materials that could also show novel properties as graphene. Indeed, many of them, such as graphitic BN layers, monolayers of group IV elements and group III-V compounds as well as transition metal dichalcogenides and Bi compounds, have been fabricated and characterized. These new 2D materials could exhibit a number of intriguing properties such as quantum spin Hall effect and nonlinear optical property that the corresponding bulk materials do not have. They also have several promising applications such as novel types of transistors, efficient catalysis for hydrogen production and spin transport devices. One of the most exciting recent developments in 2D quantum materials is the discovery and characterization of magnetism and superconductivity in exfoliated single-layer materials. These developments pave the way for an enormous influx of new quantum materials designed layer by layer through mechanical stacking. Today, given the dramatic advances in nano-characterization tools driven primarily by the study of graphene, the science of 2D quantum materials is poised to advance very quickly. Here we propose to perform the state-of-art ab initio calculations in order to provide a theoretical understanding of the fundamental properties of these emergent low-dimensional materials and also to assess their application potentials. 

There are a number of experimental groups in both universities and Academia Sinica, having strong interests in these novel materials. We plan to interact strongly with them. Indeed, using the platform provided by the Computational Material Research Thematic Group, we will organize joint experimental and theoretical meetings and workshop on various aspects of these new materials.

 

This project will be coordinated by Tay-Rong Chang with core participants Feng-Chuan Chuang, Horng-Tay Jeng, Hsin Lin and others. 
 

Topological materials have fascinated the physics community in both physics and material science, and this can be understood from two perspectives. First, it possesses great variability in classifying the topology. Second, the associated surface states (or the boundary modes due to the inherent topology in topological materials are robust against perturbations, as long as the topology is intact. Hitherto, numerous theoretical methods have been integrated to explore this paradise. 

The main research aims of the TG are as follows. Initially, minimal tight-binding models of interested topological states will be constructed. It is required that both nontrivial bulk bands and topological boundary states can be captured by the models. In the second stage, different methods, e.g., the Boltzmann transport, the Kubo formula, and the nonequilibrium Green's functions, are applied for exploring target phenomena, according to their essential nature. This procedure allows us to distill the significant physics insights for target phenomena in a fast pace. Meanwhile, articial errors and computing performance can be spotted and optimized more efficiently in dealing with a simple system. Finally, given the clues from the second stage, ab initio real-material tight-binding models of selected candidates will be passed to the second stage again. We anticipate that after proceeding the designed research flow, the crucial physics insights are not only unveiled scientifically, but also practiced in real materials.

 

This project will be led by Yang-Hao Chan with main participants Guang-Yu Guo, Horng-Tay Jeng (NTHU), Chi-Cheng Lee and others.
 

Strongly correlated materials such as transition metal oxides span a wide range of crystalline structures and host a rich variety of fascinating physical phenomena. The best known phenomena observed in transition metal oxides are high temperature superconductivity in cuprates and colossal magnetoresistance (CMR) in manganites. More recently, multiferroic materials with large magneto-electric effect have been attracting enormous research interest, mainly because of their interesting fundamental physics and also their potential technological applications. In a multiferroic material, magnetism and ferroelectricity coexist. In some multiferroic materials, magnetic order can create ferroelectricity or vice versa, i.e., strong magneto-electric effect exists. This cross-correlation gives rise to interesting prospect of controlling electric polarization by magnetic fields and manipulating magnetization by electric fields, and thus provides an exciting opportunity to construct new multifunctional devices such as next-generation memory devices that can be electrically written and magnetically read. In order to exploit these novel phenomena, however, one should thoroughly understand the mechanisms that control the physical properties of the emergent materials. Quantum mechanical calculations are an indispensable and powerful method for investigating the novel properties of materials. 

For weakly correlated solids such as simple metals and sp semiconductors, ab initio calculations on their physical and chemical properties have been routinely carried out based on (spin) density functional theory (DFT) for many-electron systems with the local density approximation/generalized gradient approximation (LDA/GGA). Nonetheless, in many transition metal oxides, d-electrons are often strongly correlated, and therefore, the improved LDA/GGA+U scheme with a Hubbard U term added, is widely used. Indeed, the LDA/GGA+U approach gives rise to a fairly good description of the physical properties such as magnetism, electricity, charge-orbital order, spin-wave dispersions and even electron-phonon below magnetic transition temperature in many transition metal oxides.

However, in many cases, the LDA/GGA+U method fails to provide a proper description of the electronic structure and related spectra at finite temperatures especially above the magnetic transition temperatures because dynamical local correlation among the d electrons is neglected here. For example, high temperature superconductor parent compound La2CuO4 is an antiferromagnetic insulator at low temperature and becomes a paramagnetic insulator above the transition temperature. The GW method predicts it to be a metal both below and above the transition temperature. The LDA/GGA+U approach also fails to give rise to an insulating band structure above the transition temperature although the antiferromagnetic insulating state at low temperature is correctly predicted. Latest research efforts have revealed that only the combination of the quasiparticle GW method which takes into account the long-range dynamical correlation with the dynamical mean-field theory (DMFT) which includes local dynamical correlation (GW-DMFT) could describe the insulating behavior of the paramagnetic La2CuO4 above the antiferromagnetic transition temperature. Nonetheless, the occurrence of the state-of-the art GW-DMFT method appears to be just the beginning of a viable solution to describe the electronic structure and related physical properties of strongly correlated real materials such as cuprate oxide superconductors and Fe-based superconductors. This is because different ways to perform QSGW calculations could lead to quite different results. Furthermore, how to best and effectively solve the quantum impurity problem in the DMFT approach remains to be investigated. Therefore, the principal goal is to combine the complementary ab initio single/quasi-particle calculation of realistic materials with ab initio many-body theory and model calculation to explore/develop efficient GW-DMFT methods and also apply them to study the fascinating properties of strongly correlated transition metal oxides.

 

(1) Meetings and workshops: Two general meetings, one in summer and another in winter, will be held to report research progress and share new ideas among the members. Distinguished overseas scholars will also be invited to give talks or lectures in these meetings. Postdoctoral researchers and graduate students are especially encouraged to attend the general meetings and report their work though oral or poster representations. In addition to general meetings, special seminars and workshops are organized on demand. Each program leader is to decide whether a seminar or workshop is to be called for on a regular basis, and whether an invited scholar is to be the focus of the gathering. The list of previous workshops is as followed.

 

1. Workshop of Quantum Materials, July 23-25, 2020

2. 18th Workshop on First-Principles Computational Materials Physics, July 6-7, 2020

3. 17th Workshop on First-Principles Computational Materials Physics, June 25-26, 2019

4. NCTS 1-day Tutorial on Wannier functions, Aug 9, 2018

 

(2) International collaborations: International collaborations will be encouraged with emphasis on research topics which will eventually lead to joint publication of high-impact articles. Several international scholars are cooperating with our members of the subproject in their research. They include Steven Louie (UC Berkeley), Arun Bansil (Northeastern University), and A. Castra Neto (National University of Singapore). We will invite these scholars to visit Taiwan or send our members to visit their groups to facilitate joint research.

 

(3) Training young researchers: We will continue to provide the popular annual training courses for first-principles computational materials research to interested students and researchers. We organize two courses each year, namely, the spring elementary hands-on ab initio calculation school (two days), and the summer advanced hands-on ab initio calculation school which lasts for one week. The spring school usually have two identical classes in order to meet high demand. The training courses are open to people of all kinds of academic disciplines. Records have shown that students, postdocs, faculty members, and professional scientists from physics, chemistry, materials science, engineering, geophysics, life sciences, and even medicine have attended the courses offered by us. It is well known that many research groups, theoretical or experimental, regularly send their students to the training courses. We will continue this important service to people in the academia and industry.

 

2020 Spring school on First Principles Computational Materials Research - Introductory Level

2020 Summer School on First Principles Computational Materials Research - Advance Level

 

(4) International Travel fund. The Group will continue to finance within its budget limits graduate students and postdocs from the members’ groups to attend seminars, workshops, and general meetings in Taiwan and abroad. We will also assist junior college faculty members who are not able to obtain grants temporarily and rely on the platform to keep up with the latest research, by providing necessary training and offering opportunities for joining current research groups. One of the regular regional/international workshops is The Asian Workshop on First-Principles Electronic Structure Calculations. This workshop is an annual event rotating among Taiwan, Japan, Korea and China. The most recent workshop, the 21th Asian Workshop was held in Korea and 22nd Asian Workshop will be in Osaka University, Japan. Our community will then organize the 23rd Asian Workshop in Taiwan. The previous workshop in Taiwan was the 19th Asian Workshop, which was held in NCTU, in Hsinchu, Taiwan, Oct. 31 to Nov. 2, 2016. Thus, in the next phase (2021-2025), we will support junior members (assistant professors, postdocs, and graduate students) to attend 24th (China 2022), 25th (Korea 2023), 26th (Japan 2024), workshop abroad in order to involve actively in this Asian community of ab initio research. We will hold the 23rd workshop (Taiwan 2021) and 27th workshop (Taiwan 2025). In addition to the international workshops and conferences, we will also send young participants to visit foreign institutes (USA, Germany, Japan, Singapore, and China) to conduct the research and experience the international environment.

 

The research topics of this TG include three of the fast growing research fields which are being tackled by many physicists, chemists, materials and life scientists worldwide. The track records of our core members and young participants have consistently shown their capabilities of publishing high-quality and high-impact research papers. The core members in this TG are not only the locally but also internationally recognized pioneers in the field of topological materials. We will continue to pursue and maintain the leadership in this research field. Meanwhile, our core members also actively serve local academic and research communities to promote the local researches based on first-principles calculations. Our core members will continuously increase Taiwan’s international visibility by publishing frontier research papers. We expect that some high quality publications will be delivered every year and the research work will be jointly published by the core members, domestic, and international collaborators.