The MathNet Korea
Information Center for Mathematical Science

세미나

Information Center for Mathematical Science

세미나

서울대학교 지구환경과학부 세미나

KAIST 화학과 세미나

Theoretical and computational approaches have made important contributions to heterogeneous electrocatalysis. Such approaches, however, assume that the catalytically-active surface for a particular class of materials is the same. We have discovered that the electrocatalytic activity of nickel phosphides and CaMnO3 toward the H2 and O2 evolution reactions, are governed by aqueous surface equilibria, i.e. the most catalytically-active surfaces are aqueous surface reconstructions. In addition to the determination of realistic surfaces for computational studies of heterogeneous electrocatalysis, we are also developing combined first-principles and machine learning techniques for the automated discovery of catalytic descriptors. Using these techniques, we discovered that the Ni-Ni bond length is an excellent descriptor for the hydrogen evolving activity of Ni2P and that it can be modulated via nonmetal surface doping, which induces a chemical pressure effect. For decades, ab initio thermodynamics has been the method of choice for computationally determining the surface phase diagram of a material under different conditions. The surfaces considered for these studies, however, are often human-selected and too few in number, leading both to insufficient exploration of all possible surfaces and to biases toward portions of the composition-structure phase space that often do not encompass the most stable surfaces. To overcome these limitations and automate the discovery of realistic surfaces, we combine density functional theory and grand canonical Monte Carlo (GCMC) into \"ab initio GCMC.\" We demonstrate ab initio GCMC for the study of oxide overlayers on Ag(111), which, for many years, mystified experts. Ab initio GCMC rediscovers the surface phase diagram of Ag(111) with no preconceived notions about the system. Using nonlinear, random forest regression, we discover that Ag coordination number with O and the surface O-Ag-O bond angles are good descriptors of the surface energy. Additionally, using the composition-structure evolution histories produced by ab initio GCMC, we deduce a mechanism for the formation of oxide overlayers based on the Ag3O4 pyramid motif that is common to many reconstructions of Ag(111). Ab initio GCMC is a promising tool for the discovery of realistic surfaces that can then be used to study phenomena on complex surfaces such as heterogeneous catalysis and materials growth, enabling reliable and insightful interpretations of experiments.

KAIST 화학과 세미나

Hydrophobic interactions are mutually attractive forces experienced by apolar solutes, such as hydrocarbons, perfluorocarbons, and inert gases, in water. It drives the formation of micelles and gas-hydrates, and crucially influences the structures and functions of lipid membranes and the proteins and ion-channels held therein. Although actively studied for nearly 30 years, a molecular understanding of the physical origin of the hydrophobic force is still lacking. Further, recent experimental observations of ultrafast transport of water molecules across hydrophobic nanosheets and nanochannels, the anomalously low dielectric constant of water confined between graphene sheets, and the formation of cubic-crystals of ice therein, comprise fascinating but poorly understood phenomena associated with water under nanoscale hydrophobic confinement. In this talk, we will show surface force measurements and extensive computer simulations that find that the attractive forces between molecularly smooth mica films coated with hydrophobic monolayers could be around 10% higher in light water than heavy water, even though macroscopic measurables, such as interfacial tensions and contact angles of those liquids are equal within. Free energy decomposition analysis revealed this to be a purely nanoscale effect, brought on by relatively enhanced (compared to the bulk liquid) zero-point energy and entropy in heavy water, compared to light water. This enhancement is due primarily to better coupling with the low frequency vibrational modes of the hydrophobic surface. These results advance interfacial nuclear quantum effects and entropy as a critical forces that informs the stability of confined water, which could possibly be exploited as a design principle for aqueous nanoscale devices.

KAIST 화학과 세미나

Metal-organic frameworks (MOFs) have attracted great attention in gas storage and separation over the past decades, owing to their high level of porosity and chemical and structural tunability. Recent increase in global demand for isotopic gases, particularly deuterium, have triggered the researches to separate physico-chemically almost identical isotopic mixture with MOFs by tuning the pore size or introducing open metal sites (OMSs) as strong binding site. Nonetheless, the difficulty of optimizing the pore size to the sub-Angstrom scale and insufficient separation efficiency of OMSs limits the synthesis of MOFs with high deuterium-selectivity. Therefore, there is a critical need for new approaches utilizing MOFs in hydrogen isotope separation. In this talk, we present two novel strategies toward well-designed MOF-based separation systems that can effectively separate deuterium from hydrogen isotopic mixture. The first strategy exploits the dynamic pore change during the breathing of a flexible MOF, MIL-53(Al), for the high selectivity for D2 over H2. During the flexible and reversible transition, the pore apertures are continuously adjusted, thus providing the tremendous opportunity to separate mixtures of similar-sized and similar-shaped molecules that require precise pore tuning. The other strategy is the simple post-modification of introducing imidazole molecules into the highly dense OMS channels of MOF-74 to optimize the aperture size and increase the internal binding energy, simultaneously. It can synergistically maximize the efficiency of deuterium separation, having a high uptake with the highest reported separation factor as high as ~26 at 77 K. We believe that our results will provide new opportunities for the intelligent design of porous materials leading to the development of other highly efficient isotope and gas separation systems.

KAIST 화학과 세미나

In this talk, I will discuss how atoms and molecules interact with intense x-ray pulses generated by x-ray free-electron lasers (XFELs). XFELs open a new era in science and technology, offering many unique opportunities that have not been conceivable with conventional light sources. Because of their very high x-ray photon fluence within very short pulse duration, materials interacting with XFEL undergo significant radiation damage, i.e., they possibly become highly ionized and then explode. Understanding of radiation damage is the key for successful XFEL experiments. I will present a theoretical framework to treat x-ray-induced processes and to simulate radiation damage dynamics, introducing two dedicated toolkits, XATOM and XMOLECULE. Then I will present two recent studies, x-ray multiphoton ionization dynamics of Xe atoms [1] and ultrafast explosion dynamics of iodomethane (CH3I) molecules [2]. The former demonstrates an interplay between resonance and relativistic effects in the heavy-atom ionization dynamics, and the latter reveals x-ray molecular ionization enhancement via intramolecular charge transfer. Both of them provide critical insights of radiation damage dynamics of molecules containing heavy atoms, for example, metalloproteins, at high x-ray intensity. With these examples, I will demonstrate how theory and experiment work together to advance XFEL science, and discuss how to apply these new findings for potential XFEL applications such as femtosecond x-ray nanocrystallography.

KAIST 화학과 세미나

Bacterial protein translocation is fine-regulated to respond to dynamic cellular requirements. In the cytosol, the hydrophobic signal sequences of membrane proteins are recognised by the signal recognition particle (SRP) and targeted to the Sec translocon at the inner membrane by interacting with the membrane-associated receptor (SR). Meanwhile, from outside the cell, only desired nutrients and substrates are transported into cells across the outer membrane pore proteins (porins). The outer membrane barrier, however, can be overcome by noxious compounds and toxic proteins, which have evolved to hijack porins. To study cell membrane systems, simplified model membranes such as supported bilayers, liposomes, and nanodiscs have been developed. Site-specific labelling for fluorescence resonance energy transfer (FRET) measurements is a simple but powerful strategy to monitor the interaction and conformational changes between labelled substrates in aqueous buffer as well as in the model membranes. Furthermore, protein dynamics can be investigated one at a time via electrical recording of a single channel embedded in a supported bilayer system. Here, I am going to talk about recent findings in two interesting protein translocation pathways at the membrane interface using FRET and single channel recording as major tools: (1) SRP-mediated targeting complex formation at the Sec translocon and (2) the directional transport of toxin-derived peptides/proteins through the bacterial envelopes. This new findings provide useful insights into the dynamics of biological systems, specifically at the membrane interface. Furthermore, this opens new opportunities for engineering essential cellular machines and establishing new methods for drug delivery.

KAIST 화학과 세미나

Natural products continue to plan an important role in the evolution of organic chemistry. This lecture will discuss the development of analogs of epidithiodiketopiperazine alkaloids as preclinical antitumor agents, and some insights in chemistry and biology that resulted from our recent studies of rearranged spongian diterpenoids.

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

POSTECH 물리학과 세미나

한국고등과학원 세미나

The Simplest Little Higgs (SLH) model is a most simple realization of the litte Higgs mechanism which aims at solving the Higgs mass naturalness problem. In this talk I will present an analysis of its scalar potential, from which a relation between the top partner mass and the mass of the additional singlet pseudoscalar is obtained. Based on theoretical constraints from the mass relation and perturbative unitarity, and recent experimental constraints from recent LHC searches, I will give a characterization of the model’s viable parameter space. I will also discuss some interesting but subtle issues regarding the phenomenological analysis of the additional singlet pseudoscalar. The scalar potential analysis and other subtle issues discussed here could also be relevant for other models in which the Higgs arises as a pseudo-Nambu-Goldstone-boson of some global symmetry breaking.

한국고등과학원 세미나

Non-Riemannian geometries typically emerge in explicit models of (loop) quantum gravity. Such exotic structures are shown to be critical in resolving a serious problem (raised by Turok et. al.) in the context of the no-boundary proposal and other theories of smooth initial conditions of the universe. The novel mathematical techniques required to calculate these quantum geometric effects will be shown to find interesting applications in other fields such as nonassociative quantum mechanics.

한국고등과학원 세미나

We study the vectorlike fermion(VLF) loop effect in the charged Higgs production and decays. As for the constraints, we investigate b to s gamma, Higgs precision and direct search bound of VLF. We pay special attention to the elector-weak constraints since they are quite strong. We find that the branching ratio of H^+ to W gamma can be enhanced by one or two order of magnitudes while H^+ to W Z still remains at the same order. We find the production channel in which the resonance production of neutral heavy Higgs boson happens and then it decays into charged Higgs can be important in broad mass range. We estimate cross sections for production and decays of charged Higgs via H^+ to W (gamma/Z) at the LHC as well as discovery significance.