# Contributions of AIMaS in the 45th Vietnam Conference on Theoretical Physics (VCTP-45)

The Vietnam Conference on Theoretical Physics (before 2019, its name is the National Conference on Theoretical Physics) is held annually on the end of July, early August, since 1976. The conference is the oldest annual scientific activity in Vietnam and which is organized by the Association of Theoretical Physics, Institute of Physics (Vietnam Academy of Science and Technology) in conjunction with other sponsors. By 2020, the Conference has been held 45 times.

From 12 to 14 of this October, the 45th Vietnam Conference on Theoretical Physics is held on Vinh Yen City, Vinh Phuc Province. There are 09 invited talk from Nguyen Ai-Viet (Information Technology Institute, Vietnam National University), Phan Duc Anh (Phenikaa University), **Nguyen Truong Thanh Hieu (Advanced Institute of Materials Science, Ton Duc Thang University), Hirobumi Mineo (Advanced Institute of Materials Science, Ton Duc Thang University)**, Shingo Takeuchi (Phenikaa University), Nguyen Thi Kim Thanh (Institute of Physics, Vietnam Academy of Science and Technology), Tran Xuan Truong (Le Quy Don Technical University), **Le Thi Cam Tu (Advanced Institute of Materials Science, Ton Duc Thang University),** Nguyen Thi Hong Van (Institute of Physics, Vietnam Academy of Science and Technology). More information can be found in https://iop.vast.ac.vn/theor/conferences/nctp/45/index.php?p=399.

It is an honor of our Advanced Institute of Materials Science (AIMaS), Ton Duc Thang University (TDTU) when there are **03 invited speakers **(over 09 invited speakers) from our Insitute. This shows the appreciation of the Vietnamese theoretical physics community for the contributions of AIMaS researchers.

**1. Dr. Hieu T. Nguyen-Truong, Head of Laboratory of Applied Physics**

**Title: **Low-energy electron inelastic mean free paths in materials

**Abstract: **The electron inelastic mean free path (IMFP) is an important quantity for studying electron transport in condensed matter and for various types of electron spectroscopy/microscopy. For decades, experimental techniques and theoretical models have been developed to determine this quantity. However, the determination of low-energy IMFPs is still a challenge. Recently, we have presented a theoretical approach to this problem [1]. Accordingly, we determine the IMFPs for energies below 100 eV within the dielectric formalism and the time-dependent density functional theory. Our calculated IMFPs for elemental solids [2] and monolayer graphene [3] agree well with experimental data. In this talk, we present an overview of the recent progress in this field and report our latest results. We show that the presented approach is general and reliable not only for bulk materials but also for two-dimensional materials.

References: [1] H. T. Nguyen-Truong, Appl. Phys. Lett. 108, 172901 (2016). [2] H. T. Nguyen-Truong, J. Phys. Condens. Matter 29, 215501 (2017). [3] H. T. Nguyen-Truong, B. Da, L. Yang, Z. Ding, H. Yoshikawa, and S. Tanuma, Appl. Phys. Lett. 117, 33103 (2020).

Link: https://iop.vast.ac.vn/theor/conferences/nctp/45/program.php?t=3&n=2&n2=0

More information about Dr. Hieu: Recently, Dr. Hieu has been awarded as Young Ta Quang Buu Prize for his work H. T. Nguyen-Truong, Appl. Phys. Lett. 108, 172901 (2016).

**2. Dr. Hirobumi Mineo, Associate Professor of Atomic Molecular and Optical Physics Research Group**

**Title: **Theoretical study of dynamic Stark-induced pi-electron rotations in low symmetry aromatic ring molecules beyond the frozen nuclear approximation

**Abstract:** Effects of vibrational modes on dynamic Stark-induced pi-electron rotations in low-symmetry aromatic ring molecules were theoretically studied under the adiabatic approximation. The lowest vibronic states in the two electronic excited states are set to be degenerate by using two UV lasers. The parameters of the two lasers (frequencies, intensities) were determined under the condition in which the lowest two vibronic states are in equal energy with each other. The resultant degenerate state is called dynamic Stark-induced degenerate vibronic state (DSIDVS), which is an essential key to generate coherent pi-electron angular momentum of aromatic ring molecules taking into account vibrational degrees of freedom. Closed forms of vibronic state wavefunctions are derived by solving the time-dependent Schrödinger equations analytically. Here the number of the vibronic excited states were restricted to extract the essential role of vibrational mode on pi-electron rotations. In this work we adopt toluene molecule, which is one of the typical aromatic ring molecules with lower symmetry.

Link: https://iop.vast.ac.vn/theor/conferences/nctp/45/program.php?t=3&n=6&n2=0

**3. Dr. Cam-Tu Le, Researcher of Atomic Molecular and Optical Physics Research Group**

**Title:** Effect of dynamic core-electron polarization on harmonic process of linear molecules**Abstract:** This talk gives a review of dynamic core-electron polarization (DCEP) effect on the harmonic process of linear molecules. High-order harmonic generation (HHG) which is one of nonlinear phenomena occurring when atoms/molecules subjected to intense ultrashort lasers [1] has become a powerful tool to probe and control molecular dynamics. In principle, the strong-field processes can be obtained from directly solving the time-dependent Schrodinger equation (TDSE). However, in practical, this method is usually handled by using some schemes such as single-active electron approximation to apply for more-than-two-electron systems. In this framework, we investigate the influence of the DCEP [2], which is induced by the driving laser, on the HHG stimulated from linear molecules, such as CO – polar molecule and CO 2 – nonpolar molecule. With the few-cycle pulses, the results show that DCEP plays an essential role for the polar molecule CO, it makes the results meet the agreement with experimental and other theoretical results. By investigating the time-dependent ionization rate, we explain completely the DCEP mechanism: it affects the harmonic intensity through the ionization at a specific instant, rather than throughout the whole pulse propagation as understood. Furthermore, based on the instantaneous ionization rate, one can predict the influence of DCEP on the harmonic intensity of polar molecules [3]. Meanwhile, for nonpolar molecules such as CO 2 , the DCEP can affect the harmonic process through a different mechanism – suppressing the distortion of the laser field on the highest occupied molecular orbital (HOMO) during the recombination phase, thus partially recovering the HOMO symmetry [4]. Consequently, this shifts and sharpens the minima in the HHG spectra, which arise from the two-center interference [5]. Furthermore, the influence of DCEP can manifest in case of multicycle pulses in which the DCEP affects the ratio of even-to-odd harmonic intensity emitted from CO due to the degree of symmetry breaking of the molecule-laser system [6].

References: [1] A. McPherson et al., J. Opt. Soc. Am. B 4, 595 (1987). [2] B. Zhang, J. Yuan, and Z. Zhao, Phys. Rev. Lett. 111, 163001 (2013). [3] C.-T. Le, V.-H. Hoang, L.-P. Tran, and V.-H. Le, Phys. Rev. A 97, 043405 (2018). [4] C.-T. Le, D.-D. Vu, C. Ngo, and V.-H. Le, Phys. Rev. A 100, 053418 (2019). [5] M. Lein, N. Hay, R. Velotta, J. P. Marangos, and P. L. Knight, Phys. Rev. Lett. 88, 183903 (2002). [6] N.-L. Phan, C.-T. Le, V.-H. Hoang, V.-H. Le, Phys. Chem. Chem. Phys. 21, 24177 (2019).

Link: https://iop.vast.ac.vn/theor/conferences/nctp/45/program.php?t=3&n=5&n2=0

More information about Dr. Cam-Tu: She is awarded as Young Scientist Prize of Vietnam Theoretical Physics Society for her works C.-T. Le, V.-H. Hoang, L.-P. Tran, and V.-H. Le, Phys. Rev. A 97, 043405 (2018) and C.-T. Le, D.-D. Vu, C. Ngo, and V.-H. Le, Phys. Rev. A 100, 053418 (2019), which is announced on the first day of this conference.

Besides these invited talks, some researchers from AIMaS also attend and present their works in this conference:

• ** Oral:**

Dr. Eibun Senaha (TPPG Group)

https://iop.vast.ac.vn/theor/conferences/nctp/45/program.php?t=2&n=4&n2=0

• ** Posters:**

Dr. Nguyen Duy Vy (AP Lab):

https://iop.vast.ac.vn/theor/conferences/nctp/45/program.php?t=1&n=2

MSc. Le Tri Dat (AP Lab):

https://iop.vast.ac.vn/theor/conferences/nctp/45/program.php?t=1&n=3

Dr. Do Minh Hoat (former Head of COLAMS)

https://iop.vast.ac.vn/theor/conferences/nctp/45/program.php?t=1&n=29

**We welcome all scientists to discuss and cooperate in research with our institute.**

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