Table of contents

The 45th Vietnam Conference on Theoretical Physics (VCTP-45) was held during 12-14 October 2020 in Vinh Yen city, Vinh Phuc province, Viet Nam.

The VCTP-45 was organized by the Institute of Physics, Vietnam Academy of Science and Technology (VAST) under the auspices of the Vietnamese Theoretical Physics Society (VTPS).

The VCTP, formerly known as NCTP, has been an annual activity of VTPS for 45 years. The VCTP is aimed to be an international conference for physicists in Vietnam, in the region and worldwide. Our mission is to foster scientific exchanges and to promote a high-standard level of research and education in Vietnam and in South East Asia.

The conference covered a wide range of theoretical physics topics from 4 major fields:

•Particle, nuclear and astro-physics,

•Molecular physics, quantum optics and quantum computation,

•Condensed matter physics,

•Soft matter, biological and interdisciplinary physics.

The conference was divided into one opening session, 10 oral sessions and 2 poster sessions separated by coffee and lunch breaks. The duration of an invited talk is 30 minutes including time for discussion. The duration of an oral talk is 20 minutes including time for discussion.

Due to the Covid-19 pandemic, the conference was organized in a hybrid mode with both offline and online participation methods. The offline participation took place at the conference site, whereas the online participation was given over video conferencing. The Zoom and Microsoft Meeting softwares were used for the online participation and for broadcasting the offline sessions online. Poster presenters were requested to upload their posters on the conference website prior to the conference sessions. In general, both participation methods worked well except some limitations of the online method, such as for taking part in discussions. Due to international travel bans/restrictions, we did not have participants coming from abroad. There were, however, 3 participants who are from Japan and Philippines but working in Vietnam.

150 participants participated in the conference including 98 offline and 52 online participants. 8 invited talks, 25 oral and 98 poster contributions were presented.

This volume contains selected papers contributed by the participants.

Editors of the VCTP-45 Proceedings

Trinh Xuan Hoang, Institute of Physics, VAST, hoang@iop.vast.ac.vn

Hoang Anh Tuan, Institute of Physics, VAST, hatuan@iop.vast.ac.vn

Vu Ngoc Tuoc, Hanoi University of Science and Technology, tuoc.vungoc@hust.edu.vn

https://iop.vast.ac.vn/theor/conferences/nctp/45

Organizer, logo, photos, List of Participants, Online participants, are available in the pdf

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

• Type of peer review: Single-blind

• Conference submission management system: Authors submit papers through the conference website. Editors contact authors and referees via email.

• Number of submissions received: 21

• Number of submissions sent for review: 20

• Number of submissions accepted: 16

• Acceptance Rate (Number of Submissions Accepted / Number of Submissions Received X 100): 76%

• Average number of reviews per paper: 1

• Total number of reviewers involved: 18

• Any additional info on review process:

• Contact person for queries (please include: name, affiliation, institutional email address) Trinh Xuan Hoang, Institute of Physics, Vietnam Academy of Science and Technology, hoang@iop.vast.vn

A discrete differential evolution (DE) method has been applied to the problem of fuel loading pattern optimization of the Dalat Nuclear Research Reactor (DNRR). A classic strategy DE/rand/l/bin was chosen for the mutation of the DE method. Numerical calculations have been performed based on the core configuration of 100 highly enriched uranium (HEU) fuel bundles with various burnup levels. Comparison of the performance between the DE method and a genetic algorithm (GA) was also carried out. The optimal LPs obtained from the two methods are significantly better than the reference core. DE is more advantageous in exploring search space and approaching a global optimum than GA.

A discrete extra dimension of size 11.8 fm would imply the existence of vector X and scalar H bosons as photon's Kaluza-Klein partners. The massive vector X can be identified with the particle X17 suggested by ATOMKI's experiment. The particle model of extended photon sector coupled to extended nucleon, electron and neutrino also implies the neutron decay to its Kaluza-Klein partner. There are several different channels triggered by X and H. In this paper ve compare the contribution from them and imply some physical consequences on the parameters of the model.

Recently, we have proposed a general method to probe the whole range of degrees of orientation based on the time-frequency profile of high-order harmonic generation (HHG) but for perfectly aligned polar molecules only. In this study, we expand the previous results by considering the imperfect alignment of an ensemble of polar molecules. The "experimental" HHGs with molecular alignment distribution are simulated by numerically solving the time-dependent Schrödinger equation. The results show that, for high and moderate degrees of alignment, the proposed method is still effective and robust. For an ensemble with low degrees of alignment, the systematic error is significant due to the inaccuracy caused by the molecules aligned at large angles to the laser electric field.

This paper investigates the recollision dynamics of the nonsequential double ionization process induced by linearly polarized laser pulses with the three-dimensional classical ensemble model. The results show that the correlated two-electron momentum distribution is contaminated by the double recollision events for sufficiently short laser wavelength. When the laser wavelength increases from near-infrared to mid-infrared, the single-recollision events are more prominent than the double-recollision one. Moreover, the mechanisms governing the nonsequential double ionization process are also thoroughly studied in the case of double-recollision.

The density of condensates of a binary mixture of Bose gases restricted by two parallel planar walls is investigated within the framework of Cornwal-Jackiw-Tomboulis effective action approach in improved Hartree-Fock approximation. It results that the density of condensates strongly depend not only on the distance between two walls but also on the interspecies interaction strength and they are equal their expectation values of the field operators after adding a term associated with the quantum fluctuations.

The melting temperature T_m of gold has been determined from ambient pressure to 1500 kbar using statistical moment method (SMM) and the Lindemann criterion. The equation of the melting curve obtained is a quadratic polynomial of the melting temperature T_m, with coefficients that are explicitly dependent on pressure P. Simple number calculation and easily verify. Numerical results for the melting temperature of gold up to 1500 kbar are in good agreement with the experimental data and the theoretical results of other authors.

Based on the general expression of the Helmholtz free energy of alloys with Ll₂ structure obtained in previous works, we calculate the equations of the long-range order (LRO) parameter, and order-disorder transition (ODT) temperature in the Cu₃Au alloy, and obtained thermodynamic quantities of Cu₃Au alloy as simple functions of temperature and LRO-parameter, at pressures P = 0. Numerical results of the linear thermal expansion coefficient, isochoric and isobaric heat capacities, isothermal and adiabatic bulk modulus, are in good agreement with experiments.

Based on the quantum kinetic equation method, the quantum Ettingshausen effect has been theoretically studied under the influence of confined acoustic phonon in a cylindrical quantum wire (CQW) with infinite potential in the presence of a strong electromagnetic wave. We considered a quantum wire in the presence of a constant electric field, a magnetic field, an electromagnetic wave (EMW) with an assumption that electron – confined acoustic phonon (CAP) scattering is essential. Analytical results obtained show that the EC depends on the amplitude and the frequency of the EMW in a non-linear way. Besides, the impact of phonon confinement on the above effect characterized by m-quantum number in the expression of the EC. The theoretical results have been numerically calculated for the GaAS/AlGaAs cylindrical quantum wire model. The obtained results show that the phonon confinement contributes to the EC quantitatively and qualitatively. On the other hand m is set to zero, the result obtained is similar to the case of unconfined phonon. Furthermore, by considering the quantum size effect, the values of the EC increases, the position of the magnetic-phonon resonance peak changes, and the number of peak resonant peak increases while the radius of quantum wire declines. These obtained results are different from bulk semiconductor and unconfined phonon case which donates to the theory of the Ettingshausen effect in low-dimensional semiconductor systems.

Based on the kinetic equation method, the quantum Peltier effect has been theoretically studied under the influence of confined acoustic phonon in doped semiconductor superlattice in the presence of the electromagnetic wave (laser radiation). There were complicated dependences of the analytical expression of the Peltier coefficient (PC) on quantities such as doped concentration of the superlattice, amplitude of the laser radiation, the cyclotron frequency of electrons and temperature of the system. In detailed consideration, the quantum number m was changed in order to characterize the influence of confined acoustic phonon. When setting the m to zero, we obtained the results that corresponded to the case of unconfined phonon. The theoretical results have been numerically evaluated and discussed for the GaAs:Si/GaAS:Be doped semiconductor superlattice (DSS). We found the oscillation of the PC according to enhancement of cyclotron frequency of electrons. Moreover, position of resonant peaks has shifted under the influence of phonon confinement. In the case of low doped concentration (n _D), the PC decreased in non-linear way; then it reached a negative constant in high n _D value. The non-linear change of the PC also has been detected when investigating its dependence on the laser amplitude. All numerical results have shown that the magnitude of the PC decreased due to the increase of phonon confinement effect. In short, the confinement of acoustic phonon caused the change of the quantum Peltier effect in DSS of GaAs:Si/GaAS:Be in quantitatively as well as qualitatively.

The acoustomagnetoelectric (AME) effect in an GaAs/AlGaAs rectangular quantum wire with infinite potential (RQWIP) is investigated under the influence of electromagnetic wave (EMW) by using quantum kinetic equation (QKE) method. We obtain the electron distribution function in the interaction with internal and external phonon. By solving the inhomogeneous differential equation, the current density $\vec{J}$ and the acoustomagnetoelectric field (E _AME) are obtained in the dependence on the EMW amplitude, external acoustic wave frequency, the system temperature and the RQWIP length. The results are numerically evaluated for GaAs/AlGaAs quantum wire. We compares it to the result obtained in case of the bulk semiconductors and others low-dimension system in order to show the difference and the novelty of the results. The E _AME depends non-linearly on the wire length L _z and exhibits an oscillatory behavior as the function of wire length, although the stability period in long wire length condition. Moreover, we survey the impact of EMW on E_AME with the dependence on the external acoustic wave frequency $\omega \vec{q}$ The result also indicates that the current density $\vec{J}$ amplifies exponentially as the temperature increases.

In this work we consider a quadron – a state of four carriers – two electrons and two holes in quantum dots with parabolic confinement. By using the Hatree-Fock method, the binding energies of the quadron and biexciton have been calculated for a full range of the electron-to-hole oscillator length ratio. The effect of the magnetic field on the binding energy of the quadron and biexciton has been also investigated. The crossing between anti-binding and binding in both the excited quadron and biexciton states for the certain ranges of the electron-to-hole oscillator length ratio has been found.

Correlation ratios between the mean square displacement (MSD), mean square relative displacement (MSRD), and correlated displacement function were studied in extended X-ray absorption fine structure spectra (EXAFS). The expressions of MSD, MSRD, and correlation function were determined using Debye models. Hardy problems due to many-particles effects were considered and replaced by a calculation based on the effective anharmonic potential, including the interaction of absorbing and scattering atoms with their nearest neighbours atoms. Based on the Debye-Waller factor, the difference between MSRD and MRD was analyzed, and their ratios have calculated. The methods were applied to fcc crystals and their alloys. Numerical results for Cu, Ag crystals and CuAg50 alloys agreed with experimental values and other studies.

We derive the electronic phase diagram of the half-filled Anderson - Falicov-Kimball model with Gaussian distribution for impurities and box distribution for Coulomb disorder via dynamical mean field theory combined with typical medium theory. We show that the metal and the Mott insulator regions shrink as the strength of Coulomb disorder increases. The influence of the difference distributions (the box and the Gaussian) for impurities is also considered.

We study a Kitaev-Heisenberg model of spin 1/2 using the Popov-Fedotov fermionization proccedure. We derive the free energy of the quantum spin system on a Bravais lattice within one-loop approximation with exact on-site constraint. We discuss the obtained magnetic excitation spectrum for a particular case of square lattice in relation to the data derived by the equation of motion method and with the result of the linear spin wave theory in Holstein - Primakoff representation.

In this report, using a kinetic Monte Carlo simulation method we considered a hierarchical assembly of planar square tetramers, constrained in a plane, via droplet evaporation. We found that colloidal tetramers undergo a structural transition from initial square-shape tetramers to diamond-like tetramers. A variety range of novel cluster configurations belongs to a particular class of geometrical shape, that has not been previously reported to the clusters of single colloids. Considering potential applications of colloidal clusters in electronics, photonic materials, our result could provide an effective way to control the formation of complex colloidal cluster structures.

A model for the dimeric form of the human TSPO (hTSPO) protein is constructed homologically using the RsTSPO dimer template. Then, Molecular dynamics simulation of 1μs is carried out on the model to investigate its stability as well as to study its various physicochemical properties. Our model shows good stability, with various important conserved residues involving in monomer–monomer interactions, in good agreements with available experimental data. Several functioning motifs are well demonstrated. This model hence can serve as a good basis for further study of this system as well as for discoveries of candidate radioligands targeting TSPO protein as a biotracer for medical imaging of brain inflamation.