Communications in Physics

Regulation of continuum harmonic generation by using near-infrared chirped pulses

Cam-Tu Le — 2023-03-31

It has been known that the broadband of continuum harmonics, a regular periodicity or a wide smooth range, is essential for generating attosecond pulses. However, the high-order rescatterings of electrons (HOR) can significantly affect this important part of high-order harmonic spectra. In the present work, we apply the method suggested in our previous studies to limit the HOR effect on the continuum harmonics by using up-chirped pulses with frequency increasing in time to regulate the high-order harmonic generation of an asymmetric molecule such as CO subjected to a near-infrared laser. To do so, we numerically calculate the high-order harmonic spectra using the time-dependent wave functions obtained within the framework of the single-active electron approximation. To analyze and regulate the HOR effect, we use the time-frequency analysis together with the kinetic energy maps obtained from the equation of electron motion in the laser electric field. These techniques reveal the high-order harmonic dynamics, especially the effect of chirped pulse on the manifestation of continuum harmonics. Based on this, we release the notice on choosing appropriate parameters to obtain the optimal continuum range of harmonics. Although the detailed analysis is given for the CO molecule, the method suggested is applicable for other polar molecules.

Generation and application of four-wave mixing in collinear high harmonic generation

Khuong Ba Dinh — 2023-04-21

We describe a thorough study of the wave-mixing procedure in the extreme ultraviolet (XUV) region involving three laser fields (800 nm, 1400 nm and 1860 nm). In addition to the phase matched HHG spectrum generated by an 800-nm laser (driving field), non-integer order wave-mixing spectra are produced when the driving field and the control field (1400 nm or 1860 nm) are collinearly focused into krypton gas. In addition, the simultaneous presence of three laser fields generates resolvable four-wave mixing (FWM) frequencies that clearly indicate the contribution of each control field. We also discuss an application of the FWM scheme to extend the HHG cutoff region and generate the XUV quasi-continuum spectrum.

Updated heat capacities of \(^{161-164}\)Dy nuclei

Huong Le — 2023-04-23

This work presents the updated heat capacities of \(^{161-164}\)Dy nuclei in the nuclear temperature region from 0 to 1 MeV. The updated heat capacities are obtained within the canonical ensemble method making use of the most recent and recommended experimental nuclear level density (NLD) data together with those calculated within the back-shifted Fermi gas (BSFG) model with energy-dependent parameters. By comparing the updated heat capacities with the un-updated ones, which are obtained by using the old experimental NLD data and the BSFG with energy-independent parameters, we found that the updated and un-updated heat capacities are almost identical at low temperature, but differ from each others at high temperature. This discrepancy can be interpreted by the damping of nuclear shell structure with increasing the excitation energy. Besides, we observe that the S-shape in the updated heat capacities is much more pronounced in even-even Dy isotopes than in even-odd ones, whereas the un-updated heat capacities do not clearly exhibit this S-shape. Therefore, the updated heat capacities should provide a more convincing evidence for the signature of pairing phase transition in nuclear systems.

Contribution of unparticles to the Møller scattering in the Randall-Sundrum model

Van Soa Dang — 2023-03-31

The influence of vector, scalar unparticle exchange and photon \((\gamma)\), \(Z\) boson, Higgs-radion \((h,\phi)\) exchange on the fundamental Møller scattering in Randall-Sundrum model is presented. We evaluated the contribution from each exchange in standard model physics and unparticle physics to the final production cross-sections with the apposite energy reach in the current accelerators and colliders. The results indicate that the effect of vector unparticles is larger than its scalar all over the range of collision energy, as much greater than Higgs \((h)\) and radion \((\phi)\) in the low energy. \(Z\) boson exchange also contributes the greatest to the process in both unpolarized and polarized conditions in higher energy regions when compared to photon exchange \((\gamma)\) which normally dominates at the lower energies.

Investigation of the radiative decay \(b \to s \gamma\) in the 3-4-1-1 model

Duy Nguyen Tuan — 2023-04-21

We investigate the branching ratio of the radiative decay \(b\to s \gamma\) in the 3-4-1-1 model with arbitrary charge parameters \(p,q\). We show that the new Higgs bosons, \(\mathcal{H}_1^\pm\), and the new gauge bosons \(W_{13,14 }^{\pm p,q}, Z_{2,3}\) contribute to this decay. Of these sources, the main contribution comes from the interaction of the singly-charged Higgs boson. If the spontaneous breaking of the enlarged gauge group symmetry down to the electroweak group is around a few TeVs, the predictions for the branching ratio Br\((b\to s \gamma)\) and meson mixing are both consistent with experimental constraints.

Investigate the potential of using reactor anti-neutrinos for nuclear safeguards in Vietnam

Son Cao — 2023-04-24

One of the most abundant man-made sources of low energy (few~MeVs) neutrinos, reactor neutrino, is not only useful for studying neutrino properties, but it is also used in practical applications. In this study, we investigate the potential of using reactor neutrino detectors for nuclear safeguards in Vietnam, specifically at the Dalat Nuclear Reactor, a future research facility, and presumably commercial reactors with 500~kW, 10~MW, and 1000~MW thermal powers, respectively. We compute the rate of observed inverted beta decay events, as well as the statistical significance of extracting isotope composition under the practical assumptions of detector mass, detection efficiency, and background level. We find that a 1-ton detector mass can allow us to detect the reactor's on-off transition state from a few hours to a few days, depending on the standoff distance and reactor thermal power. We investigate how background and energy resolution affect the precision of the extracted weapon-usable \(^{239}\)Pu isotope. We conclude that in order to distinguish the 10% variation of the \(^{239}\)Pu in the 10~MW thermal power reactor, a 1-ton detector placed 50~m away must achieve 1% background level. Increasing the statistics by using a 10x larger detector or placing it \(\sqrt{10}\) times closer to the reactor alleviates the requirement of the background level to 10%.

Design and simulation of a thermal neutron beam for neutron capture studies at the Dalat research reactor

Anh Trinh Thi Tu — 2023-04-23

This paper presents the application of the MCNP5 code to conduct a modification design and simulation of a thermal neutron beam for neutron capture studies at the Dalat Nuclear Research Reactor (DNRR). The designed configuration of the horizontal neutron channel No.2 at the DNRR, which contains a conical collimator of 240.3 cm in length, and neutron filters of crystal Al₂O₃ and Bi with alternative thickness, were simulated. A pure thermal neutron beam can be obtained at the irradiation position when a composition of crystal filter of 20 cm Al₂O₃ and 6 cm Bi is employed. The thermal and epithermal neutron fluxes are 1.02´10⁸ n/cm²/s (account for 97.92% of total neutron flux) and 0.22´10⁷ n/cm²/s (account for 2.08% of total neutron flux), respectively.

Characterization of structure and Li-ionic conductivity of La\(_{(2/3)−x}\)Li\(_{3x}\)TiO\(_{3}\) ceramics prepared by spark plasma sintering

Trong Le Dinh — 2023-04-23

In this work, La_(2/3)-xLi_3xTiO₃ (LLTO) dense ceramic samples have been prepared by high-energy ball milling and spark plasma sintering (SPS) route. The crystal structures and microstructures of the samples were characterized by X-ray powder diffraction and FE-SEM, whereas their Li-ionic conductivity properties investigated by AC impedance spectroscopy. At 21 ^oC, the LLTO ceramic samples possessed the grain conductivity and grain boundary/total conductivity of σ_g = 8.3×10^-4 S cm^-1 and σ_gb = 2.3×10^-5 S cm^-1, respectively. In the investigated temperature range from 21 ^oC to 120 ^oC, the ion conduction is governed by thermally activated mechanism. The activation energies for grain and grain boundary conductivities are E_ag = 0.26 eV and E_agb = 0.43 eV, respectively.

Monte Carlo investigation for an Ising model with competitive magnetic interactions in the dominant ferromagnetic-interaction regime

Oanh K. T. Nguyen — 2023-05-03

We apply classical Monte Carlo simulation to examine the thermodynamic properties of perovskites described by the Ising model with competitive magnetic interactions. By correspondingly adjusting the ferromagnetic-interaction and antiferromagnetic-interaction probabilities, \(p\) and \((1-p)\), in the regime \(p \ge 0.5\), the temperature dependence of magnetization, total energy, spin susceptibility, and specific heat consistently show a ferromagnetic to paramagnetic (FM-PM) phase transition at a critical temperature \(T_c\). Besides, the inverse susceptibility is confirmed to follow Curie-Weiss's law above another critical temperature \(T_{CW}\). By increasing the FM interaction probability, we have observed the FM-PM critical temperature \(T_c\) shifted to the higher value while the Curie-Weiss critical temperature \(T_{CW}\) moves to the lower. The different values between these two critical temperatures imply the inhomogeneity of the systems having phase separation, thus in agreement with the increased homogeneity with increasing \(p\).

The \(120^{0}\) ordered phase of the spin-1 \(J_{1} - J_{2}\) antiferromagnetic Heisenbeberg model on a triangular lattice

Van Hinh Nguyen — 2023-05-10

We study the effect of quantum and thermal fluctuations on the excitation energy spectrum and sublattice magnetization of the 120⁰ordered phase of the spin-1 antiferromagnetic Heisenberg model on a triangular lattice with nearest J₁and next-nearest-neighbor J₂exchange interactions. The auxiliary fermionic representation of the spin operators within a functional integral formalism with an imaginary Lagrange multiplier is employed to retain an exact constraint of single particle occupancy. Representing the classical ground state by Luttinger-Tisza ordering vector Q one may consider the fluctuation contributions to the free energy of the system in the entire range of the coupling parameters. We derived the magnon spectrum in one-loop approximation and the magnetization taking into account thermal fluctuations. The obtained results are compared with the result of the linear spin-wave approximation and experimental findings on compound NiGa₂S₄.